Patent Publication Number: US-11654221-B2

Title: Dialysis system having inductive heating

Description:
PRIORITY CLAIM 
     This application claims priority to and the benefit as a continuation application of U.S. patent application Ser. No. 16/271,417, entitled, “Systems and Methods For Priming Hemodialysis Using Multiple Fluid Sources”, filed Feb. 8, 2019, now U.S. Pat. No. 10,426,883, issued Oct. 1, 2019, which is a continuation of U.S. patent application Ser. No. 15/163,128, entitled, “Systems and Methods For Priming Hemodialysis Using Dialysis Fluid”, filed May 24, 2016, now U.S. patent Ser. No. 10/245,369, issued Apr. 2, 2019, which is a continuation of U.S. patent application Ser. No. 13/749,309, entitled, “Systems and Methods for Priming Sorbent-Based Hemodialysis Using Dialysis Fluid”, filed Jan. 24, 2013, now U.S. Pat. No. 9,364,602, issued Jun. 14, 2016, which is a continuation application of U.S. patent application Ser. No. 13/222,622, entitled, “Systems And Methods For Priming Sorbent-Based Hemodialysis”, filed Aug. 31, 2011, now U.S. Pat. No. 9,072,830, issued Jul. 7, 2015, which is a continuation application of U.S. patent application Ser. No. 10/982,170, entitled, “High Convection Home Hemodialysis/Hemofiltration And Sorbent System”, filed Nov. 4, 2004, now U.S. Pat. No. 8,029,454, issued Oct. 4, 2011, which claims priority to and the benefit of U.S. Provisional Patent Application No. 60/517,730, filed Nov. 5, 2003, entitled, “High Convection Home Hemodialysis/Hemofiltration And Sorbent System”, the entire contents of each of which are hereby incorporated by reference and relied upon. 
    
