Patent Publication Number: US-2021178046-A1

Title: Dialysis system having carbon dioxide generation and prime

Description:
BACKGROUND 
     The present disclosure relates generally to medical delivery. More specifically, the present disclosure relates to the priming of extracorporeal therapy machines. 
     Due to various causes, a person&#39;s renal system may fail. Renal failure produces several physiological derangements. It is no longer possible to balance water and minerals or to excrete daily metabolic load. Toxic end products of metabolism (urea, creatinine, uric acid, and others) may accumulate in blood and tissue. 
     Kidney failure and reduced kidney function have been treated with dialysis. Dialysis removes waste, toxins and excess water from the body that normal functioning kidneys would otherwise remove. Dialysis treatment for replacement of kidney functions is critical to many people because the treatment is life saving. 
     One type of kidney failure therapy is Hemodialysis (“HD”), which in general uses diffusion to remove waste products from a patient&#39;s blood. A diffusive gradient occurs across the semi-permeable dialyzer between the blood and an electrolyte solution called dialysate or dialysis fluid to cause diffusion. 
     Hemofiltration (“HF”) is an alternative renal replacement therapy that relies on a convective transport of toxins from the patient&#39;s blood. HF is accomplished by adding substitution or replacement fluid to the extracorporeal circuit during treatment (typically ten to ninety liters of such fluid). The 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 sufficient convective clearance). 
     Hemodiafiltration (“HDF”) is a treatment modality that combines convective and diffusive clearances. HDF uses dialysis fluid flowing through a dialyzer, similar to standard hemodialysis, to provide diffusive clearance. In addition, substitution solution is provided directly to the extracorporeal circuit, making increased ultrafiltration possible, which in turn yields convective clearance. 
     Most HD (HF, HDF) treatments occur in centers. A trend towards home hemodialysis (“HHD”) exists today in part because HHD may be performed daily, offering therapeutic benefits over in-center hemodialysis treatments, which occur typically bi- or tri-weekly. Studies have shown that more frequent treatments remove more toxins and waste products than treatments occurring less frequently, but lasting longer. A patient receiving more frequent treatments does not experience as much of a down cycle as does an in-center patient, who has built-up two or three day&#39;s worth of toxins prior to a treatment. In certain areas, the closest dialysis center may be many miles from the patient&#39;s home causing door-to-door treatment time to consume a large portion of the day. HHD may take place overnight or during the day while the patient relaxes, works or is otherwise productive. 
     In any of the above modalities, it is necessary to prime the associated equipment prior to running a treatment on a patient. Priming typically involves displacing air with a physiologically compatible solution such as dialysis fluid or saline. Air may however be difficult to remove from certain areas of a blood circuit, such as within the membranes of a dialyzer. Pockets of air may remain even after being flushed with a solution. 
     An improved priming system and method is needed accordingly. 
     SUMMARY 
     The examples described herein disclose automated systems and methods applicable, for example, to fluid delivery for: plasmapherisis, hemodialysis (“HD”), hemofiltration (“HF”) hemodiafiltration (“HDF”), continuous renal replacement therapy (“CRRT”), apheresis, autotransfusion, hemofiltration for sepsis, and extracorporeal membrane oxygenation (“ECMO”) therapies (extracorporeal therapies). Any of the extracorporeal therapies may require that the blood circuit including a blood filter (dialyzer, hemofilter, etc.) be primed prior to being connected to the patient. The present disclosure sets forth systems and methods that use carbon dioxide (“CO 2 ”) gas to initially prime the blood circuit. CO 2  gas is better able than a priming fluid to dislodge air trapped in the membranes, and indeed within the pores of the membranes, of a blood filter. CO 2  gas may also be better at flushing air out of pockets located within other areas of the blood circuit, e.g., pockets created by the housing of a drip chamber of the blood circuit, or pockets created by the sealing of connectors to tubes (or the connectors themselves), or tubes to tubes, within the blood circuit. In short, CO 2  gas is better able to enter and flush small open spaces than is typical priming fluid. 
     There are two primary embodiments for performing the CO 2  gas prime of the present disclosure. In one primary embodiment, CO 2  gas is created by the system itself but outside of the extracorporeal or blood circuit. Here, CO 2  gas is transported into the blood circuit or set. In a second primary embodiment, CO 2  gas is carried by the fluid creating the CO 2  gas into the extracorporeal or blood circuit and released. 
     In the first primary embodiment, a CO 2  gas chamber may be provided, which receives the constituent fluids that mix together to form CO 2  gas and then stores at least a portion of the CO 2  gas. The constituent fluids that mix together to form CO 2  gas may include bicarbonate (e.g., from B-concentrate) and any one of: (i) a citric acid solution, (ii) an acetic acid solution (e.g., from A-concentrate), (iii) a lactic acid solution, (iv) a malic acid solution, (v) a hydrochloric (“HCl”) acid solution, (vi) a phosphoric acid solution, (vii) any other biocompatible acid solution (e.g., an acid whose anion is present in a human body), and (v) blends thereof. The system in an embodiment uses A-concentrate and B-concentrate to make a dialysis fluid for treatment. If using A-concentrate and B-concentrate to also make the CO 2  gas generation fluid, then no additional source of acid is needed, however, the CO 2  gas generation fluid made from A-concentrate and B-concentrate will be formulated differently, e.g., have a different or lower pH, than the treatment fluid made from A-concentrate and B-concentrate. 
     If using citric acid solution, an acetic acid solution or any other acid solution other than A-concentrate, then a separate source of the additional acid is provided, then the CO 2  gas generation fluid will have a different constituent makeup than the dialysis treatment fluid. In any case, an accurate metering pump may be provided to pump, in any order, bicarbonate solution in a desired amount and any of the acid solutions in a desired amount into the gas chamber. The bicarbonate and acid mix to form CO 2  gas over the course of a short period of time. 
     A CO 2  gas line is provided in one embodiment to run from an upper portion of the CO 2  gas chamber to a desired gas injection location. The CO 2  gas line may be opened or closed during the formation of CO 2  gas (which happens very quickly, on the order of seconds) depending upon how much pressure the CO 2  gas line is rated to hold. The desired injection location may, for example, be the fresh dialysis fluid line leading to a port of the blood filter, be a substitution fluid line leading to the arterial or venous blood line, or be a line leading to a port of the machine for receiving the connected blood lines for priming. In any case, the desired injection location is positioned and arranged to deliver the CO 2  gas into the blood circuit. 
     Under certain circumstances, it may be required to lower the pressure downstream from the CO 2  gas chamber to help draw the CO 2  gas from the CO 2  priming fluid. The underpressure may be created for example by operating the blood pump in reverse to place the dialyzer and the CO 2  gas line leading to the dialyzer under a negative pressure. The blood pump may also be operated (e.g., multiple times in normal and reverse directions) during the CO 2  gas prime to help distribute the CO 2  gas to all portions of the blood circuit or set. The CO 2  gas during the CO 2  gas prime pushes air in one embodiment out of a vent provided for example with a venous air trap of the blood set. As shown below, an alternative to an underpressure, or perhaps in addition to the underpressure, the CO 2  priming fluid may be mixed or stirred to release the CO 2  gas. 
     In addition to, or alternatively to, the use of an underpressure to release CO 2  gas from the CO 2  gas generation fluid, the metering pump may also operate with a recirculation line that recirculates the CO 2  gas generation fluid to thereby stir and agitate the CO 2  gas generation fluid. Stirring and agitating the CO 2  gas generation fluid helps to release the CO 2  gas from the fluid. It is contemplated to stir and agitate the CO 2  priming fluid at a location elevationally below a location where the CO 2  gas is collected and then delivered to the blood set. 
     When the CO 2  gas prime has been completed, the system and method of the first primary embodiment may then perform an additional prime of the blood set using a priming fluid such as saline or dialysis fluid prepared for treatment. The priming fluid such as saline or dialysis fluid prepared for treatment may absorb the CO 2  gas and/or expel the gas from the blood circuit, e.g., via the vent of the venous air trap. The dual stage prime, in addition to the CO 2  gas prime, results in superior air removal. 
     The second primary embodiment, in which CO 2  gas is carried by the CO 2  gas generation fluid into the blood set where it is released, does not need a CO 2  gas chamber or separate metering pump. The CO 2  gas generation fluid may again be formed via a bicarbonate solution and any of the acid solutions discussed herein, e.g., (i) a citric acid solution, (ii) an acetic acid solution (e.g., from A-concentrate), (iii) a lactic acid solution, (iv) a malic acid solution, (v) a hydrochloric (“HCl”) acid solution, (vi) a phosphoric acid solution, (vii) any other biocompatible acid solution, and (v) blends thereof. The CO 2  gas generation fluid may be delivered to the blood circuit via any of the entry locations discussed herein, e.g., at the dialyzer port, substitution port, or arterial and venous line connection port. 
     An underpressure may again be needed to separate the CO 2  gas from the CO 2  gas generation fluid. The underpressure may be created by running different pumps at different speeds and/or by subjecting a portion of a blood line for which CO 2  gas contact is desired to a negative or intake side of a pump, e.g., a peristaltic blood pump. The blood pump may again be run in normal and reverse directions multiple times to ensure that the CO 2  gas is separated effectively from the CO 2  gas generation fluid and distributed effectively to all portions of the blood set. 
     When the CO 2  gas prime of the second primary embodiment has been completed, the system and method of the present disclosure again perform an additional prime of the blood set using a priming fluid such as saline or dialysis fluid prepared for treatment. The priming fluid such as saline or dialysis fluid prepared for treatment may absorb the CO 2  gas and/or expel the gas from the blood circuit, e.g., via the vent of the venous air trap. 
     In light of the disclosure herein and without limiting the disclosure in any way, in a first aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, an extracorporeal therapy system includes: a dialysis fluid circuit including dialysis fluid preparation structure configured to prepare a dialysis fluid for an extracorporeal therapy treatment; a blood circuit including a blood filter for use during the extracorporeal therapy treatment; a blood pump operable to pump blood through the blood circuit and blood filter; and a control unit operable with the dialysis fluid preparation structure and the blood pump, the control unit programmed to prepare a gas generation fluid different than the dialysis fluid for the extracorporeal therapy treatment, wherein the gas generation fluid generates carbon dioxide (“CO 2 ”) gas, and wherein the CO 2  gas is used to prime the blood circuit including the blood filter. 
     In a second aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the control unit is further programmed to absorb and/or flush the CO 2  gas from the blood circuit using a priming fluid, such as the dialysis fluid for the extracorporeal therapy treatment or saline, thereby further priming the blood circuit. 
     In a third aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the CO 2  gas is generated in the dialysis fluid circuit and delivered from the dialysis fluid circuit to the blood circuit. 
     In a fourth aspect of the present disclosure, which may be combined with the third aspect in combination with any other aspect listed herein unless specified otherwise, the CO 2  gas is thereafter absorbed within and/or removed from the blood circuit using a priming fluid, such as the dialysis fluid for the extracorporeal therapy treatment or saline. 
     In a fifth aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the CO 2  gas is carried by the gas generation fluid into the blood circuit. 
     In a sixth aspect of the present disclosure, which may be combined with the fifth aspect in combination with any other aspect listed herein unless specified otherwise, the system is configured to lower the pressure of the gas generation fluid carrying the CO 2  gas to cause the CO 2  gas to be released. 
     In a seventh aspect of the present disclosure, which may be combined with the fifth aspect in combination with any other aspect listed herein unless specified otherwise, the CO 2  gas and remaining fluid is thereafter absorbed within and/or removed from the blood circuit using a priming fluid, such as the dialysis fluid for the extracorporeal therapy treatment or saline. 
     In an eighth aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the gas generation fluid has a different pH than the dialysis fluid for the extracorporeal therapy treatment. 
     In a ninth aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the blood circuit includes a vented chamber enabling at least one of (i) the CO 2  gas to push air out of a vent of the vented chamber or (ii) a priming fluid, such as the dialysis fluid for the extracorporeal therapy treatment or saline, to push CO 2  gas out of the vent. 
     In a tenth aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the extracorporeal therapy system includes a CO 2  gas chamber in fluid communication with the dialysis fluid preparation structure, the CO 2  gas chamber sized and arranged to mix the CO 2  gas generating priming fluid, causing CO 2  gas to be generated. 
     In an eleventh aspect of the present disclosure, which may be combined with the tenth aspect in combination with any other aspect listed herein unless specified otherwise, the system includes a metering pump positioned and arranged to pump bicarbonate and (i) a citric acid solution, (ii) an acetic acid solution, (iii) a lactic acid solution, (iv) a malic acid solution, (v) a hydrochloric (“HCl”) acid solution, (vi) a phosphoric acid solution, (vii) any other biocompatible acid solution, or (v) blends thereof. 
     In a twelfth aspect of the present disclosure, which may be combined with the tenth aspect in combination with any other aspect listed herein unless specified otherwise, the extracorporeal therapy system includes a CO 2  gas line extending from the CO 2  gas chamber to a location where the CO 2  gas may be introduced into the blood circuit. 
     In a thirteenth aspect of the present disclosure, which may be combined with the twelfth aspect in combination with any other aspect listed herein unless specified otherwise, the location includes a line in fluid communication with a port of the blood filter or a line in fluid communication with a blood line extending from the blood filter. 
     In a fourteenth aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the extracorporeal therapy system includes a source of acid solution placed in fluid communication with the dialysis fluid preparation structure. 