    
     BACKGROUND 
     The present invention relates generally to medical treatments. More specifically, the present invention relates to medical fluid treatments, such as the treatment of renal failure and fluid removal for congestive heart failure. 
     Hemodialysis (“HD”) in general uses diffusion to remove waste products from a patient&#39;s blood. A diffusive gradient that occurs across the semi-permeable dialyzer between the blood and an electrolyte solution called dialysate causes diffusion. Hemofiltration (“HF”) is an alternative renal replacement therapy that relies on a convective transport of toxins from the patient&#39;s blood. This therapy is accomplished by adding substitution or replacement fluid to the extracorporeal circuit during treatment (typically ten to ninety liters of such fluid). That substitution fluid and the fluid accumulated by the patient in between treatments is ultrafiltered over the course of the HF treatment, providing a convective transport mechanism that is particularly beneficial in removing middle and large molecules (in hemodialysis there is a small amount of waste removed along with the fluid gained between dialysis sessions, however, the solute drag from the removal of that ultrafiltrate is not enough to provide convective clearance). 
     Hemodiafiltration (“HDF”) is a treatment modality that combines convective and diffusive clearances. HDF uses dialysate to flow through a dialyzer, similar to standard hemodialysis, providing diffusive clearance. In addition, substitution solution is provided directly to the extracorporeal circuit, providing convective clearance. 
     Home hemodialysis (“HHD”) has declined in the last twenty years even though the clinical outcomes of this modality are more attractive than conventional hemodialysis. One of the drawbacks of home hemodialysis is the need for a dedicated water treatment, which includes equipment, water connection and drainage. Installing and using those components is a difficult and cumbersome task that can require a patient&#39;s home to be modified. Nevertheless, there are benefits to daily hemodialysis treatments versus bi- or tri-weekly visits to a treatment center. In particular, a patient receiving more frequent treatments removes more toxins and waste products than a patient receiving less frequent but perhaps longer treatments. 
     SUMMARY 
     The present invention provides a system, method and apparatus that performs a daily renal replacement therapy, which combines both diffusion and convection transport from the patient. In hemodialysis, high flux membranes can in some cases backfilter fluid from the dialysate to the blood even though, on balance, net fluid flow is from the patient. That backfiltration is due to a pressure differential between the inlet/outlet of the blood and inlet/outlet of dialysate in specific areas of the dialyzer. The present invention capitalizes on that phenomenon. 
     In one embodiment, two small high flux dialyzers are connected fluidly to the cassette in series. Dialysate and blood flow in a countercurrent manner through the dialyzers and extracorporeal circuit. In one embodiment, however, the dialysate flow through the dialyzers can alternatively be co-current or in the same direction as the flow of blood through the blood circuit. A restriction is placed between the two dialyzers in the dialysate flow path. The restriction is variable and adjustable in one preferred embodiment to account for different treatment conditions or to be adjusted during a single treatment. The restriction is alternatively a simple fixed restriction, such as an orifice plate with a smaller orifice. Due to the restriction between the filters, a positive pressure is built in the venous dialyzer, causing a high degree of intentional backfiltration. Depending on the size of the restriction between the dialyzers, that backfiltration causes a significant flow of dialysate through the high flux venous membrane directly into the blood. That backfiltered solution is subsequently ultrafiltered from the patient from the arterial dialyzer. The movement of dialysate into the blood in the venous filter and removal of dialysate from the arterial dialyzer causes a convective transport of toxins from the patient. Additionally, the dialysate that does not move directly into the patient but instead flows across the membranes of both dialyzers provides a diffusive clearance of waste products. 
     The system therefore acts as a hemodiafiltration system providing both convective and diffusive clearances. The system in one embodiment is configured for home use, wherein at least a portion of the dialysate and extracorporeal flow paths is sterilized and provided in a disposable cassette, which is loaded into a home pumping apparatus. For example, the system can be a portable device that uses an integrated disposable fluid management system or cassette and a sterile, prepackaged solution to perform a hemodialysis therapy. The system in one embodiment is particularly suited for home use because of its compact size, ease of therapy setup, and lack of need for a water treatment and dialysate proportioning system. 
     Unlike current hemodialysis machines, the patient does not have to manage complicated tubing sets. The patient simply places the cassette into the renal failure therapy machine, connects solution bags to the machine and starts an automated priming sequence. When the priming is complete, the patient connects the bloodlines to the patient&#39;s body and starts the dialysis therapy. At the end of treatment the patient&#39;s blood is returned to the patient&#39;s body. The patient merely discards the ultrafiltrate (“UF”) waste and the therapy ends without the patient having to perform a complicated disinfection procedure. 
     In one embodiment, the cassette-based system operates as follows. A blood pump pulls blood from the patient, pushes it through both hemodialyzers and returns the blood to the patient. Dialysate solution is drawn from a dialysate source and heated to a desired patient temperature. Infusion pumps pump fresh dialysate from the bag into the venous dialyzer. The restriction is placed in the dialysate flow path between the two dialyzers to facilitate the backfiltration of dialysate into the bloodline via venous dialyzer. The restriction is preferably variable but alternatively fixed. 
     The flow out of the infusion pumps pushes fluid at the restriction creating a positive pressure in the venous hemodialyzer. Using a high flux membrane, the backpressure forces a portion of the dialysate, e.g., fifty percent or more, into the patient&#39;s bloodline. The rest of the dialysate flows through to the arterial dialyzer. Drain pumps remove from the flow paths an equivalent amount of fluid as delivered by the infusion pumps as well as any fluid loss that the patient has gained in the interdialytic period. The spent fluid and ultrafiltrate are then put into a drain bag or dumped to an external drain. 
     The cassette-based dialysate pumps are controlled to balance the dialysate flow to the venous dialyzer with the dialysate flow from the arterial dialyzer so that the patient fluid status is maintained. Due to that balancing capability an identical amount of fluid is ultrafiltered from the patient in the arterial hemodialyzer as is backfiltered into the extracorporeal circuit in the venous dialyzer. Ultrafiltering this fluid from the blood creates a solute drag effect providing a convective transport of toxins similar to hemofiltration. Since some dialysate flows along the fiber in the venous to arterial dialyzer there is also diffusive transport of toxins from the blood. 
     Air bubble detectors, heating elements, pressure sensors, temperature sensors, etc., are also integrated into the cassette for both the dialysate management and extracorporeal blood sides as necessary to allow for a safe treatment for the patient and reliable operation of the system. 
     Recently published studies show that ultrapure dialysate produces better outcomes when compared to standard dialysate. The prepackaged, sterilized dialysate used in one embodiment of the present invention may produce outcomes that are as good as, if not better than, ultrapure dialysate. It should be appreciated however that the present invention is not limited to the use of prepackaged dialysate bags, but instead, may use dialysate prepared on-line or at home. The advantage of the online system to the patient is to eliminate the solution bags and the space they consume. The dialysate, whether supplied from a sterilized bag or made online, may also be recirculated in one or more loops using one or more charcoal or sorbent cartridge. 
     One preferred at home generation system is described herein. That system uses a reservoir, such as a five liter bag of sterile dialysate installed in a rigid container. A shunt is placed across the dialyzers at start-up for rinsing and priming. During treatment, a sorbent cartridge that operates using an urea exchange or a binding urea is placed in the post dialyzer or ultrafilter (“UF”) loop. The sorbents may remove other substances, such as beta 2 microglobulin or phosphate, etc. A series of infusion pumps simultaneously pull dialysate from the sterile bag, through a heater, through an ultrafilter and through the shunt to the sorbent cartridge. If necessary, an infusate such as a gamma sterilized infusate that includes calcium, magnesium, and potassium is added to the dialysate reservoir. 
     After the solution is heated and ready for treatment, the blood treatment machine prompts the user to install the cassette. The blood circuit can be primed with a saline bag hooked via the arterial bloodline or by backfiltering dialysate through the blood treatment venous filter. Air bubble detectors, heating elements, pressure sensors, temperature sensors, etc., are integrated into the cassette for both the dialysate and extracorporeal blood circuits as necessary to enable a safe treatment for the patient and a system that operates reliably. 
     The patient is then hooked to the arterial and venous needles and the treatment begins. For short therapies, the dialysate flow can be relatively high, for example, three hundred ml/min for three hours or one hundred ml/min for up to eight hours. The dialysate/UF flow control pumps control the flow to and from the dialyzers. By increasing the frequency of the pumps that pull the effluent dialysate from the arterial dialyzer, the fluid accumulated in the patient in the interdialytic period is removed. Portions of the dialysate/UF flow control pumps are integrated into the cassette along with a portion of the blood pump in one embodiment or are alternately provided separate from the cassette and integrated into the machine. 
     Due to the impracticality of hanging and storing bags, solution-bag based systems are limited to a total practical amount of dialysate per treatment. The sorbent-based fluid regeneration system enables a therapy that uses more dialysate and thereby provides enhanced waste clearance. Providing an increased amount of dialysate beneficially enhances the clearance of waste products from the renal patient. For example, the sorbent cartridge could be used for a four hour treatment at two hundred to two hundred fifty ml/min dialysate flow or about fifty liters of dialysate over the entire treatment, which would provide an increased volume of dialysate and better waste clearance over other hemofiltration systems. The sorbent system is also applicable to the hemofiltration systems described herein, making even predilution HF possible. For hemofiltration, an additional reusable ultrafilter is provided to maintain redundancy of bacteria and endotoxin removal. 
     The sorbent-based regeneration system is particularly suited for home use because it eliminates the need to store numerous solution bags, eases therapy setup and does not require a connection to the patient&#39;s water tap. Also, the patient does not have to connect a tubing set. The patient instead places the cassette into the machine, adds an initial five liter bag of sterile dialysate to the reservoir and starts the automated priming sequence. When the priming is complete, the patient connects himself/herself to the blood circuit and starts the dialysis therapy. 
     The portable device, the use of prepackaged solutions or an on-line fluid generation system and the use of a disposable set each provide dialysis patients with the flexibility and freedom that previously has only been available to peritoneal dialysis patients. Because there is no dedicated water hookup and the present machines are small, it is possible for a patient using the present systems to travel and perform blood therapy dialysis sessions on the road. Many of the systems and methods described herein can be adapted to work with in-center solutions, and many of the aspects of the present invention are not limited to home use. 
     High convection hemodialysis is believed to be more effective than conventional hemofiltration because it has convective clearance in addition to the diffusive transport of toxins. The therapy is expected to provide good waste clearance of small, middle and large molecules from even end-stage renal patients. 
     The device is well-suited for use in hospitals for acute patients for situations in which a required water supply and dialysis proportioning system are unavailable. The present device is easier to set up and use in an intermittent acute setting. 
     The present invention provides multiple methods and apparatuses for not only controlling the amount of dialysate or substitution fluid that is delivered to the extracorporeal circuit or dialyzer but also for accurately controlling the amount of ultrafiltrate removed from the patient. The various alternatives can be divided into three main types. One type of control used is a pneumatic control based on Boyle&#39;s Law. Here, the fluid pumps are placed in fluid communication with a known volume of air. The system uses Boyle&#39;s Law to place into an equation a series of known or measured values to calculate accurately the amount of fluid (e.g., versus air) from a pump chamber pumped to the patient. The method and apparatus use fluid and air pressure signals generated and converted to numbers that are placed into an equation. The equation yields the fluid volume pumped per cycle or stroke of the pump. The Boyle&#39;s law system in one embodiment provides accurate information on an end stroke or pump cycle basis but not necessarily on a real time basis. The present invention also includes a system and method based on Boyle&#39;s Law that generates flow rate data on a real time basis. 
     A second large category of volumetric control includes the use of a balancing device. Many embodiments for employing such balancing device are discussed below. The balancing device embodiments may be parsed into two main sub-groups. One sub-group uses a single balancing device. Another sub-group includes dual balancing devices. 
     The present invention also teaches and discloses a plurality of different types of balancing devices. In one embodiment, the system employs one or two balancing chambers. In another embodiment, the system employs one or two balancing tubes. The balancing tubes include a tubular housing with a piston or ball-like separator within the housing. The separator acts similarly to the membrane or diaphragm of the balance chamber. 
     A third type of balancing device is one or more tortuous path. The tortuous path is defined in one embodiment by a disposable cassette as an elongated channel. The diameter or cross-sectional area of the channel is configured so that bulk movement of fresh or effluent dialysate can efficiently move an existing bulk of fluid within the tortuous path. That is, fresh dialysate in bulk moves a bulk of spent or effluent dialysate currently residing in the path to drain. In the next cycle, spent or effluent dialysate in bulk pushes the bulk of fresh fluid just introduced into the tortuous path to the patient or dialyzer. The cross-section and the length of the path are configured to minimize an amount of mixing of the fresh and spent fluids at the ends of the bulks of fluid. 
     The various volumetric balancing devices can be used with many different types of pumps, such as a peristaltic pumps, membrane pumps, gear pumps or a combination thereof. A single pump may be used with the balancing devices. Separate fresh and spent dialysate pumps may be used alternatively. Further, a separate ultrafiltrate pump is also contemplated and discussed, which enables the main pump(s) to be dedicated to pumping an equal volume of fluid to and from the patient. 
     The third major type of fluid management uses a scale to measure the amount of fluid delivered to the patient and the amount of fluid removed from the patient. In an embodiment illustrated below, fluid bags are placed on a stand, which is coupled to a shaft. At one end, the shaft couples to a rolling diaphragm. The rolling diaphragm, in combination with other apparatus, defines a closed but variable volume. As the weight in the fluid bags fluctuates, a pressure within the volume also fluctuates. A pressure sensor senses the pressure and the controller or processor of the machine processes the signal from the pressure sensor to develop a corresponding weight signal. The weight signal is then used to determine how much fluid has been delivered and or removed from the patient. In one embodiment, fresh and spent fluid bags are measured by the same weight sensing device, so that the system expects to see a net overall weight gain over time due to the ultrafiltrate removed from the patient. A load cell could also be used for this application. 
     As illustrated in detail below, the present invention provides multiple embodiments for other components of the systems and methods of the present invention, such as the fluid heater, the balancing devices, the disposable cassette, bag positioning and other important features of the present invention. For example, the present invention includes an access disconnection sensor (“ADS”), which can detect when either the arterial or venous needle has been removed inadvertently from the patient during treatment. Further, various pressure relief schemes, integrity tests, etc., are discussed herein, which are important especially for a home-use machine, which the patient may be use while sleeping. 
     It is therefore an advantage of the present invention to provide a hemodialysis, hemofiltration or hemodiafiltration system usable in a home or clinic setting. 
     It is another advantage of the present invention to provide a cassette-based hemofiltration/hemodiafiltration system, which enables a patient at home to easily set up a sterile blood therapy system. 
     It is another advantage of the present invention to improve the effectiveness of renal failure blood treatment therapy. 
     Moreover, it is an advantage of the present invention to provide a renal failure blood therapy that employs convective and diffusive modes of clearance. 
     Still further, it is an advantage of the present invention to provide a renal failure blood therapy in which both diffusive and convective clearance modes are provided and wherein the percentage use of either mode can be varied. 
     Further still, it is an advantage of the present invention to provide a cassette-based blood therapy that is configurable in the field to perform either hemodialysis, enhanced convection hemodialysis, hemofiltration or hemodiafiltration. 
     Yet further, it is an advantage of the present invention to provide a blood therapy system with one or more therapy fluid circulation loops that optimize the consumption of fresh dialysate. 
     Still another advantage of the present invention is to provide a home renal failure blood treatment therapy that is configurable to operate with multiple different types of therapy fluid sources, such as solution bags, solution preparation units or on-line dialysate generation systems. 
     It is yet a further advantage of the present invention to provide a home renal failure therapy system operable with many types of systems that control accurately the amount of fluid exchanges and the amount of fluid or ultrafiltrate removed from the patient. 
     Still further, it is an advantage of the present invention to provide improved fluid volume control devices. 
     Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the figures. 
     Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG.  1    is a schematic illustration of one embodiment of a renal failure blood treatment therapy system of the present invention that provides diffusive and convective clearance modes. 
         FIGS.  2  and  3    are perspective views of one embodiment of a disposable cassette and associated flow components for use with the blood treatment therapies described herein. 
         FIG.  4    is a schematic illustration of a renal failure therapy system that operates with a dialysate fluid generation unit. 
         FIG.  5    is a schematic illustration of a renal failure blood treatment therapy system having a therapy fluid recirculation loop. 
         FIG.  6    is a schematic illustration of one embodiment of a home use hemofiltration system of the present invention. 
         FIG.  7    is a schematic view of another embodiment of a home use hemofiltration system of the present invention. 
         FIG.  8    is a schematic view of one embodiment of a home use hemodiafiltration system of the present invention. 
         FIGS.  9  to  11    show various embodiments of a home use blood treatment therapy that employs a regeneration unit that regenerates and reuses spent dialysis fluid and fluid ultrafiltered from the patient. 
         FIGS.  12  and  13    are alternative hemodialysis and hemofiltration systems using peristaltic pumps to pump the therapy fluid. 
         FIG.  14    is an alternative hemodialysis system, wherein the flow of dialysate and blood are co-current. 
         FIGS.  15  and  16    are schematic views of one embodiment of a pneumatically controlled method and apparatus for controlling the volume of ultrafiltrate removed from the patient. 
         FIGS.  17  to  22    are schematic flow diagrams of various embodiments for controlling the volume of ultrafiltrate removed from the patient via a single balance chamber. 
         FIG.  23    is a schematic flow diagram illustrating various steps of one ultrafiltrate control method and apparatus employing a single balance tube. 
         FIG.  24    is a schematic flow diagram illustrating one embodiment for controlling the volume of fluid exchanged with the patient and the volume of ultrafiltrate removed from the patient employing a single tortuous path. 
         FIGS.  25  and  26    are schematic flow diagrams illustrating various features and advantages associated with an ultrafiltrate control method and apparatus that employs dual balance chambers. 
         FIGS.  27 A to  27 D  are schematic flow diagrams illustrating the valve operation and associated flow outcomes of another method and apparatus for controlling the volume of fluid exchanged with the patient and the volume of ultrafiltrate removed from the patient, which includes dual balance tubes. 
         FIG.  28    illustrates one alternative valve arrangement for the balance tube volume control device of the present invention. 
         FIG.  29    is a schematic flow diagram illustrating yet another embodiment for controlling the volume of ultrafiltrate removed from the patient, which includes dual tortuous paths. 
         FIGS.  30  and  31    illustrate yet a further alternative embodiment for controlling the amount of fluid that has been exchanged with and the amount of ultrafiltrate removed from the patient, which includes a weight measurement system. 
         FIG.  32    is an elevation view of one embodiment of an enhanced convection of hemodialysis filter of the present invention. 
         FIG.  33    is a schematic view of one embodiment for the variable flow restriction located between the dual dialyzers of the present invention. 
         FIG.  34    is a perspective view showing the cassette operably configured with flow actuation components of the dialysis systems of the present invention. 
         FIG.  35    is a perspective view of one embodiment for operably coupling the solution bags to the renal failure therapy machine of the present invention. 
         FIGS.  36  and  37    are perspective views of embodiments for coupling the solution bags to the renal failure therapy machine, which also show one embodiment for enabling the machine to receive the cassette of the present invention. 
         FIG.  38    is a perspective view of an alternative embodiment for pumping therapy fluid employing linear tubing pumps. 
         FIG.  39    is a perspective view of one embodiment for operably coupling the solution bags to a system using linear tubing pumps. 
         FIG.  40    is a schematic diagram showing one embodiment of a cassette of the present invention, which operates linear tubing pumps of the present invention. 
         FIG.  41    is a schematic illustration of another embodiment of a cassette of the present invention, which operates with linear tubing pumps. 
         FIGS.  42  and  43    are sectioned perspective views of different alternative implementations of one embodiment of a fluid heater of the present invention. 
         FIG.  44    is a cutaway section view illustrating one embodiment for incorporating a balance chamber into a disposable cassette. 
         FIG.  45    is a perspective cutaway view of one embodiment of the balance tube of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     The present invention provides various apparatuses and methods for a home hemodialysis (“HHD”) treatment that increases and enhances the amount of backfiltration during treatment. It is important to note that even though this system is designed for the home, it is also suitable for use in a clinic, acute renal treatment center or self-care center. The system uses a disposable fluid management system, which may include a disposable set having a disposable cassette or tubing organizer (referred to herein collectively as cassette). The cassette houses at least a portion of at least one of the dialysate and extracorporeal flow paths. In one embodiment, two small high flux dialyzers are connected fluidly and in series to the cassette. In one embodiment, dialysate and blood flow in a countercurrent manner through the dialyzers with respect to each other. A restriction is placed between the two dialyzers in the dialysate flow path. The restriction is variable and adjustable in one embodiment to account for different treatment conditions or to be adjusted during a single treatment. The restriction is alternatively fixed, such as an orifice plate with a restricting orifice. 
     Due to the restriction between the filters, a positive pressure is built in the venous dialyzer (first dialyzer to receive dialysate but second dialyzer to receive blood in countercurrent arrangement), intentionally causing a relatively high degree of backfiltration. Depending on the size of the restriction between the dialyzers, that backfiltration causes a significant flow (e.g., 10 to 70 percent of total dialysate flow) of dialysate through the high flux venous membranes and into the blood circuit. The backfiltered solution provides convective clearance. In one embodiment, ultrafiltrate is removed from the patient via the arterial dialyzer (first dialyzer to receive blood but second dialyzer to receive dialysate in countercurrent arrangement). 
     The diffusion of dialysate into the venous dialyzer and removal of dialysate from the arterial dialyzer causes a convective transport of toxins from the patient. Additionally, the dialysate that does not move directly into the extracorporeal circuit (e.g., the other percentage of the dialysate) but instead flows across the membranes of both dialyzers, providing a diffusive clearance of waste products. This system, referred to herein as an enhanced convection hemodialysis (“ECHD”) system, is similar to a hemodiafiltration system, which provides both convective and diffusive clearances. The system in one embodiment is configured for home use, wherein at least a portion of the dialysate and extracorporeal flow paths is sterilized and provided in a disposable set, which is loaded into a machine having multiple pumps, a heater, valve actuators and the like. 
     Enhanced Convection Hemodialysis (“ECHD”) 
     Referring now to the drawings and in particular to  FIG.  1   , one embodiment of the renal failure therapy system  10  of the present invention is illustrated. System  10  employs two or more high flux hemodialyzers, such as a venous dialyzer  20  and an arterial dialyzer  30 . In one embodiment, hemodialyzers  20  and  30  are relatively small, e.g., on the order of one quarter to three meters2 of membrane surface area. Dialyzers or hemodialyzers  20  and  30  are relatively high flux dialyzers, e.g., having a UF coefficient of eight milliliters of water diffused per hour per millimeters Hg pressure or greater (as used herein, the term “flux” refers to the above UF coefficient, which measures the ease of water transport through the membrane, expressed in milliliters/hour/millimeter Hg). 
     As discussed above, hemodialyzers  20  and  30  cause backfiltration in the venous dialyzer  20  of a relatively large portion of the fresh dialysate. The backfiltered dialysate and the fluid accumulated during the interdialytic period is ultrafiltered or removed from the patient  42  via the arterial dialyzer  30 . The fluid not backfiltered flows across the semi-permeable membrane in the arterial  30  and venous  20  dialyzers, enabling system  10  to provide both diffusive and convective removal of waste from the patient&#39;s blood. 
     In one home use and in-center embodiment shown in  FIG.  1   , sterile dialysate is stored in bags or containers  14 ,  16  and  18  (more than three solution bags may be used). System  10  in the illustrated embodiment employs pumps  22 ,  24 ,  26  and  28  that each operate with a respective volume measuring device  32 ,  34 ,  36  and  38 . As described in detail below, various volumetric measuring devices are used alternatively with the systems of the present invention. One measuring device is a capacitance fluid volume sensor that measures the volume of fluid pumped through one of the pumps  22  to  28 . That measurement in one embodiment informs a controller or microprocessor how much fluid (or air) has been pumped. The controller or microprocessor compares the actual amount of fluid pumped to an expected amount of fluid pumped and adjusts the pumping rates accordingly to make-up or back-off the delivery of new fluid to dialyzers  20  and  30  as needed. Alternatively or additionally, the capacitive measuring devices  32  to  38  can sense when a larger volumetric error in the system occurs and trigger, for example, an error message (e.g., when air becomes trapped in the system or a majority of a stroke length is missed). 
     It should be appreciated that the present invention is not limited to capacitive fluid volume measuring but can use instead other suitable types of volume measuring. Moreover, the present invention is not limited to volume measuring but instead can employ balancing devices that ensure a set amount of dialysate is pumped to the dialyzers, from the dialyzers and from the patient  42 . Further alternatively, fluid pump management can be accomplished on a mass basis, via one or more scale. Still further, flowrate and volume pumped can be calculated based on a number of pump strokes, such as a number of peristaltic pump revolutions based on a number of steps of a stepper motor, based on a sensed amount of movement of a linear or rotating pump actuator or via a device that operates according to Boyle&#39;s Law. All of those measuring alternatives are included in the term “volume measuring device.” Control using the volume measuring device can be closed loop, where the actual amount of fluid delivered is monitored, or open loop, where the scheme relies on the inherent accuracy of the pump and perhaps motion control feedback, such as a monitoring of number of step pulses sent to drive the motor, linear encoder feedback or rotary encoder feedback, etc. 
       FIG.  1    illustrates two pumps  22  and  24  for Pump Set  1  and two pumps  26  and  28  for Pump Set  2 . It is important to note that a single pump may alternatively be used in place of each set of pumps, e.g., one to input dialysate to the dialyzers and one to remove dialysate from the dialyzers and UF from the patient, however, that configuration would create pulsatile or uneven flow, which is less desirable. In the illustrated configuration, a first pump of each set is pulling fluid from the pump set&#39;s source, while a second pump of each set is pushing fluid towards the pump set&#39;s destination. After that set of pump strokes, the roles of the pumps in the respective sets alternate, so that the first pump (now full of fluid) pushes fluid towards the pumps set&#39;s destination, while the second pump (now empty) pulls fluid from the pump set&#39;s source. The above cycle is repeated multiple times. 
     Pump Set  1  inputs fresh dialysate from bags  14  to  18  to the system  10  and Pump Set  2  removes a volumetric equivalent of the fluid pumped by Pump Set  1  and any fluid removed from patient  42  during the course of the treatment. As illustrated, fresh dialysate is pumped via pumps  22  and  24  from sources  14 ,  16  and  18  to the venous dialyzer  20 . A restriction  40  is located between venous dialyzer  20  and arterial dialyzer  30 . Restriction  40  builds pressure in venous dialyzer  20 , so that a relatively large amount of fresh dialysate entering venous dialyzer  20  is forced through the walls of the membranes inside venous dialyzer  20  and into the extracorporeal or blood circuit  50 . The other portion of the fresh dialysate entering venous dialyzer  20  flows across the membranes inside venous dialyzer  20 , through restriction  40  and into arterial dialyzer  30 . 
     Convective clearance occurs when a volumetric equivalent of the fluid backfiltered through venous dialyzer  20  is removed from the arterial dialyzer  30 . Also, a diffusive transport of toxins occurs across both dialyzers  20  and  30  due to a diffusive gradient that exists between blood circuit  50  and the flow of dialysate. Over the total therapy, the total amount of fluid removed from the arterial dialyzer  30  is greater than the total amount of dialysate supplied to the venous dialyzer  20 , accounting for an amount of UF removal prescribed for the therapy. 
     Example 
     The following example further illustrates one preferred therapy for the present invention. In the example, pumps  22  and  24  of Pump Set  1  infuse eighteen liters of dialysate from sources  14 ,  16  and  18  over two hours. Of that volume, one hundred ml/min of dialysate is backfiltered into the patient&#39;s blood circuit  50  through the membrane walls of venous dialyzer  20 . Fifty ml/min of dialysate passes through the venous dialyzer  20 , restriction  40  and into venous dialyzer  30 . Pumps  26  and  28  of Pump Set  2  remove the total of eighteen liters of dialysate from bags  14 ,  16  and  18  plus any desired amount of fluid from the patient. Over two hours, twelve liters (100 ml/min multiplied by 120 minutes) of dialysate is backfiltered into the patient&#39;s blood through the venous dialyzer  20 . Pumps  26  and  28  of Pump Set  2  remove that twelve liters, the six liters of dialysate that is not backfiltered into blood circuit  50  plus any fluid ultrafiltered from the patient. 