     In a fifteenth aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, at least one of (i) a replacement fluid pump is used to move the gas generation fluid while priming the blood circuit or (ii) the blood pump is operated in at least one direction while priming the blood circuit with CO 2  gas. 
     In a sixteenth aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, an extracorporeal therapy system includes: a dialysis fluid circuit including dialysis fluid preparation structure configured to prepare a dialysis fluid for an extracorporeal therapy treatment; a carbon dioxide (“CO 2 ”) gas chamber in fluid communication with the dialysis fluid preparation structure; a blood circuit including a blood filter for use during the extracorporeal therapy treatment; and a control unit operable with the dialysis fluid preparation structure, the control unit programmed to prepare in the CO 2  gas chamber a gas generation fluid different than the dialysis fluid for the extracorporeal therapy treatment, wherein the gas generation fluid generates CO 2  gas in the CO 2  gas chamber, and wherein the CO 2  gas is used to prime the blood circuit including the blood filter. 
     In a seventeenth aspect of the present disclosure, which may be combined with the sixteenth aspect in combination with any other aspect listed herein unless specified otherwise, the extracorporeal therapy system includes a recirculation line in fluid communication with the CO 2  gas chamber to agitate the gas generation fluid to help release the CO 2  gas. 
     In an eighteenth aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, an extracorporeal therapy method includes: storing in a computer memory a first formula or setpoint for preparing a dialysis fluid for an extracorporeal therapy treatment; storing in the computer memory a second, different formula or setpoint for preparing a gas generation fluid for carbon dioxide (“CO 2 ”) gas priming; providing fluid preparation structure to prepare the gas generation fluid according to the second formula or setpoint; allowing CO 2  gas to be generated from the gas generation fluid; and priming a blood circuit with the CO 2  gas. 
     In a nineteenth aspect of the present disclosure, which may be combined with the eighteenth aspect in combination with any other aspect listed herein unless specified otherwise, priming the blood circuit with the CO 2  gas includes enabling the CO 2  gas to enter the blood circuit. 
     In a twentieth aspect of the present disclosure, which may be combined with the eighteenth aspect in combination with any other aspect listed herein unless specified otherwise, allowing the CO 2  gas to be generated and priming the blood circuit with the CO 2  gas includes enabling the CO 2  gas to be released from the gas generation fluid after entering the blood circuit. 
     In a twenty-first aspect of the present disclosure, which may be combined with the eighteenth aspect in combination with any other aspect listed herein unless specified otherwise, the fluid preparation structure includes a source of bicarbonate, an A-concentrate source, and at least one pump for mixing the bicarbonate and the A-concentrate with purified water. 
     In a twenty-second aspect of the present disclosure, which may be combined with the eighteenth aspect in combination with any other aspect listed herein unless specified otherwise, the second, different formula or setpoint for preparing the gas generation fluid involves (i) a mixing ratio between bicarbonate and a solution containing citric acid, (ii) a mixing ratio between bicarbonate and a solution containing acetic acid, (iii) a mixing ratio between bicarbonate and a solution containing lactic acid, (iv) a mixing ratio between bicarbonate and a solution containing malic acid, (v) a mixing ratio between bicarbonate and a solution containing hydrochloric (“HCl”) acid, (vi) a mixing ratio between bicarbonate and a solution containing phosphoric acid, (vii) a mixing ratio between bicarbonate and any other biocompatible acid solution, or (v) blends thereof. 
     In a twenty-third aspect of the present disclosure, which may be combined with the eighteenth aspect in combination with any other aspect listed herein unless specified otherwise, the second, different formula or setpoint for preparing the gas generation fluid includes a different constituency or a different make-up of a same constituency as the dialysis fluid for the extracorporeal therapy treatment. 
     In a twenty-fourth aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, a CO 2  gas chamber is provided separate from the blood filter. 
     In a twenty-fifth aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the dialysis fluid and the gas generation fluid are formed from at least one common concentrate. 
     In a twenty-sixth aspect of the present disclosure, any of the structure and functionality disclosed in connection with  FIGS. 1 to 4B  may be combined with any of the other structure and functionality disclosed in connection with  FIGS. 1 to 4B . 
     In light of the present disclosure and the above aspects, it is therefore an advantage of the present disclosure to provide an extracorporeal therapy system and method having improved priming. 
     It is another advantage of the present disclosure to provide an extracorporeal therapy system and method capable of generating its own CO 2  gas for priming. 
     It is a further advantage of the present disclosure to provide a relatively inexpensive system and method to improve priming. 
     It is still another advantage of the present disclosure to provide a system and method to improve priming that is physiologically compatible. 
     It is still a further advantage of the present disclosure to provide a system and method to improve priming that is time efficient. 
     It is yet another advantage of the present disclosure to provide a system and method to improve priming that requires little or no additional setup. 
     Still another advantage of the present disclosure is to reduce the amount of typical priming fluid, e.g., saline, which reduces cost and fluid priming time. 
     The advantages discussed herein may be found in one, or some, and perhaps not all of the embodiments disclosed herein. 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. 1A  is a schematic illustration of one system and method of the present disclosure of the present disclosure employing CO 2  gas generation and priming of the present disclosure. 
         FIG. 1B  is a schematic illustration of an alternative version of the system of  FIG. 1A  and includes structure for stirring or agitating the CO 2  priming fluid to release CO 2  gas therefrom. 
         FIG. 1C  is a schematic illustration of an alternative version for a blood circuit or set that may be used with the system of  FIGS. 1A and 1B  and includes a pressure measurement line in the blood set for receiving air displaced by CO 2  gas. 
         FIG. 1D  is a schematic illustration of the system of  FIG. 1C  showing a priming fluid sequence to purge and absorb CO 2  gas from the blood set before treatment. 
         FIGS. 2A to 2D  are schematic illustrations of another system and method of the present disclosure, employing one embodiment for CO 2  gas generation and priming of the present disclosure, which releases CO 2  gas from a CO 2  gas generation fluid within the blood circuit. 
         FIGS. 3A to 3D  are schematic illustrations of a further system and method of the present disclosure, employing another embodiment for CO 2  gas generation and priming of the present disclosure, which releases CO 2  gas from a CO 2  gas generation fluid within the blood circuit. 
         FIGS. 4A and 4B  are schematic illustrations of still another system and method of the present disclosure, employing another embodiment for CO 2  gas generation and priming of the present disclosure, which releases CO 2  gas from a CO 2  gas generation fluid within the blood circuit. 
     
    
    
     DETAILED DESCRIPTION 
     The examples described herein are applicable to any medical fluid therapy system that delivers a medical fluid, such as blood, dialysis fluid, substitution fluid, purified or sterilized water, liquid concentrate, or an intravenous drug. The examples are particularly well suited for kidney failure therapies, such as all forms of hemodialysis (“HD”), plasmapherisis, hemofiltration (“HF”) hemodiafiltration (“HDF”), continuous renal replacement therapy (“CRRT”), apheresis, autotransfusion, hemofiltration for sepsis, and extracorporeal membrane oxygenation (“ECMO”) treatments, referred to herein collectively or generally individually as an extracorporeal therapy. Moreover, the systems and methods described herein may be used in clinical or home settings. Extracorporeal therapy system  10   a  in the examples below is described as a renal failure therapy system having a machine  12  that creates online dialysis fluid for treatment. 
     Referring now to  FIG. 1A , one embodiment for a renal failure therapy system  10   a  employing an improved priming method or procedure of the present disclosure is illustrated. System  10   a  includes a machine  12  having an enclosure or housing. The housing of machine  12  holds the contents of a dialysis fluid circuit  30  described in detail below. The housing of machine  12  also supports a user interface  14 , which allows a nurse or other operator to interact with system  10   a . User interface  14  may have a monitor screen operable with a touch screen overlay, electromechanical buttons, e.g., membrane switches, or a combination of both. User interface  14  is in electrical communication with at least one processor  16  and at least one memory  18 . At least one processor  16  and at least one memory  18  also electronically interact with, and where appropriate control, the pumps, valves and sensors described herein, e.g., those of dialysis fluid circuit  30 . At least one processor  16  and at least one memory  18  are referred to collectively herein as a control unit  20 . The dashed lines extending from control unit  20  are electrical or signal lines leading to and/or from pumps, valves, sensors, the heater and other electrical equipment of system  10   a.    
     Dialysis fluid circuit  30  includes a purified water line  32 , an A-concentrate line  34  and a bicarbonate B-concentrate line  36 . Purified water line  32  receives purified water from a purified water device or source  22 . The water may be purified using any one or more process, such as, reverse osmosis, carbon filtering, ultraviolet radiation, electrodeionization (“EDI”), and/or ultrafiltering. 
     An A-concentrate pump  38 , such as a peristaltic or piston pump, pumps A-concentrate from an A-concentrate source  24  to mix with purified water from purified water line  32  via A-concentrate line  34 . Conductivity cell  40  measures the conductive effect of the A-concentrate on the purified water, sends a signal to control unit  20 , which uses the signal to properly proportion the A-concentrate by controlling A-concentrate pump  38 . The A-conductivity signal may be temperature compensated via a reading from temperature sensor  42 . 
     A B-concentrate pump  44 , such as a peristaltic or piston pump, in the illustrated embodiment pumps purified water from purified water line  32 , through B-concentrate line  36  and a B-concentrate source or bicarbonate cartridge  26  located along B-concentrate line  36 , into a mixture of purified water and A-concentrate leaving conductivity sensor  40 . Conductivity cell  46  measures the conductive effect of the B-concentrate on the purified water/A-concentrate mixture, sends a signal to control unit  20 , which uses the signal to properly proportion the B-concentrate by controlling B-concentrate pump  44 . The B-conductivity signal may also be temperature compensated via a reading from temperature sensor  48 . 
     Purified water is degassed prior to receiving the concentrates, removing bubbles from the water. The water is degassed in a chamber  50  via a degassing pump  51 , placed in fluid communication with degassing chamber  50 . A heater  52  controlled by control unit  20  heats the purified water for treatment to body temperature, e.g., 37° C. The fluid exiting conductivity cell  46  is therefore freshly prepared dialysis fluid, properly degassed and heated, and suitable for sending to dialyzer  102  for treatment. Fresh dialysis fluid for a renal therapy treatment, such as hemodialysis, hemofiltration or hemodiafiltration, may be said to be made from a formula or at least one setpoint, e.g., conductivity setpoints for sensors  40  and  46 , which is (are) specified according to a physiological constituency of the dialysis fluid that is suitable for treatment, e.g., suitable for the patient. The treatment formula or at least one setpoint may be different than ones described below to create dialysis fluid for priming, referred to herein as a priming fluid, wherein that dialysis fluid is formulated or regulated to create carbon dioxide (“CO 2 ”) gas. 
     It is worth noting that priming blood set  100  is most often done using a priming fluid, which may be hung on a stand, and which is normally a saline solution. When priming using an online fluid (dialysis machine  12  itself produces the priming fluid), there are several options depending upon the type of machine  12 . For example, the priming fluid may be dialysis fluid prepared for treatment, an A-concentrate based solution (possibly not the best alternative) or a sodium chloride fluid if machine  12  is able to separate a source of sodium from its other concentrates. Each of the examples listed above, including saline, is different than the CO 2  producing priming solutions of the present disclosure. 
       FIG. 1A  further illustrates a fresh dialysis fluid pump  54 , such as a gear pump, which delivers fresh dialysis fluid for treatment to dialyzer  102 . Control unit  20  controls fresh dialysis fluid pump  54  to deliver fresh dialysis fluid to the dialyzer at a specified flowrate. A used dialysis fluid line  56  via a used dialysis fluid pump  58  returns used dialysis fluid from dialyzer  102  to a drain  60 . Control unit  20  controls used dialysis fluid pump  58  to pull used dialysis fluid from dialyzer  102  at a specified flowrate. An air separator  62  separates air from the used dialysis fluid line  56 . A pressure sensor  64  senses the pressure of used dialysis fluid line  56  and sends a corresponding pressure signal to control unit  20 . 
     Conductivity cell  66  measures the conductivity of used fluid flowing through used dialysis fluid line  56  and sends a signal to control unit  20 . The conductivity signal of cell  66  may also be temperature compensated via a reading from temperature sensor  68 . A blood leak detector (not illustrated), such as an optical detector, looks for the presence of blood in used dialysis fluid line  56 , e.g., to detect if a dialyzer membrane has a tear or leak. A heat exchanger  72  in the illustrated embodiment recoups heat from the used dialysis fluid exiting dialysis fluid circuit  30  to drain  60 , preheating the purified water traveling towards heater  52  to conserve energy. 
     A fluid bypass line  74  allows fresh dialysis fluid to flow from fresh dialysis fluid line  76  to used dialysis fluid line  56  without contacting dialyzer  102 . A fresh dialysis fluid tube  78  extends from machine  12  and carries fresh dialysis fluid from fresh dialysis fluid line  76  to dialyzer  102 . A used dialysis fluid tube  80  also extends from machine  12  and carries used dialysis fluid from dialyzer  102  to used dialysis fluid line  56 . 
     Dialysis circuit  30  also includes an ultrafiltration (“UF”) system  70 . UF system  70  monitors the flowrate of fresh dialysis fluid flowing to dialyzer  102  (and/or as substitution fluid flowing directly to the blood set  100 ) and used fluid flowing from dialyzer  102 . UF system  70  may include fresh and used flow sensors, which send signals to control unit  20  indicative of the fresh and used dialysis fluid flowrates, respectively. Control unit  20  uses the signals to set used dialysis fluid pump  58  to pump faster than fresh dialysis fluid pump  54  by a predetermined amount to remove a prescribed amount of UF from the patient over the course of treatment. 
     An ultrafilter (not illustrated) may also be provided to further purify the fresh dialysis fluid before being delivered via dialysis fluid line  76  and fresh dialysis fluid tube  78  to dialyzer  102 . Alternatively or additionally, one or more ultrafilter may be used to purify the fresh dialysis fluid from fresh dialysis fluid line  76  to the point where the fluid may be used as a substitution fluid to perform from pre- or post-dilution hemofiltration or hemodiafiltration. 
     System  10   a  provides plural valves  92 , e.g., solenoid valves, each under the control of control unit  20  to selectively control a prescribed treatment. In particular, valve  92  in bypass line  74  selectively opens and closes the bypass line, e.g., to work with valve  92  of fresh dialysis fluid line  76  to set machine  12  into a bypass mode when, for example, the prepared dialysis fluid is incorrect in any way (composition, temperature). Bypass line valve  92  is accordingly used mainly for safety reasons. Valve  92  in fresh dialysis fluid line  76  selectively opens and closes the fresh dialysis fluid line. Valve  92  in used dialysis fluid line  56  selectively opens and closes the used dialysis fluid line. Valves  92  and  92   w  are located in purified water line  32  to selectively open and close the purified water line to purified water source  22 , to deaerate the purified water, and to deliver the purified water for mixing with the concentrates. Multiple valves  92  are also provided to operate UF system  70 . 