     The addition and removal of the twelve liters of dialysate from blood circuit  50  over the two hour therapy yields an overall convective removal according to the equation HF stdKt/V of ˜2, which has been reported to be a suitable daily amount (see Jaber B T, Zimmerman D L, Leypoldt J K. Adequacy of Daily Hemofiltration: Clinical Evaluation of Standard Kt/V (stdKt/V), Abstract Hemodialysis International Volume 7, number 1, p 80, 2003. Additionally, over the course of two hours, six liters of dialysate was used for diffusive clearance via the dialysate gradient across the membranes of dialyzers  20  and  30 . Note that the dialysate flow rates and percent convective versus diffusive could be higher or lower than those used in the example. 
     Introduction to Disposable Cassette 
     Referring additionally to  FIGS.  2  and  3   , dialyzers  20  and  30  as well as many other flow components described herein are provided in one preferred embodiment attached to a disposable cassette. Disposable cassette  100   a  can otherwise be referred to as an organizer, disposable, disposable set, etc. Disposable cassette  100   a  includes at least a portion of the extracorporeal circuit  50  and dialysate flow path  60  (see  FIG.  1   ) for the renal failure therapy treatment (e.g., all of extracorporeal circuit  50  is integrated into cassette  100   a  with the exception of the tubing going to and from the patient as illustrated in  FIGS.  2  and  3   ). Disposable cassette  100   a  provides a space efficient apparatus for handling the dialysate or therapy fluid flow portions of the many pumps and valves described herein, which are actuated pneumatically or mechanically as described below. Cassette  100   a  is therefore well suited for home use, where space, capability and resources are limited. 
     In one preferred embodiment, disposable cassette  100   a  and associated attached tubing are gamma sterilized and sealed prior to use. Alternatively, sterilization via ethylene oxide or ebeam is employed. The patient or operator opens the seal just prior to use, inserts cassette  100   a  into the therapy machine for a single use and then discards the cassette  100   a  and associated tubing. While cassette  100   a  and flow paths  50  and  60  are intended for a single use in one embodiment, cassette  100   a  and flow paths  50  and  60  could be reused with suitable disinfection and/or sterilization. 
     Incorporation of Cassette and ECHD System 
     Referring to  FIGS.  1  to  3   , beginning from the arterial access  44   a  of the patient  42 , the extracorporeal or blood circuit  50  includes a pressure sensor  46 , labeled PT 1 . PT 1  is alternatively a pressure switch with the ability to stop blood flow prior to reaching blood pump  48 . As a safety measure, system  10  in one embodiment includes a multitude of electrodes (not shown), such as two to four electrodes, which provide an access disconnection sensor, which is integrated half in the arterial line  44   a  and half in the venous line  44   b  to detect access disconnection of patient  42  from the system  10 . An alternative mechanism for detection of accidental needle disconnections is the use of a conductive blanket underneath the patient&#39;s access. The presence of blood changes the conductivity of the blanket and sets off an alarm and stops the pumps. 
     Blood pump  48  is peristaltic pump  48  in one embodiment and is located between pressure sensor PT 1  and a drip chamber  52   a  with integral pressure transducer  46 , labeled PT 2 . The drip chambers  52   a  to  52   c  remove air from the fluids passing through the drip chambers. One, a multiple of or all the drip chambers  52   a  to  52   c  in an alternative embodiment includes an associated level sensor  68   a  to  68   c . Those sensors are connected to or integrated into the associated drip chambers. Level sensors  68   a  to  68   c  sense and indicate the level or height of dialysate or therapy fluid in the dialyzer. Blood pump  48  is alternatively a volumetric pumping device other than a peristaltic pump, such as a diaphragm pump or centrifugal pump. Blood pump  48  can also be bidirectional for system priming as discussed below. Pressure sensor PT 2   46  is alternatively not associated with a drip chamber, where for example pressure transducers are used instead. Pressure sensors PT 1  and PT 2 , drip chamber  52   a  as well as the tubing  102  for peristaltic pump  48  are all connected to cassette  100   a.    
     After drip chamber  52   a , blood flows out of the housing  104  of cassette  100   a  into a the relatively small, high flux dialyzer arterial dialyzer  30 . As seen in  FIG.  2   , arterial dialyzer  30  and venous dialyzer  20  are attached to an end of housing  104  of cassette  100   a . Blood then flows from the arterial dialyzer  30  to the venous dialyzer  20 , back into housing  104  of cassette  100   a  and through a second drip chamber  52   b . Drip chamber  52   b  also has an integral pressure sensor  46 , labeled PT 3 . PT 3  is alternatively provided without a drip chamber when, for example, pressure transducers that coupled directly to the line are used instead. 
     An air bubble detector  54  labeled ABD is located downstream from drip chamber  52   b  in blood line  50 . A venous line clamp or valve  56 , labeled V 1 , which may be cassette-based or provided external to cassette  100   a , and which shuts down blood flow if air is detected in line  50  by detector  54 , is located between the air detector  54  and arterial access  44   b , which returns blood to patient  42 . An air level sensor (not illustrated) on drip chamber  52   b  is used alternatively or in addition to ABD  54 . To detect air in the blood, a level detect scheme is alternatively or additionally provided with drip chamber  52   b  or pressure transmitter  46 , labeled PT 3 . For example, an ultrasonic sensor can be placed on opposite sides of the drip chamber. The sensor generates a signal that depends upon the percentage of air in the blood that passes between a transmitting and receiving positions of the sensor. Under normal operation, when no air is present, the blood within drip chamber  52   b  resides at a relatively steady level, although level fluctuations do occur due to changes in pressure, amount of blood pumped, etc. A threshold level of blood in chamber  52   b  does exist below which the blood should not drop. When air in the blood lines is present, the blood level in the chamber  52   b  is lower than a threshold level, triggering an alarm from the alternative air/blood detector. It is important to note that an air detector and line clamp may be used on line  44   a , if required by rinse, prime or blood rinseback. 
     Dialysate flow path  60  is also located primarily in the housing of organizer or cassette  100   a . The dialysate is supplied initially in dialysate or therapy fluid supply bags  14 ,  16  and  18 . In alternative embodiments shown below in connection with  FIGS.  4  and  9  to  11   , the source is an on-line source or other type of non-prepackaged source. In the embodiment illustrated in  FIG.  1   , a minimum of one infusion bag is provided and in one preferred embodiment multiple bags, such as three sources  14  to  18  are provided.  FIG.  1    also illustrates that the system is provided initially with an empty drain bag  12 , which is filled with spent solution from the supply bag  14 ,  16  or  18  that is used first. After the first two supply bags  14 ,  16  or  18  are drained, they become the drain bags for the second and final solution bags, respectively. Because the therapy in the end removes more fluid than is inputted, each of the supply bags  14  to  18  is used to receive spent fluid and UF. The bag sequencing is controlled as illustrated by valves  56 , labeled V 8  to V 14 . 
     Dialysate or therapy solution flows from one of sources  14  to  18  to the volumetric diaphragm pumps  22  and  24  of set  1 . The volumetric accuracy of the pumps is confirmed by monitoring. As discussed above, it is desirable to use two alternating solution delivery pumps  22  and  24  to limit the amount of pulsatile flow. As a safety measure, the diaphragms of each of the pumps  22  to  28  are configured so that if they leak, the can only leak externally. Any leaks collected externally from pumps  22  to  28  is then diverted towards a moisture sensor built into the cassette  100   a , machine and/or cassette/machine interface, which senses such leak and signals: (i) an alarm; (ii) to shut down pumps  22  to  28  and  48 ; and (iii) to take any other appropriate action. 
     Suitable pneumatically and mechanically driven medical fluid pumps and diaphragms therefore are described in commonly owned U.S. Pat. No. 7,238,164, entitled Systems, Methods And Apparatuses For Pumping Cassette-Based Therapies, filed Dec. 31, 2002, the teachings of which are incorporated herein by reference. The pumps and pumping technology currently used in the HomeChoice® series of APD devices, as embodied in U.S. Pat. No. 5,431,626 and its associated family of patents, the teachings of each of which are incorporated herein by reference, are also suitable, as are various pumping technologies described in commonly owned U.S. Pat. No. 6,814,547, entitled “Medical Fluid Pump”, filed May 24, 2002, the teachings of each of which are incorporated herein by reference. 
     As discussed above, each of the pumps  22  to  28  operates individually with a volume measuring device  32  to  38 . In one preferred embodiment, volume measuring devices  32  to  38  are capacitance fluid volume sensors, indicated in  FIG.  1    by dashed lines representing the associated capacitor plates. One embodiment of a capacitance sensor is disclosed in greater detail in the U.S. patent entitled, “Capacitance Fluid Volume Measurement,” U.S. Pat. No. 7,107,837, filed on Jan. 22, 2002, incorporated herein by reference. That capacitance sensor uses capacitance measurement techniques to determine the volume of a fluid inside of a chamber. As the volume of the fluid changes, a sensed voltage that is proportional to the change in capacitance changes. Therefore, the sensor can determine whether the chamber is, for example, empty, an eighth full, quarter full, half full, full, or any other percent full. Each of these measurements can be made accurately, for example, at least on the order of the accuracy achieved by known gravimetric scales or pressure/volume measurements. Capacitance sensing, however, is simpler, non-invasive, inexpensive and is operable with continuous, non-batch, type pumping operations. 
     Generally, the capacitance C between two capacitor plates changes according to the function C=k×(S/d), wherein k is the dielectric constant, S is the surface area of the individual plates and d is the distance between the plates. The capacitance between the plates changes proportionally according to the function 1/(R×V), wherein R is a known resistance and V is the voltage measured across the capacitor plates. 
     The dielectric constant k of medical fluid or dialysate is much higher than that of air, which typically fills a pump chamber (such as pump chambers  122 ,  124 ,  126  and  128  in  FIG.  2   , which are part of pumps  22  to  28  in  FIG.  1   ) that is empty or at the end of a pump out stroke. In one embodiment, one of the capacitance plates is moveable with the volume of fluid entering or exiting the chambers  122 , yielding the changing distance, Δd, between the plates a factor in determining capacitance. Likewise the surface area, S, of the capacitance plates could be varied. In one preferred embodiment shown figuratively in  FIG.  1   , the capacitance plates  32 ,  34 ,  36  and  38  are set at a fixed distance from one another, e.g., are fixed to the rigid plastic of housing  104  of cassette  100   a . In that instance, the surface area S is also fixed, leaving the change in the dielectric constant k to account for the change in capacitance as the pump chambers  122  to  128  are filled or emptied of dialysate. 
     As at least one flexible membrane positioned within chambers  122  to  128  expands and fills with medical fluid, the overall capacitance changes, i.e., increases, creating a high impedance potential across the capacitor plates, one of which is grounded, the other of which is active. That high impedance potential is indicative of an amount of fluid in the chambers  122  to  128 . If the sensed potential does not change, or does not change enough, when it is expected to change, the system controller recognizes such lack of change as air that has become trapped in the dialysis fluid and commands appropriate actions. 
     A capacitance sensing circuit is provided, which amplifies the high impedance signal to produce a low impedance potential. The low impedance is fed back to the capacitance plates  32  to  38  and is used to protect the sensitive generated capacitance signal from being effected by outside electrical influences. The amplified potential is also converted to a digital signal and fed to a the system controller, where it is filtered and or summed. A video monitor having a graphical user interface can then be used to visually provide a volume and/or a flowrate indication to a patient or operator based on the digital signal. Additionally, the controller uses the flowrate and volume information to ensure that Pump Set  2  (pumps  26  and  28 ) withdraws the appropriate amount of fluid from arterial dialyzer  30 , namely, the amount of dialysate pumped from Pump Set  1  (pumps  22  and  24 ) plus the prescribed amount of UF removed from the patient. 
     An additional use for capacitance plates or volume measuring devices  32  to  38  is to detect a leak across pump valves V 3  and V 5 , V 2  and V 4 , V 15  and V 16  and/or V 17  and V 18 . Those valves cycle and alternate during the pump-in and pump-out strokes of pumps  22 ,  24 ,  26  and  28 , respectively and are opening and closing much more often than other valves in system  10 , such as fluid container valves V 8  to V 14 . The pump valves are therefore more susceptible to leakage than are other valves and are relatively critical to the operation of system  10 . 
     The pump valves operate in alternating pairs. For instance, to deliver fluid into pump  22 , valve V 3  is opened while valve V 5  is closed. Conversely, to push fluid from pump  22 , valve V 3  is closed while valve V 5  is opened. If both valves are either opened or closed while a pump stroke takes place, volumetric error occurs. The present invention contemplates a method and apparatus for testing valves V 3  and V 5 , using volume measuring devices  32  to  38 . 
     The valve test in one embodiment utilizes the fact that the pump has flexible fluid membranes that are crimped between a fixed volume pump chamber. When a pump-in stroke takes place, the membranes fill with fluid expanding the membrane. The corresponding pump inlet valve (e.g., valve V 3 ) is then closed, trapping fluid within the flexible pump chamber membranes. A partial pump-out stroke is attempted either via a mechanical piston or positive/negative pneumatic pressure. The pressure exerted is not enough to damage the pump components but is enough so that if either inlet or outlet valves (e.g., V 3  and V 5 ) is faulty or leaking, fluid would flow, creating a volume change that would be sensed by volume measuring devices  32  to  36 . 
     If the valves close properly, and assuming dialysate to be incompressible, the small pressure exerted should move no fluid and produce no detectable volume change. If a leak is present, a volume change occurs and is detected, causing the controller to issue an alarm condition or take other appropriate action. The above-described test can be performed at the start of therapy and/or intermittently and periodically throughout therapy, e.g., every five minutes or every one thousand strokes. The test, it should be appreciated, can at least detect which set of pump valves V 3  and V 5 , V 2  and V 4 , V 15  and V 16  or V 17  and V 18  is leaking. The test is applicable to all types of medical fluid systems, including blood therapy systems, congestive heart failure systems and peritoneal dialyzer systems. 
     The chambers  122  to  128  and housing  104  of cassette  100   a  form a first portion of a clamshell, the second portion being formed by the renal therapy machine. The first and second portions house at least one flexible membrane and the dialysate when dialysate is present. The portions are rigid and form a fixed volume in one preferred embodiment. The portions form the shape of and also house the capacitor plates  32  to  38 . That is, one of the capacitor plates is housed in cassette  100   a , while the other is housed inside the therapy machine. Alternatively, both plates are housed in the therapy machine, one on either side of the cassette. As stated above, either the cassette or machine (whichever houses the active rather than the ground capacitor plate) houses an additional guard or shield plate that provides noise protection for the high impedance signal transmitted from the active capacitor plate. 
     As an alternative to the capacitance volume sensor described above, the volume or mass of dialysate fluid flowing through the pumps  22  to  28  can be determined using other methods, such as through an electronic scale or balance. In other alternative embodiments, the mass or volume of dialysate flowed in any of the systems described herein can be sensed using various types of medical grade flowmeters, orifice plates, mass flow meters or other devices employing Boyle&#39;s Law. Further, the Fluid Management System (“FMS”) technology used in HomeChoice®, as embodied in U.S. Pat. No. 5,431,626 and its associated family of patents, the teachings of each of which are incorporated herein by reference, is also suitable for use in the present invention. A pneumatically controlled system employing this technology is discussed in more detail below. Conductivity sensors may also check for conductive and nonconductive states across the valves, detection of valve leaks is easy with this method. 
     Still further alternatively, fluid balancing chambers or match flow equalizers may be used, such as those described in U.S. Pat. No. 5,486,286, assigned to the assignee of the present invention, incorporated herein by reference, which are also employed in the System  1000 ™ produced by the assignee of the present invention. The balancing chambers or flow equalizers are integrated in the cassette in one embodiment and require a separate pump or pressurization source. The chambers or equalizers would manage fresh dialysate on one side of a diaphragm and the spent dialysate on the other side of the diaphragm, matching the volume flow of fresh and spent dialysate. A separate pump is then used to ultrafiltrate fluid from patient  42  accumulated between patient sessions. Peristaltic pumps may also be used to pump dialysate to dialyzers  20  and  30  or to any of the blood filtering devices described herein, pump an equal amount of fluid from such devices, control and pump out a prescribed amount of ultrafiltrate from the patient. One suitable peristaltic pump arrangement is illustrated below in connection with  FIG.  12   . Systems employing balancing chambers and other volumetric control devices are discussed in more detail below. 
     Referring still to  FIGS.  1  to  3   , valves  56  labeled V 2 , V 3 , V 4  and V 5  control which pump is filling and which pump is exhausting dialysate at any given time. Those valves, as well as most if not all the valves of the systems described herein have an electromechanical portion housed inside the blood treatment machine and a fluid flow portion  156 , shown in  FIG.  2   . Dialysate or renal therapy fluid exiting pumps  22  and  24  enters a heater  58 . Heater  58  is located alternatively prior to volumetric diaphragm pumps  22  and  24 . Heater  58  may be any suitable type of electrical medical fluid heater, such as a plate (electrical resistance) heater, infrared or other radiant heater, convective heater, and any combination thereof. Heater  58  is illustrated as an in-line heater. As seen in  FIG.  2   , dialysate flows through a flexible membrane heating portion  158  of cassette  100   a . The electronics and other hardware associated with heater  58  are located inside the renal failure therapy machine. Heater  58  is located alternatively to batch heat solution bags  14 ,  16  and  18 . 
     Valve  56  labeled V 6  provides a bypass that enables solution at too high or too low a temperature to be diverted to a point upstream of pumps  22  and  24  to prevent solution at too high/low a temperature from reaching the dialyzers  20  and  30  and ultimately blood circuit  50 . To that end, temperature sensor  62  labeled T 2  senses and provides feedback to the controller of system  10  indicating the temperature of dialysate leaving heater  58 . The temperature sensor  62  could be a thermocouple or IR sensor or thermistor, which is housed inside, integral with or directly adjacent to a conductivity sensor probe  63 . Conductivity sensing is temperature dependent, so it is logical to locate the two sensors  62  and  63  together or directly adjacent to each other. 
     A suitable location for the temperature sensor/conducting sensor is, for example, at sensor location T 2 , T 3  which senses the conductivity of the fluid prior to the fluid reaching dialyzers  20  and  30 . Conductivity sensor  63  may be used to test the electrolyte composition of the solution. Conductivity sensor or electrolyte sensor  63  is particularly useful when using a dual chamber version of containers  14 ,  16  and  18 , which have multiple solution components that are mixed just prior to use. 
     A pressure sensor  46  labeled PT 4  measures the pressure of the fluid flowing to venous dialyzer  20  and in one embodiment is provided in association with an additional drip chamber  52   c  that purges air through vent  64   c  and vent valve  56  labeled V 19 . Sensor PT 4  and chamber  52   c  are located alternatively prior to volumetric diaphragm pumps  22  and  24 . 
     The dialysate next flows into venous dialyzer  20 . The membranes housed inside venous dialyzer are high flux membranes as discussed above. The dialysate flow path connects to the venous  20  and arterial  30  dialyzers via the restriction  40 . Restriction  40  provides backpressure that drives a significant amount of the dialysate through the high flux membranes of the venous dialyzer  20  and directly into the blood flowing through the membranes inside venous dialyzer  20 . Restriction  40  can be set to backpressure ten to ninety percent of the dialysate entering venous dialyzer  20  into the bloodline. As discussed above, restriction  40  can be set or variable. If a fixed restriction is desired, it is possible to use a single dialyzer rather than the two dialyzers  20  and  30  shown in  FIG.  1   . A dialyzer having an internal flow restriction suitable for use in place of items  20 ,  30  and  40  shown in  FIG.  1    is described in commonly owned U.S. Pat. No. 5,730,712, entitled “Extracorporeal Blood Treatment Apparatus and Method”, incorporated herein by reference. That dialyzer as indicated is limited to having a fixed orifice. 
     As alluded to above, it is desirable for a number of reasons that restriction  40  be a variable restriction. For one reason, different patients may respond to a therapy that is more convective or more diffusive. From a cost and manufacturing standpoint, it is desirable to have a unit that can be adjusted for any patient rather than “custom” units fitted with the necessary flow restriction. Second, it is very possible that the patient and doctor will not know initially what the optimal percentage convective clearance versus diffusive clearance breakdown is, requiring some period of experimentation and optimization. Moreover, it may be desirable for a patient to perform a first treatment using a first percentage convective clearance versus diffusive clearance and later in the week, the next day or later in the same day perform a second treatment using a different percentage convective clearance versus diffusive clearance. 
     Still further, system  10  has the capability of varying the percentage convective clearance versus diffusive clearance over a single therapy session or treatment, for example in step increments or continuously. Such changes can be made as gradually or quickly as desired and span as great a range as desired, e.g., starting with 90 percent convective and ending with 90 percent diffusive. It may be determined that it is desirable to clear molecules of a particular size or range of sizes or molecules of a particular type during a certain point in the therapy, e.g., at the beginning or end. Variable restriction  40  also makes it possible to repeat certain settings or patterns of settings during a single treatment. 
     The present invention contemplates at least three levels of variability for restriction  40 . The first level can be referred to as “semi-fixed”. Here, the restriction could use a fixed orifice restriction plate, but wherein restriction  40  is configured and arranged so that the plate can be swapped out for a plate having a different sized orifice. Such swapping out would occur, however, between therapies. A second level of variability can be referred to as “manual-on-the-fly”. Restriction  40  in this instance could be a backpressure regulator or variable orifice valve with a manual adjustment that enables the patient or operator to adjust the backpressure and thus the convective versus diffusive clearance percentage. The manual adjustment could be made during a current therapy or between therapies. The third level of variability is automatic, which could be effected for example via a pneumatically operated backpressure regulator or variable orifice valve. Such pneumatically operated device receives a pneumatic signal at a controlled pressure, which sets the backpressure accordingly. The controller could be configured to output for example an analog signal, e.g., a 0-5 VDC or 4-20 mA signal, which is converted via an I/P converter to a pressure signal at a corresponding pressure. The automatic adjustment could be made during a current therapy or between therapies. 
     Referring still to  FIGS.  1  to  3   , Pump Set  2  including pumps  26  and  28  resides on the exit end of arterial dialyzer  30 . Each of the various embodiments described above for Pump Set  1 , including the pump configuration, is applicable for Pump Set  2 . Pump Set  2  is normally configured to pump at the rate of the fresh dialysate input of Pump Set  1  plus an additional amount to remove excess fluid that has accumulated in the patient&#39;s blood and tissues between treatment sessions. 
     The waste dialysate and a volumetric equivalent to the patient&#39;s fluid gained in the interdialytic period flows from arterial dialyzer  30 , through valves  56  labeled V 16  and V 18 , through pumps  26  and  28 , through valves  56  labeled V 15  and V 17 , through a blood leak detector  66  and to one of the drain bags  12  to  16 , which as discussed above are opened selectively via valves  56  labeled V 9  to V 14 . Valves  56 , detector  66  and fluid contacting portions of pumps  26  and  28  are each in one embodiment located in the housing portion  104  of cassette  100   a . The waste and a volumetric equivalent to the patient&#39;s UF may alternatively be routed after BLD  66  to a long tube placed in an acceptable drain. This alternative will not work with balance scale systems. 
     Blood leak detector  66  includes in one embodiment a light source and a photo sensor. Blood components that are not meant to be filtered through dialyzers  20  and  30  lower the light reaching the photo sensor of detector  66  if such components do travel through the membrane walls of the dialyzers into the therapy solution flow path. The controller of system  10  continuously monitors the photo sensor. Detection of a blood leak triggers an audio and/or visual alarm, stops blood pump  48  and closes venous line valve V 1 . A blood sensor, such as detector  66 , is alternatively or additionally placed in the venous line running from venous dialyzer  30  to pumps  26  and  28 . 
     In special modes, infusion pumps  22  and  24  of Pump Set  1  can infuse more solution than is removed to drain by pumps  26  and  28  of Pump Set  2 . For example, during priming, during blood rinseback or for bolus infusion, infusion pumps  22  and  24  can infuse a volume that is greater than the volume removed by pumps  26  and  28 . The special modes enable the system to fill with fluid, enable blood in line  50  at the end of therapy to rinseback to the patient  42  or for the patient  42  to receive a bolus of solution via the venous dialyzer into the post dialyzer portion of circuit  50  and through venous access  44   b  to patient  42 . 
     During priming, the arterial and venous needles  44   a  and  44   b  are connected together as seen in  FIG.  2   . The pumps of Pump Sets  1  and  2  are run until air is purged from the system, so that only (or substantially only) dialysate flows throughout the dialysate flow path  60 . When blood pump  48  begins pumping, dialysate and/or saline is backfiltered from venous dialyzer  20  into blood line  50 , priming the remainder of the extracorporeal circuit  50 . An alternative or additional form of priming is to connect a bag of saline at arterial access  44   a.    
     In one embodiment, blood is returned to the body by reversing the flow direction of blood pump  48 , which would require an additional air/blood detector and clamp, such as ABD  54  and clamp V 1  placed in line  44   a , between pump  48  and patient  42 . Blood pump  48  would run in reverse until the additional air blood sensor detected an absence of blood in line  44   a . Pump  48  would be reversed again to flow fluid in the normal direction, which would return filtered dialysate and blood to patient  42  until the absence of blood is sensed in the venous line  44   b . Alternatively, this same method of blood rinseback may be employed but the air blood sensor would only be used to confirm the absence of blood, but the rinse controlled by pre-set dialysate and/or saline volume. 
     Alternative Source—Fluid Preparation Module 
     Referring now to  FIG.  4   , an alternative system  110  is provided that operates in a very similar manner to the system  10  described above. Indeed, each of the like reference numerals shown in  FIGS.  1  and  4    have the same functionality and the same alternatives as described previously. System  110  performs convective and diffusive clearance as described above and removes the amount of fluid gained by patient  42  between therapy sessions. 
     System  110  differs from system  10  in that system  110  does not use solution bags  14  to  18  and drain bag  12 , instead, system  110  operates with and connects to a separate fluid preparation module  112 . System  110  is advantageous because patient  42  is not required to store, connect to, disconnect from and discard multiple solution bags as described above. As seen by comparing systems  10  and  110 , system  110  eliminates multiple valves  56  (V 9 , V 10  and V 12  to V 14 ) by using an on-line dialysate generation source  112 . 
     One suitable fluid preparation module  112  suitable for home use is commercially available from PrisMedical, however, other systems having a water purification pack and electrolyte cartridge to prepare the dialysate could be used. System  110  alternatively uses a large, e.g., about 120 liters, fill bag or fill container (not illustrated), which receives dialysate or therapy fluid from the preparation module  112 . System  110  is also compatible with an in-center environment, wherein a single-patient or central fluid preparation module  112  supplies a single or multiple systems  110 . The single patient or central proportioning module could prepare dialysate or substitution fluid using a proportioning system. For an in-center use, it is contemplated not to use cassette  100   a  but instead provide a machine that can be sterilized and re-used. In any of the above-described embodiments for system  110 , the system pumps waste dialysate and UF to a waste dialysate bag, waste container, drain or waste area  114 . 
     Addition of Regeneration Loop 
     Referring now to  FIG.  5   , an alternative system  210  is provided that adds a regeneration loop  212  to the dialysate flow path. As with  FIG.  4   , each of the like reference numerals shown in  FIGS.  1 ,  4  and  5    have the same functionality and the same alternatives as described previously. System  210  also performs convective and diffusive clearances as described above and removes an amount of fluid or ultrafiltrate gained by patient  42  between therapy sessions. 
     Regeneration loop  212  includes an additional pump  214 , which operates with an associated volumetric measuring device  216 . Any of the embodiments described above for pumping, measuring flow and controlling flow may be employed for pump  214  and measuring device  216 . Additional inlet and outlet valves  56 , labeled V 22 , V 23  and V 26  are provided to allow or disallow flow of spent dialysate/UF from arterial dialyzer  30  to be pumped to pump  214 . As illustrated, pump  214  can pump to the recirculation sorbent cartridge  222  or to drain. Additional outlet valves  56 , labeled V 24  and V 25 , are connected fluidly to UF pumps  26  and  28 , so that those pumps can pump selectively to drain or to the recirculation sorbent cartridge  222 . In short, any combination of pumps  26  and  28  can be used repeatedly or at different times during therapy for recirculation or ultrafiltration. 
     As illustrated, pump  214  is configured to pump spent dialysate/UF back to the inlet of arterial dialyzer  30  via line  220 . Line  220  alternatively runs to the inlet of venous dialyzer  20 , wherein the regenerated fluid is reintroduced into that dialyzer. Moreover, regenerated fluid could be pumped to both of the inlets of venous dialyzer  20  and arterial dialyzer  30 . Still further, it is possible to regenerate fluid exiting venous dialyzer  20  alternatively or additionally to the regeneration of fluid exiting arterial dialyzer  30 . 
     In system  210 , the total amount pumped through UF pumps changes due to the additional recirculation pump  214 . In the example given above, pumps  26  and  28  of Pump Set  2  were said to remove eighteen liters of dialysate added over the course of the therapy (wherein twelve liters was used for convective clearance, while six liters of dialysate was used for diffusive clearance) plus any fluid ultrafiltered from the patient. 
     Applying the eighteen liters used in the above Example to system  210 , and assuming twelve liters is used to produce convective clearance, the remaining six liters plus the volume of fluid that is recirculated through recirculation loop  212  is then used to produce diffusive clearance. If pumps  26 ,  28  and  214  are configured so that one-third of all fluid exiting arterial dialyzer  30  is recirculated, then 225 ml/min is pulled from arterial dialyzer  30 , 75 ml is passed through recirculation loop  212  and 150 ml is discharged to the drain bags  12 ,  14  and  16 . The diffusive clearance is calculated to be the six liters of single pass dialysate plus 75 ml/min of recirculation loop  212  dialysate for 120 minutes, or six liters plus nine liters, totaling fifteen liters of diffusive clearance. If pumps  26 ,  28  and  214  are each operated at 100 ml/min, one-half of all fluid exiting arterial dialyzer  30  is recirculated through recirculation loop  212  and the diffusive clearance increases to six liters plus 150 ml/min for 120 minutes or six liters plus eighteen liters, totaling twenty-four liters of total diffusive clearance. 