     Valves  92   b   1  and  92   b   2  in the illustrated embodiment may be used in a priming sequence for bicarbonate cartridge  26 . In the illustrated embodiment, bicarbonate cartridge  26  holds a powder concentrate that is contacted with purified water, dissolving the bicarbonate powder to form a saturated concentrate. In one possible priming sequence, control unit  20  causes valves  92   b   1  and  92   w  to close and valve  92   b   2  to open. Fresh dialysis fluid pump  54  is operated, creating a sub-atmospheric pressure in the cartridge. Control unit  20  then opens valve  92   b   1  to introduce purified water. Cartridge  26  is primed and when the conductivity cell  46  senses a rise in conductivity, letting control unit  20  know that the priming sequence is finished and to close valve  92   b   2 . 
     It should be appreciated that the dialysis fluid circuit  30  is simplified and may include other structure (e.g., more valves) and functionality not illustrated. Also, dialysis fluid circuit  30  illustrates one example of a hemodialysis (“HD”) pathway. It is contemplated to provide one or more ultrafilter (not illustrated) in fresh dialysis fluid line  76  to create substitution fluid for hemofiltration (“HF”). It is also contemplated to provide one or more ultrafilter in one or more line(s) branching off of fresh dialysis fluid line  76  to create substitution fluid, in addition to the fresh dialysis fluid in line  76 , for hemofiltration (“HF”) or hemodiafiltration (“HDF”). 
       FIG. 1A  further illustrates one embodiment of a blood circuit or set  100  that may be used with machine  12  of system  10   a . Blood circuit or set  100  includes a dialyzer  102  having many hollow fiber semi-permeable membranes, which separate dialyzer  102  into a blood compartment and a dialysis fluid compartment. The dialysis fluid compartment during treatment is placed in fluid communication with a distal end of fresh dialysis fluid tube  78  and a distal end of used dialysis fluid tube  80 . For HF and HDF, a separate substitution tube, in addition to fresh dialysis fluid tube  78 , is placed during treatment in fluid communication with one or both of arterial line  104  extending from an arterial access needle or cannula (not illustrated) or venous line  106  extending to a venous access needle or cannula (not illustrated). For HDF, dialysis fluid also flows through dialysis fluid tube  78  to dialyzer  102 , while for HF, dialysis fluid flow through tube  78  is blocked. 
     An arterial pressure pod  108   a  may be placed upstream of blood pump  112  (such as a peristaltic or volumetric membrane pump), while venous line  106  includes a pressure pod  108   v . Pressure pods  108   a  and  108   v  operate with blood pressure sensors (not illustrated) mounted on the housing of machine  12 , which send arterial and venous pressure signals, respectively, to control unit  20 . Venous line  106  includes a venous drip chamber  114 , which removes air from the patient&#39;s blood before the blood is returned to the patient. Venous chamber  114  may be provided with a hydrophobic or hydrophilic vent  116 . 
     In the illustrated embodiment, arterial line  104  of blood circuit or set  100  is operated by blood pump  112 , which is under the control of control unit  20  to pump blood at a desired flowrate. System  10   a  also provides multiple blood side electronic devices that send signals to and/or receive commands from control unit  20 . For example, control unit  20  commands pinch clamps ( FIGS. 2A to 2D  and clamp  140  in  FIGS. 3A to 3D ) to selectively open or close arterial line  104  and/or venous line  106 . A blood volume sensor (“BVS”, not illustrated) may be located along arterial line  104  upstream of blood pump  112 . An air detector (not illustrated) may be provided to look for air in venous blood line  106 . 
       FIG. 1A  further illustrates structure for enabling CO 2  gas to be generated in dialysis fluid circuit  30  and to be delivered to blood circuit  100  for priming purposes. In particular, dialysis fluid circuit  30  shows a bicarbonate priming line  136 , running to a first three-way solenoid valve  192   a  under control of control unit  20 . A metering pump  82  under control of control unit  20 , such as a volumetric or membrane pump, or perhaps an accurately controlled peristaltic pump or gear pump is located between first three-way solenoid valve  192   a  and a second three-way solenoid valve  192   b  under control of control unit  20 . 
     The third leg of first three-way solenoid valve  192   a  is connected fluidly to a citric acid line  128  leading to a citric acid source  28 . Citric acid for source  28  may for example be provided in a concentration of 20% to 60% by volume and in one preferred embodiment 50% by volume. The upper citric acid limit is affected by the need to avoid the formation of precipitation. An upper limit of 50% to 60% should avoid such formation. To avoid shipping water and to avoid replacing the concentrate as much as possible, the concentration should be as high as possible. Lower citric acid concentrations are otherwise theoretically useable as long as the citric acid level is not so low that bacteria is allowed to form. 
     The second and third legs of second three-way solenoid valve  192   b  in the illustrated embodiment are connected fluidly to a gas generation chamber  90  and a return line  84 . Gas generation chamber  90  may be made of metal or plastic as desired and includes an inlet for receiving fluids from metering pump  82  and an outlet in fluid communication with a CO 2  gas line  86 , which is connected fluidly to fresh dialysis fluid line  76 . Return line  84  returns fluids used to make CO 2  gas to a point upstream of dialysis fluid pump  54 , e.g., to bicarbonate B-concentrate line  36 . Return line  84  allows control unit  20  to verify via the conductivity sensors that bicarbonate concentrate is present at valve  192   b  before opening the valve to fill gas generation chamber  90 . Without such verification, the system may instead first fill gas generation chamber  90  with purified water, possibly causing an incorrect amount of concentrate to be delivered to the chamber. Return line  84  helps to ensure that bicarbonate priming line  136  is primed with bicarbonate fluid prior to infusing fluid into gas generation chamber  90 . 
     Degassing pump  51  under control of control unit  20  is in one embodiment used mainly to create low pressure in a degassing loop that includes heater  52  and degassing chamber  50  to degas the water for priming and treatment. To make CO 2  gas generation fluid in an embodiment, system  10   a  may be currently producing dialysis fluid for treatment, however, it is not a necessity that system  10   a  is doing so. Bicarbonate cartridge  26  should be primed in any case however. As illustrated, return line  84  is positioned and arranged such the control unit  20  advantageously does not have to stop B-concentrate pump  44  to provide bicarbonate solution for CO 2  gas generation. Control unit  20  causes metering pump  82  to pump through or from bicarbonate cartridge  26  (dry or liquid source) to valve  192   b  and further into return line  84 . When the bicarbonate solution reaches a point just ahead of valve  92   w , the bicarbonate solution may begin to enter the main fresh dialysis fluid line  76 , so that the bicarbonate solution is sensed by conductivity sensors  40  and  46  feeding corresponding signals to control unit  20 . 
     Control unit  20  then causes valve  192   b  to switch, so that the bicarbonate solution enters gas generation chamber  90 . Control unit  20  may cause A- and B-concentrate pumps  38  and  44  to run at fixed, e.g., slower, operating speeds, so that concentrate pumps  38  and  44  continue to prepare dialysis fluid for treatment (assuming dialysis fluid is being prepared), while preparing B-concentrate for CO 2  gas generation. In an embodiment, the dialysis fluid for treatment is used at this time (prior to treatment) to prime fresh dialysis fluid line  76  and used dialysis fluid line  56 . 
     When a desired amount of bicarbonate solution has been added to gas generation chamber  90 , the acid step is started, e.g., adding either citric acid from citric acid source  28 , A-concentrate from A-concentrate source  24  or some other acid containing fluid. Dependent on which acid fluid is used, the acid pumping sequence may vary. Using citric acid from source  28  is straight forward in the illustrated embodiment, namely, control unit  20  causes metering pump  82  to pull a desired amount of citric acid from source  28  into gas generation chamber  90 . If A-concentrate from source  24  is used instead, control unit  20  performs a priming sequence similar to that for the B-concentrate, where A-concentrate is used to prepare dialysis solution for treatment and A-concentrate for CO 2  gas generation simultaneously (e.g., where the dialysis fluid for treatment is used at this time (prior to treatment) to prime fresh dialysis fluid line  76  and used dialysis fluid line  56 ). Here again, control unit  20  may cause A- and B-concentrate pumps  38  and  44  to run at fixed, e.g., slower, operating speeds. In one embodiment, control unit  20  ensures that an acid solution is provided at the entrance of valve  192   b . Conductivity sensors  40  and  46  operating with control unit  20  are used again to detect when the acid solution has entered main fresh dialysis fluid line  76 . The acid solution is then metered into gas generation chamber  90  in the same manner as described above for the bicarbonate solution. 
     Control unit may alternatively cause the A- and B-concentrate pumps  38  and  44  to be actively controlled during the A- and B-concentrate delivery for CO 2  gas generation. Being actively controlled in an embodiment means the speed of pumps  38  and  44  may change due to feedback presented to control unit  20 . Here, control unit  20  may be able to detect when A- and B-concentrate delivery for CO 2  gas generation has been completed by noting a change to the speed or the pump revolutions over time for A- or B-concentrate pumps  38  and  44 . 
     Next, with the port to bicarbonate priming line  136  of first three-way solenoid valve  192   a  closed and its other two ports open, the port to return line  84  of second three-way solenoid valve  192   b  closed and its other two ports open, and valve  92  in CO 2  gas line  86  open or closed as desired, metering pump  82  pumps a specified amount of citric acid (e.g., in a specified concentration) from source  28  via citric acid line  128  into gas generation chamber  90  to mix with the bicarbonate solution. 
     With the pathway through second three-way solenoid valve  192   b  between metering pump  82  and return line  84  closed, and valve  92  in CO 2  gas line  86  open or closed as desired, the citric acid and bicarbonate solution may be allowed to mix for a specified period of time, e.g., a short period of time such as from a few seconds to 120 seconds, to produce a volume of CO 2  gas that will pressurize inside gas generation chamber  90 . Once the specified CO 2  gas production period has ended, and with the most downstream valve  92  in fresh dialysis fluid line  76  closed and the pathway through second three-way solenoid valve  192   b  between metering pump  82  and return line  84  closed, control unit  20  in one embodiment causes valve  92  in CO 2  gas line  86  to open (if previously closed), allowing the pressurized CO 2  gas inside chamber  90  to flow through CO 2  gas line  86 , a distal portion of dialysis fluid line  76 , and fresh dialysis fluid tube  78  into blood circuit  100  via dialyzer  102 . If valve  92  in CO 2  gas line  86  has instead been open during CO 2  gas formation, then the CO 2  gas migrates naturally along gas line  86 . 
     In one embodiment, control unit  20  causes valve  92  in CO 2  gas line  86  to be open during CO 2  gas production (to allow for CO 2  gas formation and the use of lower pressure components). In an alternative embodiment, control unit  20  may cause valve  92  in CO 2  gas line  86  to be closed during CO 2  generation. Here, a pressure gauge in signal communication with control unit  20  may be located appropriately, e.g., along gas line  86 , and/or a pressure relief valve set at an appropriate operating limit may be provided along gas line  86  to ensure that pressurized CO 2  does not exceed a limit set for gas line  86 . 
     During the CO 2  priming of the present disclosure, arterial line  104  and venous line  106  are connected together at their distal ends, forming a loop that traps the CO 2  gas within blood circuit  100 . Hydrophobic or hydrophilic vent  116  located on venous drip chamber  114  allows the heavier CO 2  gas to push the lighter air in blood circuit  100  out of the circuit. CO 2  gas is better able than a priming fluid to dislodge air trapped in the membranes, and indeed within the pores of the membranes, of dialyzer  102 . CO 2  gas may also be better at flushing air out of pockets located within blood circuit  100 , e.g., pockets created by the housing of dialyzer  102 , pockets created by the housing of venous drip chamber  114 , or even pockets created by the sealing of connectors to tubes (or the connectors themselves), or tubes to tubes, within blood circuit  100 . In short, CO 2  gas is better able to enter and flush small open spaces than is typical priming fluid. 
     In an alternative embodiment, arterial line  104  and venous line  106  may be connected to each other via a disposable piece (not illustrated) containing the venting function (e.g., venting air to atmosphere or trapping the air) instead of incorporating the venting function at venous drip chamber  114 . In a further alternative embodiment, the air and CO 2  gas are removed from blood set  100  via a pressure measurement line  120  illustrated below in connection with  FIG. 3A . 
     In an embodiment, the bicarbonate solution and citric acid reaction within chamber  90  creates a volume of CO 2  gas, which is a multiple (e.g.,  4 X) of the total volume of blood circuit  100 , including dialyzer  102  and drip chamber  114 . Thus if the total volume of blood circuit  100  is about two-hundred fifty milliliters (“mL”), the volume of CO 2  created within chamber  90  and delivered to blood circuit  100  may be about one liter. Given the ability of CO 2  gas to reach and purge small air pockets, and the multiple blood circuit volume&#39;s worth of CO 2  gas produced for priming, it is believed that virtually all of the air will be removed from blood circuit  100  via the CO 2  gas prime. 
     It should be appreciated that blood circuit  100  and in particular dialyzer  102  should be substantially dry or as dry as possible (in one embodiment completely dry) for the CO 2  gas prime. Most dialyzers are shipped in a dry state. Even if shipped wet, or if cleaned for reuse using an aqueous cleaner or disinfectant, the dialyzer may be allowed to dry before CO 2  gas prime. It is also contemplated to pump heated air from dialysis fluid circuit  30  to blood circuit  100  if needed to aid in such drying. Also, to the extent that blood pump  112 , e.g., a peristaltic blood pump, occludes arterial line  104 , a nurse or clinician may be instructed to leave the arterial line  104  disconnected from the actuator of blood pump  112  during the CO 2  gas prime. If arterial line  104  is placed in operable communication with blood pump  112 , however, it is desirable to run the blood pump during the CO 2  gas priming so that air is more readily removed from arterial line  104  and so that CO 2  gas reaches all portions of blood set  100 . 
     A priming fluid, such as dialysis fluid formulated for treatment or saline, is then used to remove CO 2  gas from blood circuit  100 . Dialysis fluid formulated for treatment is delivered in one backfiltration priming embodiment from dialysis fluid circuit  30 , e.g., via fresh dialysis fluid line  76  to blood circuit  100  via dialyzer  102 , while saline may alternatively be delivered directly to arterial line  104  or venous line  106 , e.g., via blood pump  112  or perhaps via gravity. It is contemplated that the priming will displace the CO 2  gas in two ways. In one way, the heavier priming fluid pushes the CO 2  gas from hydrophobic or hydrophilic vent  116  located on venous drip chamber  114  just like the CO 2  gas pushed the lighter air from vent  116 . In another way, the dialysis fluid or saline under higher pressure, e.g., while running blood pump  112 , will absorb or dissolve the CO 2  gas. 
     It is contemplated that the purging in combination with the absorbing or dissolving of the CO 2  gas will remove most all of the gas prior to treatment. It should be appreciated however that CO 2  is a physiologically compatible substance that is readily absorbed in the patient&#39;s bloodstream and removed from the body via breathing. Thus trace amounts of CO 2  remaining after the fluid prime are not harmful to the patient and are instead handled naturally. It is believed that the two stage, CO 2  gas and fluid prime, even further improves air removal. 
     In addition to blood set or circuit  100 , control unit  20  may also be programmed to remove any remaining CO 2  priming fluid including its constituents from gas generation chamber  90  prior to allowing the patient to connect to blood set  100  and/or prior to performing a subsequent CO 2  priming sequence. In one embodiment, control unit  20  causes metering pump  82  to run in reverse, pulling any remaining fluid from gas generation chamber  90  through three-way solenoid valve  192   b , and pushing the fluid back through three-way solenoid valve  192   a , priming fluid line  136  and B-concentrate line  36  into fresh dialysis fluid line  76 , and via fluid bypass line  74  into drain line  56  to drain  60 . CO 2  gas from CO 2  gas line  86  backfills the fluids removed from gas generation chamber  90 . Gas generation chamber  90  may therefore be filled with atmospheric CO 2  gas upon commencing a subsequent CO 2  priming sequence. Care should be taken that only CO 2  gas, and no fluid from dialyzer  102 , is pulled into gas generation chamber  90  to avoid any potential contamination. In this sequence, concentrate pumps  38  and  44  may be locked at their present flow rates. Conductivity cell  46  may be used with control unit  20  at the end of the sequence to confirm fluid removal from gas generation chamber  90 . 