     The trade-off for the increased clearance is that a sorbent cartridge  222  is required in recirculation loop  212  to clean or regenerate the spent dialysate/UF pulled exiting arterial dialyzer  30 . Depending on quantity and quality needed for the regenerated fluid, cartridge  222  may be as simple as a carbon cartridge but is alternatively a multilayer cartridge with Urease (similar to the cartridges described in U.S. Pat. Nos. 3,669,880 and 3,669,878, the teachings of which are incorporated herein by reference). Other suitable cartridges and materials therefore are discussed in commonly owned U.S. patent application Ser. No. 10/624,150, entitled, “Systems And Methods For Performing Peritoneal Dialysis” and commonly owned U.S. Pat. No. 7,208,092, entitled, “Systems And Methods For Peritoneal Dialysis”, the teachings of each of which are incorporated herein by reference. Depending on the type of sorbent used in cartridge  222 , system  210  as well as any other system described herein that uses sorbents may require a sterile infusate additive  616  on line  220  to replace electrolytes lost in the sorbent cartridge and a conductivity temperature sensor  62 ,  63  to measure the electrolytes independently of the infusion. 
     In general, the cleaning cartridges remove waste products from the spent fluid and improve the efficiency of same for causing diffusive transport of toxins. Sorbent cartridge or cleaning cartridge  22 , can employ one or more different types of cleaners or exchangers, such as an activated charcoal filter, a sorbent exchange, a chemical cleaner, a chemical exchange, a biological cleanser, a binding adsorption agent, an enzomatic reaction agent, a mechanical cleaner and any combination thereof. 
     Cassette-Based Hemofiltration System 
     Referring now to  FIGS.  6  and  7   , systems  310  and  410 , respectively, illustrate that the cassette-based home system is configurable alternatively to perform pure hemofiltration. The primary differences between systems  310  and  410  versus systems  10 ,  110  and  210  described above are that the pure hemofiltration systems do not use the venous dialyzer  20  and the restriction  40 , which may simply be removed from or bypassed in cassette  100   a  to form hemofiltration system  310  or  410 . Arterial dialyzer  30  in  FIG.  1    then operates as hemofilter  312  in system  310  or  410 . Arterial dialyzer  30 /hemofilter  312  is therefore chosen to be able to perform both roles. 
     The remainder of system  310  is configured by disconnecting the line  314  (shown in  FIG.  1   ) from venous dialyzer  20  ( FIG.  1   ) and reconnecting the line to postdilution line  316  in  FIG.  6   . Such disconnection and connection and can occur either in housing  104  of cassette  100   a  or via tubing connected to cassette  100   a . The present invention accordingly contemplates expressly the provision of a cassette that can either be factory set or be set in the field or at home by the patient for hemofiltration or for the backfiltered hemodiafiltration (“HDF”) therapy described above. 
     A check valve  326  is placed in line  314  to prevent blood from backing up into pumps  22  and  24 . A similar check valve  326  can be used in an analogous location in any hemofiltration or HDF embodiment described herein, e.g.,  FIGS.  6  to  8  and  11   . Optional shunt line  324  and valve  56 , labeled V 20 , may be used so that predilution and postdilution HF can be performed selectively individually or simultaneously with system  310  and other systems shown below. 
     System  310  as illustrated is a postdilution hemofiltration device, wherein fluid from infusion pumps  22  and  24  is injected directly into the postdilution bloodline  316 , which is located downstream of hemofilter  312 . In an alternative embodiment, fluid from infusion pumps  22  and  24  is injected directly into the predilution bloodline  318 , which is located upstream of hemofilter  312 . In such a case, the fluid in one preferred embodiment is injected at or upstream of drip chamber  52   a  to prevent air from entering filter  312 . Predilution and postdilution both have particular advantages over one another. 
     Postdilution provides better clearance per liter of substitution solution than does the predilution clearance mode. Postdilution clearance per liter of substitution fluid can, for example, be twice as effective as predilution clearance. Postdilution blood flow rate limitations, however, restrict the total amount of substitution fluid due to the risk of hemoconcentration. Predilution allows for higher clearance rates because the volume of substitution fluid is not limited by hemoconcentration. Therefore, the overall clearance over a given time can be, albeit less efficiently, greater using predilution therapy than for postdilution therapy. 
       FIG.  7    illustrates another alternative embodiment for a hemofiltration system of the present invention. System  410  of  FIG.  7    illustrates that a first dialysate line  320  extends from the output of postdilution infusion pump  22  and feeds directly into postdilution line  316 , which exits hemofilter  312 . 
     A second line  322  extends from the output of predilution pump  24  to the drip chamber  52   a  placed just in front of predilution line  318 , which extends to the input of hemofilter  312 . Check valves  326  are placed in both lines  320  and  322  to prevent blood from backing up into pumps  22  and  24 , respectively. The embodiments discussed in  FIGS.  6  and  7    have many of the same components described above in connection with  FIGS.  1 ,  4  and  5   . Those components are marked with the same element numbers and include each of the characteristics and alternatives described above for such numbers. 
     The dialysate flow path  460  is configured somewhat differently than dialysate or therapy solution flow path  60  described above. As illustrated, heater  58  is moved in front of Pump Set  1 , namely, postdilution pump  22  and predilution pump  24 . Drip chamber  52   c  likewise has been moved to be in front of infusion pumps  22  and  24  of Pump Set  1 . Drip chamber  52   c  is provided with two temperature sensors, labeled T 1  and T 2 , as illustrated. Drip chamber  52   c  also operates with vent  64   c  as described above. Heated fluid leaving heater  58  enters postdilution and predilution pumps  22  and  24 . 
     Fluid exiting postdilution pump  22  flows via line  320  to postdilution line  316 , where that fluid enters alternative blood circuit  350  to perform convective clearance. Fluid pumped from predilution pump  24  flows via predilution line  322  to drip chamber  52   a , wherein the dialysate or therapy fluid is mixed in drip chamber  52   a  with blood pumped via pump  48 . The blood and dialysate or therapy fluid thereafter flow to hemofilter  312 . 
     Assuming pumps  22  and  24  pump about the same amount of fluid over a given period of time, fifty percent of the dialysate or therapy fluid is used for postdilution clearance, while the other fifty percent, approximately, is used for predilution clearance. It is important to note that this ratio can be varied by changing the frequency of pumps  22  and  24 . The postdilution dialysate enters the patient  42  before flowing through hemofilter  312 . The predilution dialysate or therapy fluid on the other hand flows through hemofilter  312  before reaching patient  42 . 
     Any of the embodiments described herein for providing dialysate, either prepackaged or prepared on-line, is applicable to system  310  and  410  of  FIGS.  6  and  7   , as well as each of the other embodiments described herein. Moreover, the cassette described above in connection with  FIGS.  2  and  3    as well as each of the embodiments shown below for configuring the therapy machine and supply bags is additionally operable with the hemofiltration embodiments of  FIGS.  6  and  7   . The hemofiltration systems  310  and  410  are cassette-based in one preferred embodiments and are readily applicable to home use. 
     Cassette-Based Hemodiafiltration System 
     Referring now to  FIG.  8   , one embodiment of a home-based hemodiafiltration system  510  is illustrated. Systems  10 ,  110  and  210  described above provide a type of hemodiafiltration therapy having convective and diffusive transport modes caused by restriction  40  placed between dialyzer portions  20  and  30 . System  510  on the other hand provides a hemodiafiltration system  510  via a different flow configuration. Nevertheless, many of the flow components of hemodiafiltration system  510 , as before, are provided on a disposable cassette, which is inserted for a single therapy into a hemodiafiltration machine. 
     The dialysate or therapy fluid flow path  560  of hemodiafiltration unit  510  is a hybrid of the flow path  460  of system  410  described in connection with  FIG.  7    and the system  210  described in connection with  FIG.  5   . Like  FIG.  7   , a postdilution infusion pump  22  pumps dialysate directly into postdilution blood line  316  via line  320 , while predilution infusion pump  24  pumps dialysate or therapy fluid via line  322  into filter  20 ,  30 . In alternative embodiments, hemodiafiltration system  510  infuses dialysate only into predilution line  318  or postdilution line  316 . 
     Like  FIG.  5   , system  510  is also illustrated as having the additional ultrafiltrate pump  216  that pulls a portion of the spent dialysate from dialyzer  20 ,  30  and pumps that portion through recirculation line  220  and activated charcoal or other absorbent cartridge  222 . As described above, cartridge  222  regenerates some of the spent dialysate and ultrafiltrate from dialyzer  20 ,  30 , which ultimately results in the use of less fresh fluid from containers  14  to  18  per liter of diffusive clearance. Depending on the type of sorbent used in cartridge  222 , system  210  as well as any other system described herein that uses sorbents may require a sterile infusate additive  616  on line  220  to replace electrolytes lost in the sorbent cartridge and a conductivity temperature sensor  62 ,  63  to measure the electrolytes independently of the infusion. It should appreciated, however, that hemodiafiltration system  510  does not require a regeneration loop  220  or cartridge  224 . 
     Hemodiafiltration system  510  operates in a similar manner to the system  10 ,  110  and  210  described above. That is, both systems provide convective and diffusive clearance modes. In system  510 , the convective clearance occurs because lines  320  and  322  from the infusion pumps convey dialysate or therapy fluid directly into the blood circuit  350 . Check valves  326  are placed in both lines  320  and  322  to prevent blood from backing up into pumps  22  and  24 , respectively. Diffusive clearance also occurs because dialysate is additionally moved across the membranes inside dialyzer  20 ,  30 . 
     At least a portion of many of the sensors, the pump chambers, the fluid heating pathway, the fluid flow portions of valves  56  as well as many other components of system  510  are provided in whole or in part on a cassette, such as cassette  100   a . Cassette  100   a  is then loaded into a hemodiafiltration machine for a single use and then discarded. System  510  is thereby well suited for home use. 
     Recirculation 
     The systems described previously require a fluid source, such as, sterile dialysate from bags, e.g., as in  FIG.  1   , or from a fluid generation pack, e.g., as seen in  FIG.  2   .  FIGS.  9  to  11    describe systems that are applicable to any of the therapies described herein (e.g., using convection and/or diffusive clearance modes). The systems of  FIGS.  9  to  11   , however, use a recirculating sorbent system with various filters to produce an ultrapure dialysate source. 
     Referring now to  FIGS.  9  to  11   , various sorbent-based regeneration systems are illustrated.  FIG.  9    shows a sorbent-based regeneration system  610  that performs the back-filtered convection and diffusion described in systems  10 ,  110  and  210  above.  FIG.  10    shows the system ( 610  of  FIG.  9  or  710    of  FIG.  11   ) being shunted at start-up for rinsing and priming. System  710  of  FIG.  11    is a hemofiltration system using sorbent-based regeneration, which is applicable to pre- and postdilution type HF systems as well as the HDF system  510  described in  FIG.  8   . 
     In the system  610  of  FIG.  9   , patient  42  uses an initial five liter bag of sterile dialysate, which is installed in a rigid container to form a reservoir  612 . Alternatively, five liters of water and concentrate powders or liquids are mixed inside reservoir  612  to form an initial therapy solution. 
       FIG.  10    illustrates that a shunt  614  is placed across dialyzers  20  and  30  at the beginning of treatment. A sorbent cartridge  222  is placed in the dialysate flow path  620  downstream of shunt  614 . Cartridge  222  is, for example, any of the types of sorbent systems described above in connection with system  210  of  FIG.  5   . An infusate  616  including, e.g., calcium, magnesium and/or potassium is pumped via infusate pump  618  into reservoir  612  as necessary to replenish ions that are removed via the sorbent cartridge  222 . 
     Heater  58  heats the solution leaving reservoir  612 . After the solution is heated, system  610  prompts the user or patient  42  to install a disposable, sterile cassette, such as cassette  100   a  described above. At least a portion of the air bubble detectors  54 , heating elements of heater  58 , pressure sensors  46 , temperature sensors  62 , etc., are integrated into the cassette in both the dialysate and extracorporeal blood flow paths as necessary to allow for a safe treatment for the patient and reliable operation of system  610 . The blood circuit  50  is primed with a saline bag connected to the arterial bloodline or via backfiltering dialysate or saline through venous dialyzer  20 . 
     The patient is connected to the arterial and venous access lines  44   a  and  44   b  respectively, and treatment begins. For short therapies, the dialysate flow can be relatively high, such as three hundred ml/min for three hours or one hundred ml/min for up to eight hours. Dialysate pumps  22  and  24  and UF pumps  26  and  28  control flow to and from dialyzers  20  and  30 . By increasing the pumping rate of pumps  26  and  28  that remove the effluent dialysate from arterial dialyzer  30 , the fluid accumulated in the patient in the interdialytic period is removed. The fluid flow portions of dialysate/UF pumps  22  to  28  are integrated into the cassette along with the extracorporeal circuit in one embodiment. Alternatively, those components are maintained separately from the cassette and are integrated into the machine. 
       FIG.  9    shows two volumetric devices  22  and  24  for dialysate flow and two for 26 and 28. Alternatively, one pump is employed on the input and one on the output, however, such configuration could create pulsatile flow, which is less desirable. 
     Fresh dialysate flows initially to venous hemodialyzer  20 . A restriction  40  placed between dialyzers  20  and  30  builds backpressure in dialyzer  20 , so that a relatively large amount of the dialysate is backfiltered into blood circuit  50 , with the remaining portion of the dialysate flowing to arterial dialyzer  30 . System  610  in that manner provides diffusive as well as convective clearance as has been described herein. 
     Used dialysate and UF pulled from arterial dialyzer  30  is then circulated through the sorbent cartridge  222 . Cartridge  222  removes waste products from the spent dialysate/UF fluid. The cleaned fluid is pumped to reservoir/bag  612 , where infusate  616  is added to replace the electrolytes removed by the sorbent cartridge  222 . 
     The majority of dialysate flow path  620  is located within the cassette. The cassette is single use in one embodiment but is alternatively reusable with suitable disinfection and/or sterilization. Most all components of the extracorporeal circuit  50  may be integrated into the cassette except, e.g., the tubing extending to and from the patient. The extracorporeal circuit  50  of system  610  is similar to the circuit  50  described above in systems  10 ,  110  and  210 . 
     The dialysate/infusate is heated as it exits reservoir  612  and flows past a temperature/conductivity sensor  62 . If the solution is too hot, too cold or otherwise outside of a defined physiological range, a bypass valve  56  provided with ultrafilter  626  is closed and a purge valve  56  in bypass line  628  is opened to bypass dialyzers  20  and  30 . During that bypass, both the infusate and UF pumps  22  to  28  may be stopped. To facilitate the bypass and a smooth, steady flow of fluid to/from reservoir  612 , a second circulation pump  624   b  may be employed. 
     When the solution is within the defined temperature/physiological range, the solution passes through reusable ultrafilter  626 , which employs a molecular weight cutoff that filters bacteria. Ultrafilter  626  also filters and absorbs endotoxin. The filtration of system  610 , including ultrafilter  626 , is intended to provide dialysate in as pure a form as possible. Ultrafilter  626  may also be a microfilter, if the microfilter can remove acceptable amounts of bacteria and pyrogens. 
     From ultrafilter  626  the dialysate or therapy solution is pumped to infusion pumps  22  and  24 . Flow measuring devices  32  to  38  monitor the volume of the fluid pumped by pumps  22  to  28 . Pumps  22  to  28  are configured as described above to leak to an external point. Any leaks are diverted into a moisture sensor built into the cassette and/or cassette/machine interface, so that corrective action is taken upon detection of a leak. 
     Fluid flows from infusion pumps  22  and  24  through a small 0.2 micron microfilter  630  in one embodiment. Filter  630  is integrated into the cassette and provides additional filtration of bacteria and endotoxin. The dialysate flows from filter  630  to venous dialyzer  20 , which employs high flux membranes. The dialysate flow path  620  connects the venous and arterial dialyzers via a restriction  40  between the two dialyzers. Restriction  40  provides backpressure to drive a significant amount of the dialysate directly into the blood circuit  50  inside venous dialyzer  20 . The remainder of the dialysate flows to arterial dialyzer  30 . 
     UF pumps  26  and  28  are provided on the exit side of the arterial dialyzer  30 . Those pumps are normally configured to pump at the rate of the fresh dialysate plus an additional amount to remove the fluid accumulated in the patient between treatment sessions. The used dialysate fluid and UF fluid is then circulated to the sorbent cartridge  222  and cleaned before returning to reservoir  612  and receiving an infusate  616  of e.g., calcium chloride, magnesium chloride, potassium chloride and possibly sodium acetate. As described above in connection with system  10 , pumps  22  to  28  may operate differently for priming, for bolus infusion or for blood rinseback. 
       FIG.  11    illustrates a system  710 , which replaces dialyzers  20  and  30  with a hemofilter  312 . System  710  is configurable to provide predilution, postdilution or both types of HF therapies via valves  56  and pre and postdilution flow lines  712  and  714 , respectively. Pre and post dilution HF eliminates the need for an anti-coagulant. System  710  can employ multiple ultrafilters  626  and multiple bypass lines  628  as illustrated for redundancy. Multiple filters in series ensure that if one filter becomes compromised or otherwise does not function properly, the other filter in the series ensures proper filtration. The filters each have a rated log reduction of bacteria and endotoxin. Thus, if bacteria levels reach a high enough point, some bacteria could be carried through the first filter in a series to the second filter in the series, and so on. 
     Systems  610  and  710  include a number of alternative embodiments. Ultrafilters  626  and/or microfilter  630  may or may not be reusable. Pumps  22  to  28  and flow measuring devices  32  to  38  include any of the alternatives described above in connection with system  10 , such as the matched flow equalizers such as in the System  1000 ™, produced by the assignee of the present invention. Any of the alternatives may be at least partially integrated with the cassette or provided elsewhere in the dialysis machine. A further alternative method is to use other volumetric pumping technology, such as piston pumps (with some piston pumps, depending upon if the piston exposes the solution to air, the ultrafilter needs to be placed after the pumps in the fresh dialysate loop to prevent the solution from becoming contaminated). Still further, flow monitoring could be employed instead of the volumetric pumps. Here, flow sensors measure flow and provide flowrate feedback to one or more pumps located upstream and/or downstream of the dialyzers  20 ,  30  or hemofilter  312 . 
     Systems Using Peristaltic Pumping 
     Referring now to systems  810  and  910  of  FIGS.  12  and  13   , respectively, alternative medical fluid treatment systems using peristaltic pumps  820  and  830  to pump the dialysate fluid from bags  14 ,  16  and  18  and ultrafiltrate from a blood filter are illustrated.  FIGS.  12  and  13    are simplified with respect to the figures illustrating earlier systems. It should be appreciated that many of the components and devices shown above in those systems are also used in systems  810  and  910  as appropriate. It is unnecessary to repeat the inclusion of each of those components and devices in  FIGS.  12  and  13   . Moreover, elements in  FIGS.  12  and  13    listed with like element numbers with respect to those shown above operate the same as described above and include each of the alternatives for those element numbers described above. 
     System  810  of  FIG.  12    illustrates a hemodiafiltration system using inline hemodialyzers  20  and  30 , separated by restriction  40 , as described above. Blood flows from arterial access line  44   a  of extracorporeal circuit  50  via peristaltic pump  48 , through arterial dialyzer  30 , through venous dialyzer  20 , into venous drip chamber  52   b , through blood leak detector  54  and clamp or valve  56  and venous access line  44   b  back into patient  42 . Dialysate flows from one of the source bags  14 ,  16  or  18  through drip chamber  52   c  and past heater  58 . In system  810 , peristaltic pumps  820  and  830  are used to drive the dialysate or therapy fluid from the source bags to venous dialyzer  20 . 
     Valves  56   a  to  56   h  are configured and arranged to enable either peristaltic pump  820  or peristaltic pump  830  to perform either of the fluid infusion or fluid removal tasks, namely, to infuse fluid into venous dialyzer  20  or to pull ultrafiltrate from arterial dialyzer  30 . Peristaltic pumps are inherently less accurate than the volumetric diaphragm pumps described above as well as other types of pumps or volumetric devices, such as fluid balancing chambers. Due to this inaccuracy, peristaltic pumps may have to be combined with a balance scale or another balancing method. Peristaltic pumps are, however, easy to sterilize and maintain in an injectible quality state, the pumps are generally hearty, robust and also provide built-in clamping when the pump stops pumping because the pump head pinches closed the tubing wrapped around the head. The pumps are also well accepted by the dialysis community. The valve arrangement of valves  56   a  to  56   h  and the use of the peristaltic pumps is advantageous for the above reasons. 
     The inaccuracy inherent in peristaltic pumps is repeatable especially when the pumps are rotated in the same direction. Systems  810  and  910  provide dual pumps  820  and  830  and valves  56   a  to  56   h  that are opened and closed to enable the same pump  820  and  830  to be rotated in the same direction for the same number of pump-in strokes and pump-out strokes. That feature cancels most error associated with the pumps. The pumps then perform additional pump out strokes to remove the desired amount of ultrafiltrate. 
     It should be appreciated that the above canceling can also be achieved by running one pump in one direction for the appropriate number of strokes and alternating the valves to sequentially pump-in and pump-out with the single peristaltic pump. Such an arrangement creates pulsatile flow, however, which is less desirable than a steady flow from dual pumps  820  and  830 . Therapy time is reduced as are the chances of hemoconcentrating the patient. 
     Valves  56   a  and  56   b  enable dialysate heated by heater  58  to flow to either peristaltic pump  820  or  830 . Valves  56   c  and  56   d  in turn enable fluid to flow from either pump  820  or  830  to venous dialyzer  20 . Valves  56   e  and  56   f  enable ultrafiltrate to be pulled from arterial dialyzer  30  to either peristaltic pump  820  or  830 , respectively. In turn, valves  56   g  and  56   h  enable the ultrafiltrate pulled from dialyzer  30  to be pumped via either valve  820  or  830 , respectively, to drain bag  12 ,  14  or  16 . 
     The operation of dialyzers  20  and  30  in combination with restriction  40  does not change in system  810  from their operation described above in connection with system  10  of  FIG.  1   . The dual operating pumps  820  and  830  enable a continuous flow of fluid into and out of dialyzers  20  and  30 . Importantly, as with the membrane pumps  22  to  28  described above, the tubing used with peristaltic pumps  820  and  830  can be sterilized with methods such as gamma, ebeam or ethylene oxide, and operated without compromising such sterilization. 
     Flow or volume measuring devices  840  and  850  are each provided to operate with a respective pump  820  or  830 , respectively. Devices  840  and  850  can provide tachometer feedback, for example, measuring the speed of rotation of the peristaltic pump head in one example. In another example, measuring devices  840  and  850  count to the number of strokes made by the head of peristaltic pumps  820  and  830 . In a further alternative embodiment, ultrasonic, mass flow, vortex shedding, or other type of flow measurement technique is used to measure the amount of fluid entering or exiting pumps  820  and  830 . Various embodiments showing peristaltic pumps in combination with one or more balancing chamber or volumetric control device are illustrated in detail below. 
     System  910  of  FIG.  13    illustrates a hemofiltration version of system  810  described in  FIG.  12   . System  910  is similar in all respects to system  810  except that hemofilter  312  replaces hemodialyzers  20  and  30  and restriction  40  of system  810 . Also, the inlet line  314  extending from valves  56   c  and  56   d  is connected to line  824  extending from hemofilter  312  to venous drip chamber  52   b  in system  910 . In system  810  of  FIG.  12   , line  314  as illustrated is connected instead to the inlet of venous dialyzer  20 . Line  328  in both systems  810  and  910  exits the relevant blood filtering device and flows to valves  56   e  or  56   f . Thus, the functioning of valves  56   a  to  56   h  does not change from system  810  to system  910 . That is, valves  56   a  and  56   b  operate as inlet dialysate or substitution valves in both systems. Valves  56   c  and  56   d  operate as outlet dialysate valves in both systems. Valves  56   e  and  56   f  operate as ultrafiltrate inlet valves in both systems. Valves  56   g  and  56   h  both operate as ultrafiltrate outlet valves in both systems. System  910  optionally provides a bypass line  828  and shunt valve  56   i  that enables system  910  to perform pre or postdilution hemofiltration as described above. 
     Any of the alternative embodiments for providing a sterile solution or for regenerating used solution described above are applicable to systems  810  and  910 . Further, each of the components described above, such as valves  56 , drip chambers  52  (collectively referring to drip chambers  52   a ,  52   b  and  52   c ), heater  58 , etc., or those portions thereof that contact the fluids used in the systems, can be provided in a disposable cassette in systems  810  and  910 . In particular, shown below are machines that house the flow devices as well as the disposable cassette. Those machines show that a majority of the peristaltic blood pump is located within the machine, with the peristaltic pump head located outside of the machine. Such arrangement is applicable to systems  810  and  910 , which use multiple peristaltic pumps. The cassette can have multiple tubing portions that the patient or operator wraps around the externally located peristaltic pump heads for use. 
     Co-Current Flow 
     Referring now to system  950  of  FIG.  14   , an alternative medical fluid treatment system using co-current flow is illustrated. System  950  of  FIG.  14    includes many of the same components described above, for example, in connection with system  10  of  FIG.  1   . Many element numbers shown in  FIG.  14    are the same as the element numbers shown in previous embodiments. Those like element numbers in  FIG.  14    operate the same as described above for those numbers and include each of the alternatives described previously for same. 
     System  950  operates in a similar manner to system  10  of  FIG.  1   , both of which include dual dialyzers  20  and  30 , and a restriction, such as variable restriction  40 , placed between the dialyzer portions. System  10  of  FIG.  1   , it should be appreciated, is a counter-current flow system. That is, dialysate line  314  in  FIG.  1   , which receives therapy fluid from pumps  22  and  24 , in turn feeds the therapy fluid into venous dialyzer  20 . The fluid flows through venous dialyzer  20 , variable restriction  40  and through arterial dialyzer  30 . At the same time, blood flows initially into arterial dialyzer  30 , continues through blood circuit  50 , through venous dialyzer  20  and eventually into patient  42 . System  950  of  FIG.  14   , on the other hand, includes output dialysate line  952  instead of line  314  in  FIG.  1   . Dialysate line  952  carries fresh and heated therapy fluid into arterial dialyzer  30  instead of venous dialyzer  20 . The dialysate in system  950  therefore flows from arterial dialyzer  30 , through variable restriction  40 , into venous dialyzer  20  and out venous dialyzer  20  to ultrafiltrate pumps  26  and  28 . Blood leak detector  66  is alternatively placed upstream of pumps  26  and  28  as illustrated in  FIG.  14    or downstream of those pumps as illustrated in  FIG.  1   . 
     Co-current flow of dialysate via line  952  of system  950  is beneficial in one respect because, as with predilution hemofiltration, dialysate is introduced into arterial dialyzer  30  at the start of the blood filtration portion of blood circuit  50 , and may, therefore, help to prevent hemoconcentration of the patient&#39;s blood. Variable restriction  40  operates to backfilter therapy fluid inside arterial dialyzer  30  into extracorporeal circuit  50 . Afterwards, blood and therapy fluid flow into venous dialyzer  20  via bloodline  50  and are subjected to diffusive clearance via the non-backfiltered dialysate that flows from arterial dialyzer  30  into venous dialyzer  20  through restriction  40 . The roles of dialyzers  20  and  30  are reversed in system  950  with respect to system  10  of  FIG.  1   , wherein the clearance mode in venous dialyzer  20  is primarily diffusive, while the clearance mode in arterial dialyzer  30  is primarily convective. 
     Operation of system  950  is otherwise substantially similar to that described above in connection with system  10  of  FIG.  1   . While system  950  is operable with supply bags  14  to  18  and drain bag  12 , any of the above-described embodiments for supplying fresh dialysate are alternatively operable with system  950 . Further, system  950  is operable with the regeneration sorbent system described above in connection with system  210  of  FIG.  5   . Still further, co-current flow can be provided in connection with the hemodiafiltration system  510  of  FIG.  8   . Still further, the volumetric diaphragm pumps  22  to  28  can be replaced by peristaltic pumps  820  and  830 , in accordance with the teachings described above in connection with system  810  of  FIG.  12   . 
     Ultrafiltrate Control—Boyle&#39;s Law 
     Referring now to  FIGS.  15  and  16   , a method of determining the volume of fluid pumped through a membrane pump is illustrated. Pumps  22  and  24  described above are shown for example. As discussed herein, pumps  22  and  24  include pump chambers defined at least partially by a rigid cassette, such as cassette  100   a . The cassette includes a flexible membrane or sheeting. Another portion of the pump chamber is defined in one embodiment by the renal replacement therapy machine into which the cassette is inserted. In  FIGS.  15  and  16   , pump  22  includes a membrane  252 . Pump  24  includes a membrane  254 . Positive and negative tanks  268  and  270  move membranes  252  and  254  to pump fluid via positive and negative pressure via valves  274 ,  276 ,  278  and  280  as needed. The pneumatic system also includes reference reservoirs  256  and  258 . Reservoir  256  communicates with air residing on the non-fluid side of membrane  252  of pump  22 . Likewise, reference reservoir  258  communicates with air residing on the non-fluid side of membrane  254  of pump  24 . 
     Reference reservoirs  256  and  258  have a constant and known volume. In the equations shown below the volumes of reservoirs  256  and  258  are designated as V 1  reservoir and V 2  reservoir. In the example, the volumes of pressure sensors that measure V 1  reservoir and V 2  reservoir are 20 ml. The blood therapy treatment unit also has pressure sensors that measure the pressure inside reference reservoirs  256  and  258 . In  FIG.  15   , when valves  260  and  262  are closed and vent valves  264  and  266  leading to sound absorbers  286  and  288  are open, the pressure inside reservoirs  256  or  258  reaches atmospheric pressure or approximately 15 psia. In  FIG.  16   , when vent valves  264  and  266  are closed and reservoir valves  260  and  262  are opened, the pressure inside pump chamber  1  equalizes with the pressure inside reservoir  256 . The pressure inside pump chamber  2  equalizes with the pressure inside reservoir  258 . 
     The cassette is also configured such that a pressure sensor housed within the blood therapy unit measures the initial and final air fluid pressures, inside pumps  22  and  24 . In the equations shown below, the fluid pressure inside pump  22  is designated as P 1  chamber. The fluid pressure inside pump  24  is designated as P 2  chamber. The fluid pressures vary from an initial pressure to a final pressure. Likewise, the pressures P 1  and P 2  within reservoirs  256  and  258  designated as P 1  and P 2  reservoir, respectively, vary from an initial pressure to a final pressure. 
     The volume of air within either one of the pumps  22  or  24  (volume V 1  for pump  22  which is supposed to be full is shown for example) is calculated via Equation 1 as follows: 
     