     The above example described the preparation of CO 2  gas using a separate citric acid supply  28 . In an alternative embodiment, citric acid supply  28  and associated citric acid line  128  are removed, and an A-concentrate from A-concentrate source  24  is used instead. A-concentrates vary in physiological composition from manufacturer to manufacturer and from type to type. It is believed however that most if not all A-concentrates will work at least to some degree in combination with bicarbonate to create CO 2 . One suitable A-concentrate for preparing CO 2  gas has a dilution ratio if 45 (e.g., having 135 mmol/l of acetic acid). 
     With the removal of citric acid supply  28  and associated citric acid line  128 , A-concentrate may be pumped from A-concentrate source  24 , through a portion of A-concentrate line  34  and A-concentrate priming line  134 , to the same port of three-way solenoid valve  192   a  that connected formerly to citric acid line  128 . In the illustrated arrangement of  FIG. 1A , A-concentrate may therefore be metered via metering pump  82  to gas generation chamber  90  in the same manner described above for citric acid. 
       FIG. 1A  illustrates an either/or scenario in which either (i) citric acid supply  28  and associated citric acid line  128  are present while priming line  134  is not present, or (ii) A-concentrate priming line  134  is present to use A-concentrate from A-concentrate source  24  instead, while citric acid supply  28  and associated citric acid line  128  are not present. It is expressly contemplated however for system  10   b  to provide both options at once, in which both citric acid supply  28  and associated citric acid line  128  and A-concentrate priming line  134  are provided. Here, a three-way valve under control of control unit  20 , e.g., similar to three-way solenoid valves  192   a  and  192   b , may be added at the juncture of lines  28  and  134  to allow control unit  20  to open either a path including citric acid supply  28  and associated citric acid line  128  or a path including A-concentrate priming line  134  and A-concentrate source  24 , as desired by the operator. 
     Like above, with the fluid pathway through three-way solenoid valve  192   b  to metering pump  82  opened, and valve  92  in CO 2  gas line  86  open or closed as desired, the A-concentrate and bicarbonate solution may be allowed to mix for a specified period of time, e.g., for enough time to allow a downstream underpressure to be formed to pull CO 2  gas out of the mixture, producing a volume of CO 2  gas that will pressurize inside gas generation chamber  90 . Once the specified CO 2  gas production period has ended, and with the most downstream valve  92  in fresh dialysis fluid line  76  closed and the fluid pathway through second three-way solenoid valve  192   b  to metering pump  82  opened to allow for circulation within the loop including lines  84  and  136 , control unit  20  causes valve  92  in CO 2  gas line  86  to open (if previously closed), allowing the pressurized CO 2  gas inside chamber  90  to flow through CO 2  gas line  86 , a distal portion of dialysis fluid line  76 , and fresh dialysis fluid tube  78  into blood circuit  100  via dialyzer  102 . If valve  92  in CO 2  gas line  86  has instead been open during CO 2  gas formation, CO 2  gas migrates naturally along CO 2  gas line  86  as discussed above. Blood circuit  100  is then primed with CO 2  gas as described above. 
     It should be noted that when using A-concentrate to create carbon dioxide gas, it is likely that an underpressure needs to be created downstream from gas generation chamber  90  for the CO 2  to be released from the mixed solutions. For example, with dialyzer  102  being dry, control unit  20  may cause blood pump  112  to run in reverse to pull a negative pressure through arterial line  104  leading to dialyzer  102 , the dialyzer itself, fresh dialysis fluid tube  78  leading to dialyzer  102 , CO 2  gas line  86 , and a distal portion of dialysis fluid line  76 . Also, system  10   a  using A-concentrate to create carbon dioxide gas is configured to mitigate or prevent the creation of precipitation, such as calcium carbonate. With A-concentrate, the amount (or perhaps a concentration of the concentrate leading to the amount) of acid in the solution is selected to create an amount of CO 2  gas needed for priming. 
     It is also contemplated that when using A-concentrate for the acid solution, a surplus of the A-concentrate may be needed. Without the surplus, it is more likely that precipitation will form in gas generation chamber  90 , which is undesirable. Assuming the use of a bicarbonate concentrate having a concentration of 1000 mmol/l (e.g., from B-concentrate in liquid form stored in a canister), pumping 10 mL (see table below) of that B-concentrate into the chamber will produce 10 mmol of sodium bicarbonate. In comparison, pumping 85 ml of  45   x  A-concentrate (as is done in the table below) will produce about 11.475 mmol of HAc. But if instead BiCart® powdered bicarbonate cartridge solution provided by the assignee of the present disclosure is used, the concentration of 10 mL of resulting bicarbonate fluid leaving the cartridge is in the range of 1200 mmol/l depending upon temperature. Here, to minimize precipitation, approximately 1200/1000 (20% more) 45× A-concentrate may be added, resulting in about 100 mL of A-concentrate and 13.5 mmol HAc. 100 mL of A-concentrate and 10 mL of BiCart® produced bicarbonate solution will then yield about 330 to 350 mL of CO 2  gas depending on the temperature of the mixture. To produce instead a desired lesser amount of CO 2  gas (e.g., 250 mL), the amount of sodium bicarbonate is lowered to in turn lower the A-concentrate needed. 
     In addition to citric acid and A-concentrate, other acidic solutions may be combined with the bicarbonate solution to produce CO 2  gas. One such acidic solution is marketed under the tradename SelectBag®, which is an acetic acid containing concentrate developed by the assignee of the present disclosure. The SelectBag® solution may be placed at the location of either citric acid bag  28  or A-concentrate container  24  and used according to the corresponding sequence described above. 20 mL of SelectBag® solution (example used in the table below) contains approximately 12 mmol HAc. The amount of sodium bicarbonate should be in the same range to avoid precipitation using SelectBag® solution. 
     Citric acid from source  28  includes no calcium chloride in one embodiment and therefore does not yield a precipitate. Alternatively, when using a dialysis concentrate that contains calcium chloride, for example, system  10   a  ensures that enough concentrate is provided, so that the pH is held low enough to prevent the formation of precipitation. The SelectBag® solution, for example, contains calcium and acetic acid. When using SelectBag® solution, system  10   a  again ensures that enough concentrate is added to prevent precipitation due to the calcium. Another possible solution is SelectBag® Citrate solution provided by the assignee of the present disclosure. SelectBag® Citrate solution contains calcium and citric acid. When using SelectBag® Citrate solution, system  10   a  yet again ensures that enough concentrate is added to prevent precipitation, despite the fact that the solution contains citric acid (even though the citric acid improves the situation because citric acid binds with calcium, lowering the concentration of free ionized calcium, and reducing the risk for precipitation). It is only when a precipitation yielding component such as calcium chloride is not present, such as with pure citric acid, that precipitation is not possible. Even here however, system  10   a  provides enough acid to yield the correct or desired amount of CO 2  gas. 
     Other suitable acid solutions or blends thereof include a lactic acid solution, a malic acid solution, a hydrochloric (“HCl”) acid solution, (vi) a phosphoric acid solution, and (vii) any other biocompatible acid solution. Any one or more of the above-listed acid solutions may be mixed or blended with each other as desired for use with system  10   a  (and any of the systems described herein). It should also be appreciated that while the mixing of bicarbonate solution and an acid solution has been described as delivering bicarbonate solution first, then acid solution, the order may be reversed for any of the embodiments described herein. 
     As discussed above, it is contemplated to create and deliver a volume of CO 2  gas that is a multiple of the total volume of blood set  100  to ensure proper removal of air. The following table shows experimental data performed using saturated B-concentrate mixed alternatively with 45× A-concentrate (mix one part of the concentrate and 44 parts purified water to make 45 parts of ready fluid), SelectBag® solution, and 50% by volume citric acid. Assuming blood set  100  to be 250 mL in total volume, the liquid volumes specified in the table would need to be increased three to seven times to achieve sufficient multiples of the blood set volume. The use of citric acid for example would require 50 to 60 mL of total liquid volume to achieve a  4 X multiplier of the blood set volume. 60 mL of total liquid volume is believed to be a very reasonable amount. 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                 vol. sat. B conc. 
                 vol. 45x A conc. 
                 vol. SelectBag 
                 vol. 50% citric 
                 vol. CO 2   
               