       
         
           
             
               
                 
                   
                     V 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                     ⁢ 
                     
                       ( 
                       
                         air 
                         , 
                         
                           full 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           chamber 
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           ( 
                           
                             
                               P 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               reservoir 
                             
                             , 
                             initial 
                           
                           ) 
                         
                         - 
                         
                           ( 
                           
                             
                               P 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               reservoir 
                             
                             , 
                             final 
                           
                           ) 
                         
                       
                       
                         
                           ( 
                           
                             
                               P 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               chamber 
                             
                             , 
                             final 
                           
                           ) 
                         
                         - 
                         
                           ( 
                           
                             
                               P 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               chamber 
                             
                             , 
                             final 
                           
                           ) 
                         
                       
                     
                     × 
                     V 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                     ⁢ 
                     
                       ( 
                       reservoir 
                       ) 
                     
                   
                 
               
               
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     The volume of air for an empty chamber for either one of the pumps  22  or  24  (shown in this example for pump  24  or V 2 ) is calculated according to Equation 2 as follows: 
     
       
         
           
             
               
                 
                   
                     V 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                     ⁢ 
                     
                       ( 
                       
                         air 
                         , 
                         
                           empty 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           chamber 
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           ( 
                           
                             
                               P 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               reservoir 
                             
                             , 
                             initial 
                           
                           ) 
                         
                         - 
                         
                           ( 
                           
                             
                               P 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               reservoir 
                             
                             , 
                             final 
                           
                           ) 
                         
                       
                       
                         
                           ( 
                           
                             
                               P 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               chamber 
                             
                             , 
                             final 
                           
                           ) 
                         
                         - 
                         
                           ( 
                           
                             
                               P 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               chamber 
                             
                             , 
                             final 
                           
                           ) 
                         
                       
                     
                     × 
                     V 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                     ⁢ 
                     
                       ( 
                       reservoir 
                       ) 
                     
                   
                 
               
               
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     Each of the pressures for each of the pumps  22  and  24  shown in Equation 1 is measured via a suitably placed transducer. The final air pressure within the reservoirs  256  and  258  is also measured. The final pressure of air within the chambers, which should equal the final reservoir pressure can be double checked. The measured pressures satisfy the numerators and denominators in Equations 1 and 2. As discussed above, the volumes of the reservoirs V 1  and V 2  are constant and known. 
     For each pump then, Equation 3 calculates the volume pumped for a stroke as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           Volume 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           fluid 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           pumped 
                         
                       
                     
                     
                       
                         
                           for 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           pump 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           or 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                     
                   
                   = 
                   
                     
                       V 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       V 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                       ⁢ 
                       
                         ( 
                         
                           air 
                           , 
                           
                             empty 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             chamber 
                           
                         
                         ) 
                       
                     
                     - 
                     
                       V 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       V 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                       ⁢ 
                       
                         ( 
                         
                           air 
                           , 
                           
                             full 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             chamber 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
               
             
           
         
       
     
     The fluid volume pumped for a stroke of a pump is equal to the volume of air when that pump chamber is empty or void of fluid less the volume of air in that pump chamber when the chamber is expected to be full of fluid. It should be appreciated that the Equations 1 to 3 that are derived from Boyle&#39;s law compensate for air bubbles that may be present in the dialysate and for instances where membranes  252  and  254  may not travel fully to one side or the other of the pump chambers of pumps  22  and  24 , respectively. 
     The above-described method provides an accurate, after-the-fact, measurement of the volume of fluid that has been moved by either one of the pumps  22  and  24 . By using the volumetrically controlled pumps, an exact amount of fluid can be exchanged with the patient and an exact amount of ultrafiltrate can be removed from the patient by setting the fluid removal pumps, e.g., pumps  26  and  28 , to pump faster or more volume than the fluid inlet pumps  22  and  24  (see for example, in  FIGS.  1 ,  4 ,  6 ,  7   ). Because the volume for each stroke can be calculated, the amount of fluid removed from the patient can be summed and controlled. 
     It should be appreciated that Equations 1 to 3 described above could be used in a machine that mechanically moves membranes  252  and  254 . In such case, positive and negative pressure tanks  268  and  270  would not be needed, however, separate reference reservoirs  256  and  258  as well as a test pressure tank  272  are needed. Test pressure tank  272  may be employed even in the present embodiment so that pressure tanks  268  and  270  may be operated independent from the volume control. 
     Calculating the volume of fluid pumped according to Equations 1 to 3 provides information on how much volume has been moved per pump stroke. The equations do not provide real time information of actual fluid flow. That is the valve opening and closing, sequence in  FIGS.  15  and  16    occurs between pump strokes, when valves  274 ,  276 ,  278  and  280  are closed, isolating the pumps from the positive and negative pressure sources. When the pumps are pumping fluid, reference reservoirs  256  and  258  are isolated from the pump. 
     If fluid flow stops or occurs at a flow rate that is greater than a desired flow rate, the pneumatic system may not detect this until after the undesired fluid flow rate has occurred. In blood therapy systems, such as dialysis, hemofiltration or hemodiafiltration, if the withdrawal of the fluid from circulating blood exceeds about thirty percent of the blood flow rate, the blood thickens and may clog the dialyzer or hemofilter fibers. If the dialyzer or filter becomes clogged, therapy may have to be terminated and the patient may lose an amount of blood trapped in the extracorporeal circuit. 
     The apparatus shown in  FIGS.  15  and  16   , however, provides a solution for real-time flow rate data for both blood flow and dialysate infusion and removal. The real-time flow rate is again calculated using principals of Boyle&#39;s law. As described above, equations one and two calculate the volume of air within the pump chambers  22  and  24  when those chambers are either full or empty. In this method, valves  260  and  262  to reference reservoirs  256  and  258  are closed and the appropriate valves to positive pressure tank  268  and negative pressure tank  270  are opened. For example, valve  274  may be opened to supply positive pressure to pump  22  to push fluid from that pump. At the same time, valve  280  may be opened to pull a vacuum on pump  24  to draw fluid into the pump. Since the volumes of air in the pump chambers are known from Equations 1 and 2, those volumes are added to the known volumes of air in pressure reservoirs  268  and  270  (e.g., 500 ml) to form total initial volumes. The pressures are measured as the membranes  252  and  254  move due to the supplied pressures. The change in pressure over time corresponds to a change in volume one time, which yields a flowrate. 
     In the following equations, the total initial volume in pump  22  and the respective pressure chamber is V 1  total, initial=V 1  chamber, initial plus Vpos/neg tank. The total volume in pump  24  and the respective pressure chamber is V 2  total, intial=V 2  chamber, initial plus Vpos/neg tank. The pressure of the pump  22  system as measured at the positive or negative tank is initially Ppos/neg, tank, intial. The pressure of the pump  24  system as measured at the positive or negative tank is initially Ppos/neg tank, initial. The pressure of either system at any time T is Ppos/neg tank, time T. The volume in either pump at time T is therefore as follows: 
     
       
         
           
             
               
                 
                   
                     V 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     or 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     V 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     total 
                   
                   , 
                   
                     
                       time 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       T 
                     
                     = 
                     
                       
                         
                           
                             
                               P 
                               
                                 pos 
                                 / 
                                 neg 
                               
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             tank 
                           
                           , 
                           initial 
                         
                         
                           
                             
                               P 
                               
                                 pos 
                                 / 
                                 neg 
                               
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             tank 
                           
                           , 
                           
                             time 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             T 
                           
                         
                       
                       * 
                       V 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       or 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       V 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       total 
                     
                   
                   , 
                   initial 
                 
               
               
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
               
             
           
         
       
     
     The fluid moved by either pump at time T is therefore as follows:
 
 V   fluid  moved by pump 1 or 2= V 1 or  V 2 total, time  T−V 1 or  V 2 total, initial   EQUATION 5
 
     Knowing the time T and the volume of fluid moved by pump  22  or  24  at time T, the flow rate on a real time basis may be calculated, displayed and used to control the renal failure therapy systems of the present invention. 
     Ultrafiltrate Control—Single Balance Chamber 
     Each of the systems  10 ,  110 ,  210 ,  310 ,  410 ,  510 ,  610 ,  710  and  950  that employ membrane pumps, such as pumps  22 ,  24 ,  26  and  28  are capable of metering out precise amounts of fluid, which can be controlled as described above for example via Boyle&#39;s Law. For manufacturing and cost reasons, however, it may be desirable to use a different type of pump to move spent and effluent dialysate. For example, peristaltic pumps, such as the blood pump  48  described above, may more easily integrate into a disposable cassette or tubing set because the disposable part of a peristaltic pump is essentially a loop of tubing. The accuracy of peristaltic pumps, however, may not alone be precise enough for pumping dialysate in systems, such as hemofiltration, hemodialysis and hemodiafiltration, in which a prescribed amount of ultrafiltrate or effluent dialysate needs to be removed from the patient. 
     Patient  42  between dialysis or hemofiltration treatments gains water depending on the extent of kidney loss and fluid intake. Many people suffering kidney failure do not have the ability to urinate. Over the time between dialysis treatments, those patients accumulate fluid. The patient&#39;s total fluid weight gain can vary over different treatments based on the amount of fluid the patient has consumed between treatments and the amount of time between treatments. Therefore, the systems and methods of the present invention need to have a controllable and accurate way of removing whatever amount of fluid is needed to be taken from the patient during the home treatment. Because home patients can treat themselves more often, the amount of fluid that needs to be removed will be typically less than that for in-center treatments. Nevertheless, the home dialysis machine needs to be able to remove the amount of fluid gained between treatments. 
     Referring now to  FIGS.  17  to  22   , various systems  300   a  to  300   f  (referred to herein collectively as systems  300  or generally as system  300 ) employing a single balance chamber  340  are illustrated. Systems  300   a ,  300   b ,  300   c ,  300   d , and  300   e  each operate with a peristaltic dialysate pump  370 . As discussed above, a peristaltic pump is desirable for a cassette-based system because the cassette portion of the pump consists primarily of a looped tube that fits around the pumping head housed by the renal failure therapy machine. 
     Balancing chamber  340  provides the level of volumetric accuracy provided by the membrane pumps discussed above. The majority of systems  300  use peristaltic pump  370  to drive the dialysate, while balancing chamber  340  meters a precise amount of dialysate to the dialyzer, hemofiltration line, etc. Balance chamber  340  in turn meters a pressurized amount of ultrafiltrate from the dialyzer or hemofilter. System  300   f  of  FIG.  22    shows one alternative embodiment, which combines balance chamber  340  with one of the fresh dialysate membrane pumps  22  or  24  and one of the effluent dialysate membrane pumps  26  or  28  discussed above. 
     One primary difference between systems  300   a  to  300   d  is the modality or type of therapy with which balance chamber  340  and peristaltic dialysate pump  370  are used. System  300   a  of  FIG.  17    uses a single dialyzer  20  or  30 . In system  300   a , the modality performed is a primarily diffusive hemodialysis treatment unless the dialyzer has an internal restriction as mentioned previously. However this dialyzer requires a high flux membrane. Longer and narrower dialyzers will increase the percentage of backfiltration. Also a dialyzer having an internal flow restriction suitable for use, such as described in commonly owned U.S. Pat. No. 5,730,712, entitled “Extracorporeal Blood Treatment Apparatus and Method”, is incorporated herein by reference. That dialyzer as indicated is limited to having a fixed orifice. The modality or therapy of system  300   b  of  FIG.  18    is the advanced convection hemodialysis (“ECHD”) treatment provided by arterial and venous high flux dialyzers  20  and  30 , respectively, which are separated by variable restriction  40 . The modality or treatment provided by system  300   c  of  FIG.  19    is the convective treatment, hemofiltration, wherein substitution fluid is pumped directly into venous line  44   b , and wherein ultrafiltrate is removed via a hemofilter  312 . 
     System  300   d  of  FIG.  20    illustrates balance chamber  340  operating in combination with a hemodiafiltration modality. As discussed above, hemodiafiltration combines the diffusive clearance of hemodialysis with the convective clearance of hemofiltration. As seen in  FIG.  20   , a dialyzer  20  or  30  is provided. Also, a separate line  320 , coupled with an additional peristaltic pump  380 , feeds dialysate or substitution fluid directly into venous line  44   b .  FIGS.  17  to  20    illustrate that the volumetric control of ultrafiltration via single balance chamber  340  can be provided for many different types of modalities, such as hemodialysis, ECHD, hemofiltration and hemodiafiltration. The remainder of the description may in certain cases be specific to dialysis or ECHD. It should be appreciated, however, that those teachings are applicable to each of the systems  300  shown in  FIGS.  17  to  20   . 
     Viewing any of the systems  300 , effluent or spent dialysate flows from a dialyzer  20 ,  30  or hemofilter  312  through effluent line  328  and valve V 5  to peristaltic dialysate pump  370 . While pump  370  in one preferred embodiment is a peristaltic pump, pump  370  can alternatively be of any desired variety, such as a piston-driven diaphragm pump, a pneumatic pump or a gear pump. The output of fluid from pump  370  flows via valve V 4  to a spent side  342  of the balance chamber  340 . Similar to the flexible membrane in the membrane pump, balance chamber  340  is separated into a spent compartment  342  and a fresh compartment  344  via a flexible membrane  346 . As discussed herein, valves  56 , such as valve V 4 , may be any suitable type of valve, such as a standard solenoid valve or a volcano-type valve formed partially in the cassette, which is the same or similar to that used in a HomeChoice® system. 
     Balance chamber  340  is a passive volumetric metering device. The same or substantially the same amount of fluid is pushed out of balance chamber  340  as is received into balance chamber  340 . Pumping effluent dialysate into spent compartment  342  in turn pushes membrane  346 , which forces an equal amount of fresh dialysate to exit fresh compartment  344  and travel through valve V 1  in line  314  and into dialyzer  20 ,  30  or into venous line  44   b  depending on the modality used.  FIGS.  17  to  20    are not meant to describe each of the flow components that would be associated with the respective system  300 . For example, if balance chamber  340  pushes substitution fluid through valve V 1  and inlet line  314 , a suitable check valve would be placed in line  314 , which would prevent blood from backing into balance chamber  340 . When enough effluent dialysate enters spent chamber  342  via valve V 4 , so that membrane  346  traverses all the way or substantially all the way towards the chamber wall of fresh compartment  344 , valves V 1 , V 4  and V 5  shut off. 
       FIGS.  17  to  20    show a pressure relief  332  located between the inlet and outlet of dialysate pump  370 . In one embodiment, pressure relief  332  includes a check valve that cracks or relieves at a specific pressure. Alternatively, pressure relief  332  includes a valve seat that relieves pressure at a preset value. For example, a spring tension can control the amount of force or pressure within the pressure relief line that is needed to crack or open pressure relief  332 . When system  300  is used with a disposable cassette, the opening of the valve or seat is configured so that the relieved dialysate is collected and does not contact any of the components within the renal failure therapy machine. 
     In an alternative embodiment, dialysate pump  370  is placed upstream of heater  58 . In such case, pressure relief  332  can extend from the inlet of dialysate pump  370  to fresh dialysate inlet line  334  upstream of valve V 3 . In yet another alternative embodiment, pressure relief  332  incorporates sterile dialysate bags or substitution bags  14  to  18 . That configuration is desirable because it prevents inline heater  58  from overheating fluid when idle, e.g., during an ultrafiltration stroke. 
     A cycle in which effluent fluid is removed from the dialyzer or hemofilter and fresh fluid is sent to the patient or dialyzer has been described. A next cycle sends fluid to drain. Here, heated and fresh dialysate from one of supplies  14 ,  16  or  18  flows through valve V 6 , dialysate pump  370 , valve V 3  and into dialysate compartment  344  of balance chamber  340 . Valves V 1 , V 4  and V 5  are closed. The receipt of fresh dialysate into compartment  344  pushes flexible membrane  346 , causing an equal amount of spent or effluent dialysate to drain via valve V 2  and drain line  338 . Depending on the point in time in the therapy in which this drain cycle takes place, spent effluent can be sent to drain bag  12  or one of the used supply bags  14  or  16 . Once all of the spent dialysate in chamber  342  is emptied through valve V 2  and drain line  338 , all valves V 1  to V 6  are shut off. The fill with spent fluid and pump to patient cycle may then be repeated via the cycle described above. 
     It should be appreciated that the two cycles just described ensure that an equal amount of fluid is sent to the patient and taken from the patient. A UF sequence is described below in which fluid is taken from the patient but not sent to the patient. Calculating the total volume of ultrafiltrate moved is readily done in the illustrated systems  300 . The cumulative volume of the UF cycles is added to determine the total amount of fluid removed from the patient. 
     In one embodiment, pump  370  is run at a slower speed when fresh dialysate is pumped to the dialyzer or patient than when dialysate is pumped from the patient. The difference in speed increases the time that fresh dialysate is flowing to the dialyzer. For hemodialysis, the speed difference increases the diffusion time by increasing the time that dialysate is flowing along the hollow fibers within the dialyzer. The increased time also benefits HF, HDF and ECHD by producing a more gradual ultrafiltration of the patient. The gradual ultrafiltration reduces the risk of hemoconcentration. 
     To remove ultrafiltrate, system  300  begins from an all valves closed position and opens valves V 2 , V 3  and V 5 . Pump  370  causes effluent dialysate to fill the fresh compartment  344  with spent dialysate. That action moves membrane  346  and forces an equal amount of spent fluid previously removed from the patient in spent chamber  342  to be pushed through valve V 2  and line  338  to one of the drain bags. Because the source of fluid used to push this amount of fluid to drain is used dialysate, the amount of used dialysate pumped into fresh compartment  344  is also removed from the patient as ultrafiltrate. That is, there is a small net loss of fluid from the patient during this cycle. In one embodiment, the ultrafiltrate cycle just described is timed to occur every so often during the previously described pump to patient and pump to drain cycles, so as to remove an overall net amount of ultrafiltrate that has collected in the patient between treatments. That net amount is entered into the machine at the start of therapy. 
     One potential drawback of the single balance chamber  340  and single dialysate pump  370  approach is that when spent dialysate is pulled from the dialyzer or hemofilter through line  328  and line  336  via pump  370  into the spent chamber or compartment  342 , a small amount of fresh dialysate is also pushed into spent compartment  342 . That small amount of fresh dialysate is the amount that remains in the tubing leading from valve V 6 , bending around peristaltic pump  370 , and extending further along line  328  towards valves V 3  and V 4 . While the single pump and single balance chamber system is desirable from the standpoint of having a cassette that is simple and relatively inexpensive, it may not be desirable to lose fresh dialysate especially if bagged sterilized dialysate is used. It should be appreciated, however, that if the dialysate is made online, the drawback is less of a concern. 
     Referring now to  FIG.  21   , system  300   e  includes an additional dialysate pump  390 , which is dedicated to removing spent or effluent fluid from the dialyzer or hemofilter. Dialysate pump  370  in turn is dedicated to pumping fresh dialysate. Dialysate pump  390  in one embodiment is a peristaltic pump, however, pump  390  may be of any of the types described above for dialysate pump  370 . Moreover, while the alternative pump configuration of system  300   e  is shown for simplicity in combination with a single dialyzer  20  or  30 , the pumping configuration of system  300   e  is compatible with any of the modalities set forth in  FIGS.  17  to  20   . 
     In the alternative pump arrangement of system  300   e , pump  390  pumps spent fluid through line  328 , valve V 4  and into the spent compartment  342  of single balance chamber  340 . That action causes membrane  346  to move and push an equal amount of fresh dialysate from fresh chamber  344  through valve V 1 , line  314  and into the dialyzer or patient. At the end of the pump to patient cycle, all valves shut off. Afterwards, valves V 2  and V 3  open allowing fresh dialysate pump  370  to pull fresh, heated dialysate from one of the supplies, through line  330 , through valve V 3  and into fresh compartment  344 . That action moves membrane  346  to push spent dialysate from spent compartment  342  through valve V 2  and line  338 , to one of the drain bags. 
     Each of the alternative configurations for the placement of pressure relief  332  is equally applicable to the dual dialysate pump system  300   e . In a further alternative embodiment (see  FIG.  23   ), pressure relief  332  is located instead from the outlet of dialysate pump  370  across to the inlet side of heater  58 . Here, pressure relief  332  connects to line  330  between supply bags  14  to  18  and heater  58  and line  330  downstream of pump  370 . 
     To remove ultrafiltrate from the patient via the dual dialysate pump system  300   e , with the spent compartment  342  full of effluent dialysate, valves V 2 , V 3  and V 5  are opened. Spent fluid pump  390  pumps effluent fluid through line  328 , valve V 5 , line  348  and valve V 3  into fresh compartment  344 . Such action causes membrane  346  to move and push effluent fluid from compartment  342  through valve V 2 , line  338  and into one of the drain bags. Because the source of matching fluid for the balance chamber is used dialysate, that amount of matching fluid is removed from the patient as ultrafiltrate. 
     It should be appreciated that after the ultrafiltrate stroke, the next action is to again pump spent fluid from the dialyzer or hemofilter through valve V 4  into spent chamber  342 . That action causes membrane  346  to move and in turn pump one balance chamber volume worth of spent fluid from fresh compartment  344  (used previously to push the volume of ultrafiltrate) through line  314  to either the dialyzer or the patient. The spent dialysate still provides a clearance benefit to the patient, especially with respect to larger molecules, such as j 32 M. This action also extends the life of a certain amount of the dialysate, which is beneficial especially in the case of a home treatment using sterilized and bagged fluid. 
     Referring now to  FIG.  22   , an alternative hybrid system  300   f  is illustrated. System  300   f  provides the single balance chamber  340  in combination with a dialysate fill pump  22 ,  24  and an ultrafiltrate removal pump  26 ,  28 . In an embodiment, the fill and removal pumps are membrane pumps as described above. The volumetric pumps eliminate the need for the additional valve V 5  and ultrafiltrate line  348  in  FIG.  21   . Otherwise, the two systems are very similar, including the dedicated dialysate removal line  328  operating with pump  26 ,  28  and a dedicated dialysate fill line  330  operating with a dedicated pump  22 ,  24 . 
     As with the other systems, system  300   f  is operable with any of the modalities discussed herein and is illustrated only for convenience in combination with a single dialyzer  20 ,  30 . The advantage of system  300   f  is that there is no mixing of fresh and spent dialysate at the balancing chamber. It should be appreciated that even in  FIG.  21   , with a separate dialysate pump  390 , a small amount of fresh solution will be mixed with spent dialysate during the ultrafiltrate cycle in which pump  390  pushes fluid through line  328 , valve V 5 , line  348  and a small portion of line  330  and valve V 3  into fresh compartment  344 . In  FIG.  22   , ultrafiltration is performed by opening valve V 6  and pulling a predetermined amount of spent dialysate through pump  26 ,  28 . Valves V 3  and V 4  are opened and all other valves are closed. Here, pump  26 ,  28  pushes spent dialysate through line  328  and valve V 4  into the spent compartment  342  of single balance chamber  340 . That action moves membrane  346 , which pushes fresh dialysate from fresh compartment  344  back through valve V 3  and line  330 . Afterwards, all valves are closed for an instant. Then valves V 2  and V 3  are opened, enabling pump  22 ,  24  to push fresh dialysate into fresh compartment  344 , forcing spent dialysate from compartment  342  to move through drain line  338  into one of the drain bags. 
     It is necessary in renal replacement therapies, such as hemodialysis to provide a bolus of fresh solution to the patient for various reasons. For instance, the patient may need a bolus or volume of fluid if the patient becomes hypovolemic (abnormally low volume of circulating blood) or hypotensive (low blood pressure). To provide a bolus of solution for system  300   f , fresh dialysate pump  22 ,  24  expels a predetermined amount of fluid, while valves V 3  and V 4  are opened and all other valves are closed. The fresh dialysate travels through line  330 , valve V 3  and into fresh compartment  344  of balance chamber  340 . That action causes membrane  346  to move and push fluid back through line  328  and valve  324  into effluent dialysate pump  26 ,  28 . Afterwards, all valves are closed. Then, valves V 1  and V 4  are opened and effluent dialysate pump  26 ,  28  pushes used dialysate into spent chamber  342  of balancing chamber  340 . That action causes membrane  346  to move, pushing fresh solution from fresh chamber  344  into the dialyzer. Since no ultrafiltration is removed in this cycle, the amount of fluid sent to the dialyzer represents a net gain or bolus of fluid for the patient. This process can be repeated as many times as necessary to provide a patient with an overall net gain in fluid, if needed. 
     Previous  FIG.  21    also illustrates one embodiment for providing a bolus of fluid to the patient. Here, an additional line  352  and valve V 6  are provided. To provide the bolus, valves V 3  and V 6  are opened, while valves V 1 , V 2 , V 4  and V 5  are closed. Fresh dialysate pump  370  causes fresh dialysate to fill through valve V 3  into fresh chamber  344  of balance chamber  340 . An equivalent amount of spent fluid is pushed via that action and membrane  346  out of balance chamber  340 , through line  352  and valve V 6  into line  314  and dialyzer  20 ,  30 . Again, since no ultrafiltration is removed in this cycle, the fluid sent to dialyzer  20 ,  30  represents a net gain or bolus of fluid. It should be appreciated that spent or effluent dialysate, which is still sterile, is suitable for the purpose of providing a bolus of fluid to the patient. 
     In an alternative embodiment, system  300   e  of  FIG.  21    can provide a bolus of solution by opening valves V 1 , V 4  and V 5 . Valve V 3  is closed. Fresh dialysate pump  370  pumps fresh dialysate into spent compartment  342 . Then all valves are closed for an instant. Afterwards, valves V 3  and V 6  are opened and fresh dialysate pump  370  pumps dialysate into fresh compartment  344 , forcing the fresh fluid in spent compartment  342  to flow through bolus line  352 , valve V 6  and line  314  into the dialyzer. System  300   e  is also restored to balancing mode. 
     A number of alternative embodiments may be used with systems  300   a  to  300   f . Any of the dialyzers discussed herein, such as the single filter disclosed in U.S. Pat. No. 5,730,712, assigned to the assignee of the present invention, may be used. Furthermore, the single dialyzer discussed below in connection with  FIG.  32    may also be used. Arterial line  44   a  in an embodiment includes an air sensor and clamp  54  for automatic blood rinseback. Additionally, any of the fluid preparation and recirculation embodiments discussed above may be implemented with the single balance chamber systems  300 . Moreover, any of the alternative embodiments listed above for systems  10 ,  110 ,  210 , etc., may be applicable to systems  300 . 
     Systems  300   a  to  300   f  also include electrodes or contacts  354  and  356 , which are used with an access disconnection sensor (“ADS”). ADS contacts  354  and  356  are incorporated respectively in arterial line  44   a  and venous line  44   b . If one of the arterial or venous lines becomes disconnected from the patient, an electrical impedance is changed. The break of the loop is sensed, blood pump  48  is shut down and corresponding clamps are closed. An alternative mechanism for the detection of accidental needle disconnection is the use of a conductive blanket underneath the patient&#39;s access. Any spillage of blood changes the conductivity of the blanket, setting off an alarm and stopping the pumping of blood and dialysate. 
     Ultrafiltrate Control—Single Balance Tube 
     The principles described above in  FIGS.  17  to  22   , covering systems  300 , are applicable to different types of balancing apparatuses contemplated by the present invention. Each of systems  300  employs a single balance chamber  340 . Referring to  FIG.  23   , an alternative system  400  employs an alternative balancing device  360 . One embodiment for a balancing tube  360  is shown and discussed in more detail below in connection with  FIG.  45   . In general, balance tube  360  includes a cylindrical or otherwise tubular member. Inside such member resides a piston, ball or other separator  366  that fits snugly within the tube or cylinder. Balance tube  360  includes a tube or cylinder having a fresh portion  362  and a spent portion  364 . Separator  366  fits snugly within the tube and moves back and forth between the fresh side  362  and spent side  364  of the tube. 
     System  400  of  FIG.  23    is configured in a similar manner to system  300   e  of  FIG.  21   . Each component marked with an identical element number performs the same function and includes each of the same alternatives described above in system  300   e . The primary difference between system  400  and system  300   e  as noted is the use of the balance tube  360  as opposed to balance chamber  340 . 
     Valves V 1  and V 4  are opened, while valves V 2 , V 3 , V 5  and V 6  are closed for the pump to dialyzer or patient cycle in system  400 . Spent dialysate pump  390  pumps effluent dialysate through line  328  and valve V 4  into the spent side  364  of balance tube  360 . That action causes separator  366  to move towards the fresh side  362  of balance tube  360  and push a like amount of fluid out through line  314  and valve V 1  into dialyzer  20 ,  30  or directly to the patient (as before, system  400  of  FIG.  23    is applicable to any of the modalities discussed herein). 
     In the pump to drain cycle, valves V 2  and V 3  are opened, while valves V 1 , V 4 , V 5  and V 6  are closed. Fresh dialysate pump  370  pumps fresh fluid through line  330  and valve V 3  into the fresh side  362  of balance tube  360 . That action causes separator  366  to move towards the spent side  364  of balance tube  360 . A like amount of fluid is forced out of spent side  364 , through drain line  338  and valve V 2  to one of the drain bags. 
     For the ultrafiltration cycle of system  400 , valves V 2 , V 3  and V 5  are opened, while valves V 1 , V 4  and V 6  are closed. Prior to this cycle, effluent dialysate resides within balance tube  360  and separator  366  is pushed all the way to the fresh side  362  of the balance tube  360 . Next, spent dialysate pump  390  pulls effluent dialysate from the dialyzer or hemofilter through line  328 , through ultrafiltrate line  348  and valve V 5 , through fill line  330  and valve V 3  into the fresh side  362  of balance tube  360 . That action causes separator  366  to move towards spent side  364 , pushing an equal volume of fluid out through valve V 2  and drain line  338  to one of the drain bags. Because the fluid sent to drain is matched with effluent dialysate from the dialyzer or ultrafilter, the fluid sent to drain constitutes fluid removed or ultrafiltered from the patient. 
     For a bolus of fluid to the patient, valves V 3  and V 6  are opened, while valves V 1 , V 2 , V 4  and V 5  are closed. In essence, no fluid can be drawn from the dialyzer or hemofilter. Instead, fresh dialysate pump  370  pumps fresh dialysate through line  330 , through valve V 3  and into the fresh dialysate side  362  of balance tube  360 . Such action causes separator  366  to move towards side  364  of balance tube  360 . A like volume of fluid is pushed from balance tube  360 , through bolus line  352  and valve V 6 , through fill line  314  into dialyzer  20 ,  30  or directly into the venous line  44   b . Because the fluid delivered to the dialyzer or patient is not matched with an amount of fluid removed from the dialyzer or hemofilter, the fluid delivered to the dialyzer or patient constitutes a net fluid gain or bolus for the patient. Such procedure is repeated as necessary until the patient receives a needed amount of fluid. Any of the alternative bolus embodiments described above in connection with  FIG.  21    may also be used with system  400  and balance tube  360 . Other features of balance tube  360  also applicable to system  400 , such as end stroke sensors, are shown below in connection with  FIG.  28   . 
     Ultrafiltrate Control—Single Tortuous Path 
     Referring now to  FIG.  24   , a further alternative flow balancing device is illustrated by system  450 . System  450  employs a single tortuous path  470 . System  450  includes many of the same components described above, such as drain bag  12 , supply bags  14  to  18 , fresh dialysate pump  370 , heater  58 , spent dialysate pump  390  and blood pump  48 . System  450  is shown in use with the ECHD dual dialyzers  20  and  30 , separated by a variable restriction  40 . It should be appreciated that system  450  may be operated with any of the modalities described herein. Other components with like element numbers are also shown. 
     The primary difference between system  450  and the previous single balance device systems is the use of a tortuous path  470  as opposed to a confined volume that is divided by a separator, such as a membrane or moving ball or piston. The advantage of system  450  is that to place tortuous path  470  in a cassette is relatively simple compared with either the volumetric membrane pumps or the balance chambers and tubes described above, which each require a flexible sheeting or membrane to be sonically welded, chemically adhered or otherwise fused to a rigid plastic cassette. 
     Tortuous path  470  as seen in  FIG.  24    includes a combination of ultrafiltrate line  328  and dialysate input line  330 . Fluid line  328 , 330  is sized to provide as best a bulk transport of fluid as possible, while attempting to minimize pressure drop. That is, a tortuous path  470  in an embodiment is a U-shaped, V-shaped or rectangular-shaped channel in the cassette, which is relatively long and thin or of a small diameter or cross section. The goal of tortuous path  470  is to allow one bulk infusion of fluid, such as fresh dialysate, to move a bulk of fluid already existing in the flow path to a desired place, such as spent dialysate to drain. 
     A drawback of tortuous path  470  of system  450  is the potential for fresh dialysate and spent dialysate to mix within the tortuous path as opposed to moving as bulk fluids. The configuration of the path is refined so that such mixing is minimized and occurs as much as possible only at the interface between the fresh and used dialysate, leaving the middle of the bulk of either fluid relatively unmixed and consistent. To this end, measures may be taken to maintain the flow of both fluids in either a laminar or turbulent state as desired to minimize mixing. For the online systems described herein especially, tortuous path  470  offers a viable solution, wherein the cost and complexity of a cassette or volumetric control system is reduced. 
     To perform the fill to dialyzer or patient cycle in system  450 , fresh dialysate is pumped via dialysate pump  370  through line  330  and valve V 2  up to closed valves V 7  and V 9 . Next, valves V 5  and V 9  are opened, while valves V 2  and V 7  are closed. Spent dialysate pump  390  pulls effluent dialysate from arterial dialyzer  30  through line  328 , valve V 5 , tortuous path line  328 ,  330  and up to valve V 9 . That bulk transport of fluid pushes the fresh dialysate residing within tortuous path line  328 ,  330  through valve V 9 , through fill line  314  and into venous dialyzer  20  or venous line  44   b.    
     After the fill cycle takes place, tortuous path line  328 ,  330  is filled with effluent or spent dialysate. The drain cycle may then take place. Here, valves V 5  and V 9  are closed, while valves V 2  and V 7  are opened. Fresh dialysate pump  370  pumps fresh, heated dialysate through valve V 2 , line  330 , through tortuous path line  328 ,  330  and up to the point of valve V 9  or V 7 . That bulk transport of fluid in turn pushes spent dialysate through drain line  338  and valve V 7  into one of the drain bags. 
     The ultrafiltrate cycle takes place as follows. With the tortuous path line  328 ,  330  filled with ultrafiltrate, valves V 5  and V 7  are opened, while valves V 2  and V 9  are closed. Spent dialysate pump  390  pulls fluid from arterial dialyzer  30  through line  328 , valve V 5  to fill tortuous path line  328 ,  330 . That amount of fluid is then moved through valve V 7 , line  338 , to drain. Because the amount of fluid moved to drain is matched at least substantially by effluent or spent dialysate, the patient experiences a net loss or ultrafiltration of fluid. 
     To provide a bolus of fluid to the patient, with the tortuous path line  328 ,  330  full of fresh or effluent fluid, valves V 5  and V 7  are closed, while valves V 2  and V 9  are opened. Fresh dialysate pump  370  pumps fresh dialysate through line  330  and fills tortuous path line  328 ,  330 . A same volume or substantially the same volume of fluid flows through valve V 9 , fill line  314  and into venous dialyzer  20 . Because the patient or dialyzer has received an amount of fluid without a corresponding amount of fluid being withdrawn from arterial dialyzer  30 , patient  42  experiences a net gain or bolus of fluid. 
     Ultrafiltrate Control—Dual Balance Chambers 
     One potential problem with the single balancing device embodiments just previously described is pulsatile flow. The single balancing device systems can compensate the pulsatile nature of the flow somewhat by slowing the flowrate of fresh fluid to the dialyzer relative to the flowrate of fluid from the dialyzer. Other solutions are provided by system  500  of  FIG.  25    and other dual balance device systems shown below. These systems provide two balance chambers, two balance tubes or two tortuous paths that operate in parallel and at alternating cycles so that flow is delivered to the dialyzer or patient as it is being removed from the dialyzer or hemofilter. System  500  includes many of the same components described above, which are shown with like numbers that do not need to be re-described. Further, system  500  is shown in operation with the ECHD dual high flux dialyzers  20  and  30  and variable restriction  40 . It should be abundantly apparent however from the previous descriptions that system  500  can operate with any of the modalities described herein. 
     System  500  includes first and second balance chambers  340   a  and  340   b , which are each the same in one embodiment as balance chamber  340  described above in connection with  FIGS.  17  to  22   . Balance chambers  340   a  and  340   b  may be referred to herein collectively as a flow equalizer. 
     In the illustrated embodiment, dialysate pumps  370  and  390  are peristaltic pumps. They may alternatively be membrane pumps or other types of pumps described herein. Fresh dialysate pump  370  is shown upstream of heater  58 , which is different from the single balance device configurations. Either configuration is possible for either of the single and double balance device systems. Further, each of the valves used in system  500  may be configured in a cassette or be any type of valve as discussed herein. 
     In a first exchange cycle, one of the balance chambers  340   a  or  340   b  fills with fresh solution and at the same time delivers an equal volume of spent dialysate to drain. In that same first cycle, the other balance chamber  340   a  or  340   b  fills with effluent dialysate and at the same time pushes a like volume of fresh dialysate to the dialyzer  20  or the patient according to the modality. Then, in a second cycle, the balance chambers  340   a  and  340   b  alternate functions so that the balance chamber that previously delivered fresh dialysate to the patient now delivers spent dialysate to drain, while the balance chamber that previously delivered spent dialysate to drain now delivers fresh dialysate to the dialyzer or patient. 
     Based on the foregoing description of the operation of balance chamber  340  in connection with  FIGS.  17  to  22   , it is not necessary to repeat the valve description for each of the balance chambers  340   a  and  340   b  of system  500 . One important aspect to distinguish, however, is that there is a short dwell time at the end of each exchange cycle when all valves are closed to ensure that the two balance chambers  340   a  and  340   b  are in sync for the next cycle. 
     The flow equalizer or balance chambers  340   a  and  340   b  are used differently than in other systems employing a flow equalizer from the standpoint that there is not a separate UF removal device in system  500 . That is, in other systems employing a flow equalizer or dual balance chambers, the balance chambers are dedicated to removing an amount of fluid from the dialyzer, while at the same time filling the dialyzer with a like amount of fluid. System  500 , on the other hand, uses balance chambers  340   a  and  340   b  for that purpose and also to remove a net amount of fluid or ultrafiltrate from patient  42 . The valve operation for removing a net loss or ultrafiltration of fluid from the patient includes opening valves V 1 , V 2 , V 6 , V 7 , and V 9 , while closing valves V 3 , V 4 , V 5 , V 8  and V 10 . This valve configuration pushes effluent dialysate to drain by pushing the fresh dialysate from balance chamber  340   b  to balance chamber  340   a.    
     The systems herein including system  500  having dual balancing chambers  340   a  and  340   b  enable an ultrafiltrate removal rate to vary over time, which is sometimes referred to as an ultrafiltrate profile. For example, if an ultrafiltrate cycle is typically performed after each five exchange cycles, one could change the rate at which ultrafiltrate is removed from the patient by increasing or decreasing the frequency of cycles. This could result, for example, in more fluid being removed during a first part of therapy than a second. In the present invention, the processor of the renal failure therapy machine may be configured to run an algorithm, which enables the patient to select a profile, a treatment time and an overall volume to be removed. The algorithm automatically calculates an ultrafiltrate frequency profile that achieves, according to the profile, an entered net cumulative ultrafiltrate volume over an entered treatment time. Those parameters may be entered through a patient data card or through a secure data connection. 
     System  500  can also provide a bolus of solution to the patient when needed. Valves V 2 , V 3 , V 7 , V 8  and V 10  are opened and valves V 1 , V 4 , V 5 , V 6  and V 9  are closed. Pump  370  is run forcing one balance chamber bolus of dialysate and/or substitution fluid to the dialyzer or patient. 
     In any of the embodiments described herein, it is important that the valves of the systems are checked to ensure that they open and close properly. In one embodiment, the valves are checked periodically throughout treatment using conductive sensing. That is, if fluid escapes from the system via a faulty valve or tear in a cassette membrane, conductive sensors that measure a flow of electricity across a liquid can send an alarm and trigger appropriate action. Further, with a cassette, temperature sensing may be employed, for example, by applying a thermistor, IR sensor or thermocouple on one side of the sheeting of the cassette. Here, the temperature sensor is attached to the blood therapy instrument and, for example, contacts the sheeting membrane so as to obtain a quick reading of the temperature of the dialysate. 
     Prime and Rinseback 
     Referring now to  FIG.  26   , it is necessary to prime the extracorporeal circuits of the present invention with sterile solution prior to connecting patient access line  44   a  and venous access line  44   b  to the patient. To do so, the ends of the arterial and venous lines are connected together at connection  358 . In one embodiment, fresh dialysate pump  370  and effluent dialysate pump  390  run and pump fluid through balance chambers  340   a  and  340   b  (or through any of the single or dual balance devices discussed herein) until dialysate or substitution fills the dialysate circuit. The blood therapy machine then enters a bolus mode. In one embodiment, blood pump  48  runs in reverse until venous drip chamber  52  fills with fluid. Excess air in the line and drip chamber vents through a transducer protector or vent  64  provided with or in communication with drip chamber  52 . Transducer protector or vent  64  in one embodiment is a 0.2 micron hydrophobic membrane. 
     In the next step of this first priming method of the present invention, blood pump  48  runs in its operational direction until half the volume of the drip chamber is moved. Then, blood pump  48  runs in the reverse direction again until drip chamber  52  is again filled and vented. The pump then runs again in the normal operation direction enough to move half a drip chamber volume worth of fluid in the normal operating direction. In each cycle, dialysate or substitution fluid is back-filtered through dialyzer  20 ,  30  (or different filter for a different modality), adding to the total volume of fluid in the extracorporeal circuit over each cycle period. This first priming method cycles back and forth as described until the extracorporeal circuit is completely filled with dialysate or substitution fluid. It should be appreciated that this priming method applies to any of the modalities described herein, any of the pumping arrangements described herein and any of the volumetric control methods described herein. 
     In a second priming method, a separate saline or priming fluid bag  368  is connected to the extracorporeal circuit via saline line  372 . In the illustrated embodiment, saline line  372  tees into the extracorporeal circuit at two places, upstream and downstream of blood pump  48 . Valves V 11  and V 12  are positioned in saline line  372  so as to allow saline to flow selectively to one of or both of the teed connections upstream and downstream of blood pump  48 . Arterial access line  44   a  is again connected to venous access line  44   b  via connection  358 . 
     In the operation of the second priming method of the present invention, valve V 11  located downstream of pump  48  is opened, enabling blood pump  48  to run in reverse and pump saline from bag  368 , through saline line  372 , through valve V 11  through access line  44   a , through connection  358 , through access line  44   b , and into drip chamber  52 . Blood pump  48  pumps saline until drip chamber  52  is full and air is purged via vent  64 . Next, valve V 11  and air detector clamp  53  are closed and valve V 12  is opened, enabling blood pump  48  to pull saline from bag  368  and push that volume of fluid in the normal operating direction downstream of pump  48 , venting air through vent  64 . This cycle continues until the extracorporeal circuit is fully primed. It should be appreciated that this second priming method is equally applicable to any of the modalities, pumping regimes, and volumetric control methods discussed herein. 
     Modifications to either of the first and second priming methods can also be made to provide a blood rinseback to patient  42 . This is done at the end of therapy to return any blood in the extracorporeal line to the patient. The primary difference for blood rinseback is that access lines  44   a  and  44   b  are connected to patient  42  instead of to each other via connection  358 . For example, using saline  368  or other suitable source, valve V 11  is opened and pump  48  runs in reverse to rinseback blood to the pre-pump portion of arterial line  44   a . An air detector  54  in that portion of arterial line  44   a  detects any air in the blood or saline and clamps the circuits if such air is detected. Pump  48  runs for an appropriate amount of time to ensure that blood has been fully rinsed back to the patient through the pre-pump portion of arterial line  44   a.    
     Next, valve V 11  closes and valve V 12  opens, enabling pump  48  to pull saline from supply  368  and operate in the normal direction. Pump  48  pumps saline or other suitable fluid from source  368  through the remaining portion of arterial line  44   a , through dialyzer  20 ,  30  (depending on modality) and through venous line  44   b  including drip chamber  52 . The rinseback returns blood from those portions of the extracorporeal circuit to patient  42 . In an embodiment, saline sensors on the arterial and venous lines  44   a  and  44   b , respectably, cause an alarm if the extracorporeal circuit is not clear or transparent after a preset amount of rinseback. After blood is fully rinsed back to the patient, the patient is instructed to disconnect from the renal failure therapy system of the present invention. 
     The first priming method described above may also be adapted for blood rinseback. Here either dialysate or saline is back-filtered through the dialyzer or other modality filter. Blood pump  48  is run in the reverse and forward cycles described above in connection with the first priming method. Pump  48  may be run at a slower speed for blood rinseback so as to limit an amount of mixing between saline and blood. The saline or other solution needed to fully rinseback the blood to the patient is thereby minimized. 
     In an alternative method for priming system  500  or rinsing back blood to the patient, one of the line clamps  54  in the extracorporeal circuit is closed and saline or dialysate is pumped via one or both dialysate pumps  370  and  390  into the extracorporeal circuit until drip chamber  52  fills to a preset level, such as ¾ full. After the drip chamber  52  is filled to the preset level, the dialysate or saline infusion is stopped, and pumps  370  and  390  no longer pump fluid into the extracorporeal circuit. Then, line clamp  54  is opened. Blood pump  48  circulates the dialysate through the extracorporeal circuit. If needed, line clamp  54  may be clamped again to repeat the process. 
     In a further alternative prime or rinseback embodiment, saline bag  368 , dialysate from a supply or drain bag, saline line  372 , valve V 12  and the portion of line  372  leading to the extracorporeal circuit between clamp  54  and blood pump  48  are used. Here, valve V 11  in  FIG.  26    is not needed. Dialysate or saline is pumped via one or more of the dialysate pumps  370  and  390  through dialyzer  20 , 30  with blood pump  48  running in the reverse direction and valve V 12  closed so as to prime or rinseback the arterial line  44   a . Then, valve V 12  is opened and saline or dialysate is pulled from supply bag  368  with pump  48  running in the normal operating direction to prime or rinseback venous line  44   b . This method uses dialysate or saline pumped through the dialysate circuit as well as a dialysate or saline source running directly to the extracorporeal circuit. This embodiment eliminates valve V 11  shown in system  500 . 
     It should be appreciated that each of the forgoing methods of prime and rinseback may be used in any of the forgoing modalities, pump configurations and volumetric control schemes. Further, those of skill in the art may be able to determine additional valving operations to achieve an effective prime and rinseback using the apparatuses and methods of the present invention. 
     Ultrafiltrate Control—Dual Balance Tube 
     While the present invention sets forth multiple embodiments for balancing devices, it is believed that the balancing tubes provide a good trade-off between ease of manufacturing, cost and effectiveness. The balancing chambers shown previously for example in  FIGS.  25  and  26    are time-tested and proven to effectively meter and control ultrafiltrate in blood kidney failure therapies, such as hemodialysis. The sheeting and chambers associated with balance chambers, while certainly manufacturable, present a more complicated cassette than simply one having valve chambers, tubing for peristaltic pumps and tubes for the balance tubes of the present invention. 
     The tortuous path embodiment, while perhaps involving the simplest cassette, may not be as desirable with respect to efficient use of fresh dialysate (due to the tendency of the fresh and effluent dialysates to mix). Again, this potential drawback is not as much of a concern when dialysate is made online. The balance tubes may offer the best solution however for home use with fresh dialysate bags. 
     Referring to  FIGS.  27 A to  27 D , different flow cycles pertinent to volumetric control of dialysate using dual balance tubes are illustrated. It should be appreciated that the layout of valves V 1  to V 10  with respect to balance tubes  360   a  and  360   b  is the same as the layout of valves V 1  to V 10  with respect dual balance chambers  340   a  and  340   b  in  FIGS.  25  and  26   . One can therefore readily visualize balance tube  360   a  being used in place of balance chamber  340   a  and balance tube  360   b  being used in place of balance chamber  340   b  in  FIG.  25   . 
     The cycle shown in  FIG.  27 A  is a first dialysate exchange cycle. Here, valves V 1 , V 4 , V 5 , V 8 , V 9 , and V 10  are open while valves V 2 , V 3 , V 6  and V 7  are closed. At the start of this cycle balance tube  360   a  is filled with fresh dialysate and separator  366   a  is located at least substantially at the end of spent portion  364   a . Also, balance tube  360   b  is filled with effluent dialysate and separator  366   b  is located at least substantially at the end of fresh portion  362   b  of balance tube  360   b . In this first cycle, fresh dialysate pump  370  pumps fresh dialysate through line  330 , line  330   b  and valve V 5  into fresh dialysate portion  362   b  of balance tube  360   b . The force of fluid entering fresh portion  362   b  pushes separator  366   b , which in turn pushes spent dialysate through open valve V 8 , line  338   b , manifold  338  and valve V 9  to one of the drain bags. 
     At the same time spent dialysate pump  330  pushes effluent dialysate from a dialyzer or hemofilter through manifold  328 , line  328   a , valve V 4  and into the spent portion  364   a  of balance tube  360   a . The force of fluid entering spent portion  364   a  of balance tube  360   a  causes separator  366   a  to move towards the fresh portion  362  of balance tube  360   a . In turn, fresh dialysate is pushed through valve V 1 , line  314   a , manifold  314  and valve V 10  to a dialyzer or the extracorporeal circuit, depending on the modality used. It should be appreciated from the valving description of  FIG.  27 A  that one of the balancing chambers is metering fresh fluid to the patient, while the other balancing chamber is metering spent fluid to drain. 
       FIG.  27 B  shows separators  366   a  and  366   b  at the fresh end  362   a  and spent end  364   b  of balance tubes  360   a  and  360   b , respectably (at the end of travel of the cycle shown in  FIG.  27 A ). At this moment all valves V 1  to V 10  are closed. The all valves closed sequence ensures that balance tubes  360   a  and  360   b  and valves V 1  to V 10  are in sync for the next fluid transport cycle. 
     Referring now to  FIG.  27 C , an opposite fluid transport cycle of that shown in  FIG.  27 A  is illustrated here beginning from the valve conditions shown in  FIG.  27 B , namely, with balance tube  360   a  filled with effluent dialysate and balance tube  360   b  filled with fresh dialysate. The opposite flow now occurs in which balance tube  360   a  meters spent fluid to drain, while balance tube  360   b  meters fresh fluid to the dialyzer or extracorporeal circuit. In this cycle, valves V 2 , V 3 , V 6 , V 7 , V 9 , and V 10  are open, while valves V 1 , V 4 , V 5  and V 8  are closed. Fresh dialysate pump  370  pumps fresh dialysate through manifold  330 , line  330   a  and valve v 3  into the fresh portion  362   a  of balance tube  360   a . Such action causes separator  366   a  to push spent dialysate through valve V 2 , line  338   a , manifold  338  and valve V 9  to drain. At the same time, spent dialysate pump  390  pumps spent dialysate from a dialyzer or hemofilter through manifold  328 , line  328   b , valve V 6  and into the spent or effluent portion  364   b  of balance tube  360   b . Such action causes separator  366   b  to push fresh dialysate through valve V 7 , line  314   b , manifold  314  and valve V 10  to the patient or dialyzer. 
     After the cycle of  FIG.  27 C  is completed each of the valves closes with the balance tubes in the same state shown in  FIG.  27 A , so that the above three cycles shown in  FIGS.  27 A and  27 C  can be repeated. It should be appreciated that the all valves closed state of  FIG.  27 B  occurs for a relatively short period of time, so that the flow of fluid to the patient or dialyzer and from the dialyzer or hemofilter is substantially non-pulsatile. Such non-pulsatile flow is advantageous versus the relatively pulsatile flow of the single balance device systems because (i) treatment is administered more efficiently and (ii) the fresh and spent pumping cycles may be carried out at the same speed reducing the risk of pulling too much fluid from the patient. 
     Referring now to  FIG.  27 D , one embodiment for performing ultrafiltration with the dual balance tubes  360   a  and  360   b  of the present invention is illustrated. It should be appreciated that the state of separators  366   a  and  366   b  and the fluids held within balance tubes  360   a  and  360   b  is the same as in  FIG.  27 A . Instead of performing the exchange cycle, however, the valve arrangement shown in  FIG.  27 D  is employed. Here, valves V 1 , V 4 , and V 7  to v 9  are opened, while valves V 2 , V 3 , V 5 , V 6  and V 10  are closed. In the ultrafiltration cycle only used dialysate pump  390  is run. Pump  370  may stop or run through recirculation line  332 . Pump  390  pumps effluent fluid through manifold  328 , line  328   a  and valve V 4  to push separator  366   a  from spent portion  364   a  of balance tube  360   a  towards fresh portion  362   a  of the tube. That action causes fresh dialysate through valve V 1 , line  314   a , manifold  314 , line  314   b  and valve V 7  into balance tube  360   b . Fluid entering balance tube  360  in turn pushes separator  366   b , forcing effluent fluid through valve V 8 , line  338   b  and manifold  338  to drain through valve V 9 . The fluid sent to drain represents ultrafiltrate because during that cycle no corresponding amount of fluid is sent to the patient or dialyzer. 
     This ultrafiltrate cycle may be varied in frequency relative to the fluid exchange cycles to vary the rate ofultrafiltrate removal over time. It should be appreciated that a bolus of fluid may be given to the patient in a similar manner, with incoming fresh dialysate pushing effluent dialysate via a separator from one balance tube to the other, forcing the separator in the other balance tube to push fresh solution towards the dialyzer or extracorporeal circuit depending on modality. The patient or dialyzer gains fluid without a corresponding loss of fluid from the patient, resulting in a bolus of fluid. 
     Referring now to  FIG.  28   , an alternative valve configuration for balance tube  360   a  of the present invention is illustrated. Here, a pair of tees  374  are mated or sealed to the ends  362   a  and  364   a  of balance tube  360   a . Valves V 1  to V 4  are placed in the same configuration relative to the inlets and outlets of tube  360   a  shown in  FIGS.  27 A to  27 D . Here, only one pathway to each end  362   a  and  364   a  of balance tube  360   a  is needed. As in  FIGS.  27 A to  27 D , valve V 2  controls whether effluent dialysate is delivered to the drain or the drain bag through line  338 . Valve V 4  controls whether effluent dialysate from the dialyzer or hemofilter enters balance tube  360   a  through line  328   a . Valves V 2  and V 4  are both located at the spent dialysate end  364   a  of balance  360   a . Valve V 3  controls whether fresh dialysate from one of the supply bags enters balance tube  360   a  through line  330   a . Valve V 1  controls whether dialysate leaves balance tube  360   a  through line  314   a . Valves V 1  and V 3  are both located at the fresh dialysate end  362   a  of balance  360   a.    
       FIG.  28    also illustrates that a pair of sensors  376 , such as optical sensors, are positioned in the instrument so as to detect and ensure that separator  366   a  has traveled to the appropriate end  362   a  or  364   a  of balance tube  360   a . For example if fluid is expected to be received from the dialyzer through line  328   a  and V 4 , the logic in the renal failure therapy machine will expect to see a beam of light of the sensor  376  at end  362   a  broken and then reestablished once separator  366   a  passes sensor  376  and reaches the end of its stroke. If the beam of light is either not broken or not reestablished the machine knows that separator  366   a  has not traveled to its appropriate destination for the given cycle and sends an appropriate signal. Alternative sensors, such as proximity, capacitance, Hall Effect, ultrasound or others may be employed instead of the illustrated optical sensors  376 . These sensors may also be employed to check valve function. Here, if separator  366   a  moves due to a valve being open when that valve is supposed to be closed, the valve is detected to have a leak. 
     Ultrafiltrate Control—Dual Tortuous Path 
     Referring now to  FIG.  29   , another dual balance device embodiment is illustrated. Here the balance chambers and balance tubes shown previously in  FIGS.  25  to  28    are replaced by a pair of tortuous paths  470   a  and  470   b . Tortuous paths  470   a  and  470   b  are placed in between valves V 1  to V 8  as seen also in  FIGS.  25  and  26   . Indeed, the operation of valves V 1  to V 8  in  FIGS.  25 ,  26  and  29    operate identically to continuously send fluid to the patient, send spent fluid to drain and remove ultrafiltrate from the dialyzer or hemofilter. As before, the dual tortuous paths  470   a  and  470   b  may be implemented with any modality and with any of the different types of pumps described herein. To push fresh fluid to dialyzer  20 ,  30 , tortuous path line  328   a ,  330   a  or line  328   b ,  330   b  is filled with fresh dialysate. Either valves V 1  and V 4  for tortuous path  470   a  or valves V 6  and V 7  for tortuous path  470   b  are opened. Pump  390  pumps spent dialysate through either line  328   a ,  330   a  or line  328   b ,  330   b  to push the corresponding bulk of fresh dialysate to the dialyzer. Then either valves V 2  and V 3  or valves V 5  and V 8  are opened to push spent fluid to drain. 
     In one preferred embodiment, the tortuous paths  470   a  and  470   b  are alternated so that one path delivers dialysate to the dialyzer during one cycle and the other tortuous path delivers dialysate to the dialyzer during the same cycle. The roles of paths  470   a  and  470   b  are then reversed. While one path is delivering dialysate to the dialyzer, the other is filling with fresh solution and delivering spent dialysate to drain. Each of the tortuous paths  470   a  and  470   b  is built to have a length and diameter that attempts to minimize the amount of mixing between fresh and spent fluids, so that the fluids tend to move in bulk to their desired destination. 
     To remove ultrafiltrate, fresh fluid from one line  328   a ,  330   a  or  328   b ,  330   b  can be moved to in turn displace spent fluid from the other line to drain. For example, valves V 1  and V 4  of tortuous path  470   a  may be opened so that spent dialysate enters line  328   a ,  330   a  and displaces fresh dialysate through open valve V 7  into line  328   b ,  330   b  of tortuous path  470   b . Valve V 6  is opened and spent dialysate is moved through line  572  to drain. If needed, a valve may be added after dialysate pump  390  so that spent fluid does not flow back into pump  390  during the ultrafiltrate cycle. 
     As illustrated, a separate ultrafiltrate pump  570  may be added to system  550  or to any of the forgoing systems. Ultrafiltrate pump  570  enables tortuous paths  470   a  and  470   b  to operate continuously to send fluid to and take equal amounts of fluid from the dialyzer or hemofilter. The ultrafiltrate pump  570  removes dialysate through ultrafiltrate line  572  to one of the drain bags. It is believed that removing the ultrafiltrate function from the tortuous paths  470   a  and  470   b  may reduce mixing of the fresh and spent fluids. The additional ultrafiltrate pump  570  can also be run in reverse with pump  390  to provide a bolus of fluid to a patient in need same. 
     It should appreciated that any of the dual balancing device systems described herein can employ the ADS contacts  354  and  356  and associated electronics to detect when one of the access lines  44   a  or  44   b  is inadvertently disconnected from the patient during treatment. Further, any system can employ one of more of the various pressure reliefs  332  shown in  FIGS.  25 ,  26  and  29    and described previously. Furthermore, the heater may be placed before or after fresh dialysate pump  370 . Again the pumps may be of any of the varieties described herein. Moreover, any of the dual balance device systems may be used with any of the fluid preparation modules described above as well as the recirculation loops. The systems may also employ noninvasive temperature measuring devices to measure the temperature of fluid within a disposable cassette. 
     Ultrafiltrate Control—Weight Scales 
     Referring now to  FIGS.  30  and  31   , a further alternative method of controlling the amount of dialysate exchanged and ultrafiltrate removed is to do so by measuring the weight of fluid within supply and drain bags  12  to  18 . For convenience only supply/drain bags  14 ,  16 , and  18  are shown in  FIG.  30   . It is well known to use weight to control a renal failure therapy process. A single scale can be employed that accounts for both fresh fluid lost and spent fluid gained. Here, because a net volume of fluid is removed or ultrafiltered from the patient, the system expects to see an increase in weight over time. Alternatively, a first scale for the fresh bags and a second scale for the drain bags are used. Two signals are produced and summed to determine the amount of ultrafiltrate accumulated for any give point in time. The system of  FIGS.  30  and  31    uses a single scale, however, the dual scale approach may be used instead. 
     The import of  FIGS.  30  and  31    is to show one apparatus by which a scale or weight measuring device may be implemented into the various systems described herein. In  FIG.  30   , a blood treatment machine  140  is illustrated. In the illustrated embodiment, blood machine  140  accepts a cassette at cassette loading portion  142 , which is on a front, angled part of machine  140 . Other embodiments of a machine that can accept a disposable cassette and employ a scale are shown below in  FIGS.  35  to  39   . Bags  14 ,  16  and  18  are loaded onto stand  144 . Stand  144  is coupled to a shaft  146 . 
       FIG.  31    shows an enlarged view of the cutaway in  FIG.  30    and that shaft  146 , stand  144  and the bags are supported by a foot  152  that rests on a table of wherever machine  140  is placed for treatment. Shaft  146  is movable linearly within a linear bearing  148 . A cap  154  having a plurality of anti-rotation pins  162  is fitted to the end of movable shaft  146 . Pins  162  reside within mating slots or grooves defined in the housing of machine  140 . Pins  162  and the mating slots or grooves enable shaft  146  to move linearly but not rotationally with respect to machine  140 . 
     A seat  164  seals one end of a rolling diaphragm  168  between the seat and cap  154 . A housing  176  coupled to foot  152  and the machine frame seals the other end of rolling diaphragm  168  between housing  176  and the frame of machine  140 . Housing  176 , rolling diaphragm  168  and seat  164  form a closed volume chamber. The rolling diaphragm enables the volume to remain closed and also enables shaft  146  to fluctuate up and down due to the varying weight within the supply end drain bags. The rolling diaphragm  168  may be made of any suitable deformable but impermeable material, such as rubber or plastic sheeting. The volume of air within the closed volume chamber pressurizes due to the weight of the bags  14  to  18  and supporting apparatus. The amount of pressure indicates or varies with the amount of liquid in bags  14  to  18 . 
     A pressure sensor, which may be any suitable type of sensor (not illustrated), is provided for example within opening  178  defined by seat  164 . The pressure sensor senses the amount of pressure within the closed volume chamber. The sensor sends a signal to a processor or a controller within machine  140 , which processes that signal and determines the corresponding weight in bags  14  to  18 . 
     The weight control system is desirable because it removes the need for the volumetric control devices described above. The cassette for machine  140  is much simpler, including mainly valve flow paths. One disadvantage of the weight system is that it requires the patient to load the bags properly onto stand  144 . The stand and assembly described in connection with  FIGS.  30  and  31    may also add weight and size to the overall device. The home renal failure therapy machine of the present invention is desirably small and light, so that a person can travel or maneuver the device easily within or outside of the home. 
     ECHD Filter 
     Referring now to  FIG.  32   , one embodiment for an ECHD filter is illustrated by filter  600 . As incorporated above, one suitable ECHD filter is described in U.S. Pat. No. 5,730,712, assigned to the assignee of the present invention. Filter  600  like the filter described in the patent is provided in a single unit. Filter  600  however differs from the one in the patent in that it allows for operation with a variable restriction  40 . 
     Filter  600  includes a housing  602  corresponding to venous dialyzer  20  and a housing  602  corresponding to arterial dialyzer  30 . Housing  602  may be made of any suitable material, such as a cylindrical, rigid plastic. Fibrous, semi-permeable membranes are loaded within the venous section  20  and the arterial section  30 . Those membranes are potted at the outside ends of housings  602  via a potting  604  according to any method known to those of skill in the art. The membranes are potted at the inside ends of each of the venous  20  and arterial  30  sections of filter  600  via a potting  606 . 
     A blood entry cap  608  is fixed in a sealed manner to housing  602  so that blood may enter cap  608  via a blood tube, be dispersed within the cap and enter the inside of the hollow semi-permeable fiber membranes of arterial section  30 . At the same time, blood is blocked from entering housing  602  on the outside of hollow fiber membranes via potting  604 . 
     Blood travels through filter  600  via the arrow shown in  FIG.  32   . That is, blood travels upward through the arterial portion  30  of filter  600  and out internal potting  606  of the arterial portion  30 . Blood then enters intermediate chamber  642 . The intermediate chamber  642  is a band or outer tube that is secured sealingly to the internal ends of housings  602 . 
     Blood then enters the second set of hollow semi-permeable membranes housed within venous portion  20  of filter  600 . The blood enters those fibers and is prevented from entering housing  602  of venous portion  20  outside the fibers via internal potting  606  at the internal end of housing  602  of venous portion  20 . Blood flows through the venous portion of the membranes, through an outer potting  604  and into a blood exit cap  632 . Blood exit cap  632  in turn couples sealingly to a tube that carries the blood away from filter  600  within the extracorporeal circuit. 
     Housing  602  of venous portion  20  includes a dialysate entry port  634  and a dialysate exit port  636 . Likewise, housing  602  of arterial portion  30  includes a dialysate inlet port  638  and a dialysate exit and ultrafiltrate port  640 . Ports  634 ,  636 ,  638  and  640  may be of any suitable type for mating sealingly with a medical fluid tubing. Port  634  receives dialysate from the dialysate supply. Port  640  enables dialysate and ultrafiltrate from the patient to be pulled out of filter  600 . The effluent dialysate stream exists filter  600  via port  640 . 
     Variable restriction  40  is placed in fluid communication with ports  636  and  638 . The restriction may be made more or less restrictive so as to backfilter greater or lesser amounts of fresh dialysate into the hollow fiber membranes located in housing  602  of venous portion  20 . As described above, the clearance of filter  600  is convective and diffusive. Filter  600  achieves one desired goal of the present invention, namely, to provide an overall effective treatment of small, middle and large molecules of a patient&#39;s waste via both convective and diffusive clearance modes. Housings  602 , caps  632 ,  608 , the potting material, the porous fibers and the ports may be made of any suitable materials. 
     Apparatus for Providing Variable Flow Restriction 
     Referring now to  FIG.  33   , one embodiment for variable flow restriction  40  is illustrated. While it is contended that there are likely many different ways to provide a repeatable and accurate variable flow restriction, variable restriction  40  of  FIG.  33    provides one suitable configuration. System  40  includes a stepper motor  954 , which is coupled to a lever arm  956  via a coupler  958 . Stepper motors are known in the art as highly accurate and repeatable positioning devices that can receive signals from a microprocessor that commands stepper motor  954  to turn a precise distance, and perhaps at a desired acceleration and velocity. In  FIG.  33   , stepper motor  954  is used primarily to position lever arm  956  to a precise position with respect to a fixed surface  960 . 
     A tube section  962  shown also in  FIGS.  1 ,  4 ,  5 ,  9 ,  12  and  14   , connects dialysate flow between dialyzers  20  and  30 .  FIG.  33    illustrates that section  962  is held in place against surface  960  via bracket  964 . Lever arm  956  as seen in  FIG.  33    is currently in a position that enables full flow through tube section  962 . That is, in the configuration illustrated in  FIG.  33   , very little dialysate would backflow through the membranes of one of the dialyzers  20  or  30 . As lever arm  956  is rotated in a counterclockwise direction as seen in  FIG.  33   , tube section  962  deforms and increasingly decreases in cross-sectional area, causing the amount of restriction in device  40  to continuously increase. Indeed, lever arm  956  could be rotated to a point that would virtually restrict all flow through tube section  962 , forcing virtually all of the therapy fluid to enter the extracorporeal circuit  50  through the membranes of one of the dialyzers  20  or  30 . 
     Importantly, stepper motor  954  is accurate and repeatable. That is, stepper motor  954  can be commanded to rotate lever arm  956  to virtually the same position time and time again. Because tube section  962  is held in the same position via bracket  964  relative to lever arm  956  and fixed surface  960 , lever arm  956  accurately and repeatedly creates the same amount of restriction through line  962  when the arm  956  travels to the same commanded position. The programmable nature of stepper motor  954  also enables restriction  40  to have virtually any desired restriction profile that varies over the duration of therapy as desired by the patient, physician or other operator. Such variable restriction profiles are described above and can be stored as programs within a memory device of the controller of the systems described herein, such that one of the variable restriction profiles can be called upon and implemented as desired. 
     Interfacing Between Cassette, Blood Treatment Machine and Solution Bags 
     Referring now to  FIG.  34   , cassette  100   a  (shown above in  FIGS.  2  and  3   ) is shown in an operable position interfaced with a number of the flow devices that are located inside of the blood treatment machine. Cassette  100   a  as illustrated includes a housing  104 . Attached to housing  104  are a number of flow components, which are provided either in part or completely on or in cassette  100   a . As illustrated, dialyzers  20  and  30  are attached to housing  104 . The tubing  102  extends so as to be able to loop around a pump head portion of blood peristaltic pump and connects fluidly to housing  104  of cassette  100   a . The arterial and venous patient lines  44   a  and  44   b  respectively also are attached to or communicate with cassette  100   a . As illustrated in  FIG.  34   , patient access lines  44   a  and  44   b  are initially connected together to preserve the sterilization of air within those lines. A number of sensors, such as pressure sensors  46  are further integrated with cassette  100   a.    
     For reference, drain container  12  and solution bags  14  to  18  are shown in one possible proximal position to cassette  100   a  in  FIG.  34   . Bags  12  to  18  connect via tubes (not illustrated) to bag ports  132  to  138 , respectively, extending from housing  104  of cassette  100   a . Ports  132  to  138  are also shown in  FIGS.  2  and  3   .  FIGS.  2  and  3    also show a number of additional ports. For example, ports  106  connect to dialyzers  20  and  30 . Ports  108  connected to peristaltic pump  102  shown in  FIGS.  2  and  12   .  FIGS.  2 ,  3  and  12    also show a number of additional ports  116 , which are connected to filters  20 ,  30  as noted in connection with  FIGS.  2  and  3   . Additional ports, such as ports  116 , and valve portions  156  can be added to cassette  100   a  to operate and communicate with sorbent cartridge  222  of  FIGS.  5  to  8   . 
       FIG.  34    also illustrates a number of the devices that are housed inside the blood treatment machine. For example,  FIG.  34    illustrates a number of valves  56 , which are operably connected to cassette valve positions  156  shown in  FIG.  2   . The fluids at all time flow through the sterile cassette  100   a , which is disposable. The mechanics and electronics of valves  56 , on the other hand, are placed inside the machine and reused. In a similar manner, heater  58  couples operably to fluid heating portion  158  of cassette  100   a  shown in  FIG.  2   .  FIG.  34    also shows drip chambers  52  (referring collectively to chambers  52   a  to  52   c , e.g.) as well as temperature sensors  62  operable with cassette  100   a . Further, infusion pump actuators of pumps  22  and  24 , shown in  FIG.  12   , are coupled operably to pump chambers  122  and  124  as seen in  FIG.  2   . Likewise, ultrafiltrate pump actuators or pumps  26  and  28  are coupled operably to pump chambers  126  and  128  shown in  FIG.  2   . 
     Referring now to  FIG.  35   , the flow devices of  FIG.  34    are shown this time housed inside blood treatment machine  150 . Blood treatment machine  150  is a machine that performs any of the systems and therapies described herein.  FIG.  35    illustrates that in one embodiment, drain bag  12  and solution bags  14  to  18  are stored in operation in a two-by-two arrangement on top of machine  150 . Machine  150  also shows the relative placement of cassette  100  within machine  150 . In particular, bag ports  132  to  138  extend upwardly from the top of the machine in relatively close proximity to bags  12  to  18 . Ports  116  (e.g., attaching to the dialyzers or hemofilters, the sorbent cartridge or attaching drip chambers  52 , etc.) extend from the side of machine  150 . 
       FIG.  35    also illustrates that peristaltic pump blood line  102  extends outside machine  150  and mates with the pumping head portion of the peristaltic pump  48 , which is housed mainly inside machine  150 , but which has a rotating head that is located outside machine  150  to receive tube  102 . Cassette  100   a  slides almost entirely inside machine  150 , leaving dialyzers  20  and  30 , peristaltic line  102 , patient access lines  44   a  and  44   b  and ports  116  outside of machine  150 . 
     Machine  150  includes a graphical user interface  160  that enables the patient  42 , nurse or other operator, to begin therapy, monitor therapy, receive status messages from the therapy, as well as collect data for post-therapy analysis of the patient&#39;s treatment and status. Graphical user interface (“GUI”)  160  allows patient  42  or other operator to select the desired therapy and to adjust the desired or necessary fluid loss or UF volume for each treatment. GUI  160  receives prescription entries via the packetized or checked data packets via memory card, flash memory, modem, internet connection, or other suitable local area or wide area mode of data communication. The electronic and software architecture running GUI  160  is redundant in one preferred embodiment, so that monitoring and controlling any critical function is executed through separate hardware and software. 
     GUI  160  in one embodiment includes a touch screen that enables the patient  42  or operator to enter desired parameters. In an alternative embodiment, GUI  160  uses electromechanical devices, membrane switches, voice activation, memory cards, or any combination of the above-described input devices. In one embodiment, GUI  160  is run via multiple processors, such as a supervisory/delegate processor system. A separate processor is provided for monitoring and checking that the critical functions of the machine are being performed correctly. That is, while one processor is dedicated to controlling the flow devices of the system to achieve the desired therapy, another processor is provided to check that the hardware processor and the associated flow devices are operating properly. 
       FIGS.  36  and  37    illustrate an alternative blood treatment machine  170 , which differs from machine  150  primarily in the arrangement of drawing bag  12  and solution bags  14  to  18 . In particular, machine  170  uses a carousel-type arrangement  172  that enables containers  12  to  18  to hang vertically. 
       FIG.  36    illustrates cassette  100   a  removed from machine  170 . Machine  170  defines slot  174  shown in  FIG.  36   , which enables cassette  100   a  to be inserted into machine  170 , as illustrated by  FIG.  37   . As illustrated, machine  170  employs GUI  160  described above in connection with  FIG.  35   .  FIGS.  35  to  37    illustrate that it is possible to configure the support of solution bags  12  to  18  in multiple ways. 
     Referring now to  FIGS.  38  to  41   , an alternative blood treatment machine  180  employs linear tubing pumps to move one or both the dialysate and blood instead of the pumps described above for such fluid transport. Indeed, it is possible to use any one of a multitude of different types of pumping technologies for either the dialysate flow path or the patient&#39;s blood circuit. For example, as shown in  FIG.  34   , peristaltic pumps, such as pump  48 , used earlier for the blood circuit can be used instead of the volumetric pumps  22  to  28  described above for the dialysate flow path. The peristaltic pumps, like pump  48 , are located mainly in the blood therapy machine and receive tubes outside the machine, similar to tube  102 , but which pump dialysate or therapy fluid. 
     Machine  180  of  FIG.  38    illustrates a similar type of alternative, which uses a series of adjacently placed round driver fingers  182  that run generally perpendicular to dialysate or therapy flow tubes, which are located within alternative cassette  190 . Linear fingers  182  compress dialysate tubes  184  sequentially in a manner similar to the rollers in a peristaltic pump to compress and move fluid within flexible dialysate tubes  184  of cassette  100   b  through such tubes and to the desired destination for the fluid. High flux dialyzers  20  and  30  connect to alternative cassette  100   b  as described above and in one embodiment extend from one side of machine  180  as illustrated. One or more motors  186  are provided to rotate cams that drive linear fingers  182  according to the prescribed sequence. 
     Referring now to  FIG.  39   , one embodiment of the linear tubing system is illustrated. Here, drain bag  12  and a plurality of solution bags  14 ,  16 ,  18  and  188  are supported by a tabletop  192 . Tubing connections, such as via tubes  194  and  196 , are made between the alternative cassette  100   b  and the bags  12  to  18  and  188 . Cassette  100   b  is positioned into a slot  198  defined by machine  180 . Machine  180  also includes GUI  160  described above. 
     Referring now to  FIGS.  40  and  41   , cassette  100   b  and an alternative cassette  100   c  illustrate schematically and respectively various embodiments for configuring the cassettes of the present invention to operate with linear tubing pumps. Cassettes  100   b  and  100   c  both operate with drain bag  12  and solution bags  14  to  18  and  188 . Both cassettes  100   b  and  100   c  include a number of sensors, such as blood leak detector  66 , a plurality of pressure sensors  46  and a plurality of air/water level sensors  68 . Both cassettes  100   b  and  100   c  operate with externally mounted high flux dialyzers  20  and  30  as discussed above. A restriction  40  is placed in the dialysate path between the arterial and venous dialyzers. 
     Cassettes  100   b  and  100   c  both include linear tubing portions  184  shown above in  FIG.  38   .  FIGS.  40  and  41    illustrate one advantage of the linear tubing pumps of the present invention, namely, that the driver fingers  182  associated with machine  180  are operable with linear tubing portions  184  of cassette  100   b / 100   c  for both the blood and dialysate flow paths, eliminating the need for having two types of pumping systems. 
     Cassette  100   c  of  FIG.  41    includes an additional linear tubing portion  184  that is connected fluidly with recirculation line  220 , which leads to an activated charcoal or sorbent cartridge  222 . Recirculation line  220  also extends from cartridge  222  into the dialysate input and of high flux dialyzer  30 . The flow of dialysate to venous dialyzer  20  and from arterial dialyzer  30  is monitored in connection with the linear tubing pumps in one embodiment via a flow measuring device that measures flow at the input line  202  into venous dialyzer  20 , which senses how much fresh dialysate is supplied from bags  14 ,  16 ,  18  and  188 . A flow measuring device also measures the flow leaving arterial dialyzer  30  via line  204  that leads via the leak detector  166  to drain bag  12 .  FIG.  41    shows a branch line  206  which selectively allows a portion of the spent dialysate or UF to be shunted via recirculation line  220  to charcoal or sorbent cartridge  222  and then back into arterial dialyzer  30 . 
     Inductive Heater 
     Referring now to  FIGS.  42  and  43   , two embodiments for the heater  58  of the present invention are illustrated by heaters  58   a  and  58   b , respectively. As discussed, heater  58  may be any suitable type of medical fluid heater such as a plate heater, infrared or other type of radiant heater, convective heater, or any combination thereof. Heater  58   a , is an inductive heater or heater with an inductive coil. Inductive heater  58   a  is configured integrally or connected fixedly to a disposable cassette, such as cassette  100 . Inductive heater  58   b , on the other hand, connects to the disposable cassette  100  via a pair of tubes and is located apart from the main body of cassette  100 . 
     As seen in  FIG.  42   , a portion of cassette  100  is shown. Cassette  100  defines fluid flow path  76  and fluid flow path  78 . In the illustrated embodiment, fluid flow path  76  is the inlet to inductive heater  58   a . Fluid flow path  78  is the outlet of fluid heater  58   a . That is, a fresh dialysate pump can pump fluid to flow path  76  and into a fluid chamber  74   a  defined by heater housing  72   a . The heated fluid then flows from fluid chamber  74   a  through flow channel  78  for example to a dialyzer or volumetric balancing device. 
     Regarding inline heater  58   b , fluid flows via a dialysate pump through a tube (not illustrated) connected sealingly to inlet port  82 . Fluid flows out of heater  58   b  to the disposable cassette through a tube (not illustrated) connected sealingly to outlet port  84  and a similar port located on the main body of the disposable cassette. 
     Heaters  58   a  and  58   b  each include a heating element or inductive coil  80 . Heater element  80  is inserted into each of the fluid flow channels  74   a  and  74   b . In an embodiment, heater element  80  is substantially cylindrical and when placed within the substantially cylindrical housings  72   a  and  72   b , respectively, creates an annular fluid flow path that flows longitudinally down the outside of heater element  80  and up the inside of heater element  80  before leaving heater  58   a  or  58   b . Heater elements  80  can be corrugated or otherwise have fin-like structures to increase the surface area of the heating element with respect to the fluid flowing through heaters  58   a  and  58   b.    
     In an embodiment, heater element  80  is a or acts as a shorted secondary coil of a transformer. The closed or looped element does not allow energy to dissipate electrically, instead is converted to heat. A transformer located in the machine includes a primary coil. The primary coil can be powered by an AC high frequency supplier. 
     The fluid heaters  58   a  and  58   b  incorporate one or more temperature sensors located so that the temperature of the liquid flowing through the heater can be monitored. The temperature sensors in one embodiment are infrared temperature sensors. Heater element  80  in an embodiment is made of a non-corrosive metal, such as stainless steel. 
     In operation, cold or room temperature dialysate is pumped into the induction heaters  58   a  or  58   b  along the outside of heater element  80 , around the bottom of heater element  80  and then along the inside of heater element  80 , finally exiting the heater. In an embodiment, the disposable cassette, such as cassette  100  is inserted such that the heating cavity defined by housing  72   a  is as positioned directly on the primary coil located within the renal therapy machine. When energized, the primary coil magnetically induces a current into the shorted coil  80 , causing the element  80  and surrounding fluid to heat. The primary coil serves a secondary purpose of centering and steadying the cassette within the renal failure therapy machine. 
     In one implementation, the surface area of the element  80  may be around or less than ten square inches to heat dialysate from five degrees Celsius to thirty-seven degrees Celsius at a flow rate of approximate 150 milliliters per minute. The heater may have a dimension of about 1 inch (25.4 mm) in diameter by 1.5 inches (38.1 mm). Other sizes, shapes and/or multiple coils  80  may be used alternatively. 
     Cassette with Balance Chambers 
     Referring now to  FIG.  44   , a portion of cassette  100  shown in cross-section illustrates one embodiment for providing a cassette-based balance chamber  340  of the present invention. Cassette  100  (including each of the cassettes  100   a  to  100   c ) includes an upper portion  96 , a lower portion  98  and a flexible sheeting  346 . In an embodiment, portions  96  and  98  are made of a suitable rigid plastic. In an embodiment, flexible membrane or diaphragm  346  is made of a suitable plastic or rubber material, such as PVC, non DEHP PVC, Krayton polypropylene mixture or similar materials. 
     The sheeting  346  is welded or bonded to one half  96  or  98 . Excess sheeting is trimmed. The two portions  96  and  98  are then bonded at a mating interface between the portions. This captures the sheeting  346  between portions  96  and  98 . Portions  96  and  98  are configured so that the welding of sheeting  346  is constrained between portions  96  and  98 . Portions  96  and  98  thereby sandwich the flexible membrane or diaphragm  346  of the cassette. 
     Using the same nomenclature from  FIGS.  17  to  21    for the inlet and outlet flow paths to balance chamber  340 , upper portion  96 , which receives and dispenses fresh dialysate, defines an inlet flow path  334  and an outlet fresh fluid flow path  314 . Likewise, lower portion  98 , which receives and dispenses effluent dialysate defines and inlet effluent path  336  and an outlet effluent  338 . Those fluid paths are in fluid communication with the like numbered fluid lines shown in  FIGS.  17  to  21   . 
     When balance chamber  340  is full of fresh fluid, a valve located upstream of the balance chamber and fresh fluid path  334  is closed. To push dialysate to the patient or dialyzer, a valve communicating with inlet effluent line  336  is opened as is a valve communicating with fresh dialysate delivery line  314 . That valve configuration enables pressurized effluent fluid to push membrane or diaphragm  346  away from the opening of effluent inlet  336  and towards the top of chamber  340 , thereby dispelling fresh dialysate within chamber  340  to a dialyzer or patient. 
     Balance chamber  340  may be oriented horizontally as shown or vertically. If vertically, the inlets are preferably located below the outlets to better enable air to escape from the fluid. Also, the ports may be combined to a single port for each chamber, similar to the alternative valve configuration of  FIG.  38    for the balance tube. The single ports may be located closer to or directly adjacent to the interface between portions  96  or  98  as desired. 
     In another embodiment (not illustrated) the portion of cassette  100  that provides a balance chamber does not include upper and lower rigid portions  96  and  98 . Instead that portion of cassette  100  includes three-ply or three separate flexible membranes. When the cassette is loaded into the renal failure therapy machine, the machine pulls a vacuum on the two outer membranes, causing the outer membranes to be sucked against the machine walls defining the balance chamber. This configuration reduces the amount of rigid plastic needed and is believed to be simpler and cheaper to produce. In an alternative configuration, the pressures in the balance chamber cavities push the sheeting to conform to the cavities, negating the need for a vacuum. The outer plies may have ports formed integrally with or connected sealingly to the plies to mate with inlet and outlet dialysate lines. 
     Balance Tube 
     Referring now to  FIG.  45   , one embodiment of the balance tube  360  is illustrated. As discussed above and using like nomenclature, balance tube  360  includes a separator  366 , which functions similar to the flexible membrane  346  of balance chamber  340 . In the illustrated embodiment, separator  366  is a ball or spherical object that moves snuggly within a cylindrical housing  382 . A pair of caps  384  and  386  are provided on either end of cylindrical housing  382 . Caps  384  and  386  seal to cylindrical tubing  382  via outer O-rings  388 . Separator or ball  366  seals to caps  384  and  386  via inner O-rings  392 . In an alternative embodiment, caps  384  and  386  are permanently or hermetically sealed to cylindrical tube  382 . Ports  394  and  396  are formed integrally with or are attached to caps  384  and  386 , respectively. Ports  394  and  396  seal to mating tubes via any mechanism known to those with skill in the art. 
     In an embodiment, cylindrical tube  382  is translucent or transparent, so that an optical sensor can detect if ball or separator  366  has properly reached the end of travel. Ultrasonic or other types of sensors may be used alternatively. The assembly could be made of two pieces of injection molded plastic that mate in the center of the tubes with the separator  366  installed prior to mating. Mating may be done by solvent bond, ultrasound or other techniques known to one of skill in the art. Tube  382  may also be a simple extrusion with molded end caps applied by a secondary operation. 
     Ball or separator  366  is sized to fit snuggly but smoothly within the interior of cylinder  382 . A small amount of mixing between fresh and effluent fluid may occur without substantially affecting the performance of the system. In an alternative embodiment, a cylindrical piston type separator is provided. In either case, separator  366  may have additional sealing apparatus, such as wipers or deformable flanges that help to enhance the sliding or rolling seal as the case may be. 
     Each of the components shown in  FIG.  45    for balance tube  360  may be made of plastic or other suitable material. In an embodiment, balance tube  360  is a disposable item, which may be formed integrally with cassette  100  or attached to the cassette via tubing, similar to heaters  58   a  and  58   b  of  FIGS.  42  and  43   . It is important to note that the O-rings and fittings are not be necessary if injection molded caps or assemblies are used. In addition, sensors such as ultrasonic or optical sensors, for the positioning of the separator can eliminate a need for sealing at the end of the tube. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.