               
                 liquid mL 
                 liquid mL 
                 liquid mL 
                 acid liquid mL 
                 gas mL 
               
               
                   
               
             
            
               
                 10 
                 85 
                   
                   
                 290 
               
               
                 10 
                   
                 10 
                   
                 150 
               
               
                 10 
                   
                 20 
                   
                 230 
               
               
                 10 
                   
                   
                 2 
                 250 
               
               
                 10 
                   
                   
                 2 
                 210 
               
               
                   
               
            
           
         
       
     
     Please note that the above data is for mixing at atmospheric pressure and a temperature of around 30° C. to 35° C. It should also be understood that the numbers above are illustrated for the purpose of showing that enough CO 2  gas may be produced using a reasonable amount of CO 2  gas generation fluid constituents. The actual amounts used in practice will likely be adjusted when precipitation is taken into account. Moreover, reducing downstream pressure, e.g., by reducing the pressure in arterial line  104 , reduces the amount of CO 2  gas needed (e.g., half the quantity needed at half atmospheric pressure in the bloodlines). It is accordingly contemplated to maintain a low pressure in blood set  100 , e.g., at atmospheric or below, during the CO 2  gas priming. 
     Moreover, the example above using 10 mL® solution will likely create significant precipitation and thus may not present a realistic solution. The two citric acid examples having two different input volumes yielding different gas volumes show that test results may vary, for example, based upon environmental temperature and whether or not an underpressure is provided with the reaction. The underpressure may yield gas volume results that exceed what is theoretically possible at atmospheric pressure. 
     The formulations for the CO 2  gas priming mixtures are very likely going to be different than the formulations for treatment fluid, either because different constituents are used or because the same constituents are used in different proportions. Regarding the formulations generally, mixtures may be prepared accurately in two different ways, either by (i) using a formula for the mixture and accurately adding the constituents according to the formula or (ii) using a sensor reading indicative of the overall constituency of the mixture, e.g., conductivity, and sensor feedback to control unit  20  to servo to a desired sensor reading setpoint for a desired overall constituency. The different CO 2  gas formulations of the present disclosure may be prepared using either mixing accuracy technique. 
     The above table shows formulas, e.g., (a) formulas 1:8.5 for bicarbonate/A concentrate, (b) formulas 1:1 and 1:2 for bicarbonate/SelectBag® solution, and (c) formula 5:1 for bicarbonate/citric acid. As discussed above, it is conceivable to use conductivity sensors, setpoints and servo feedback to meet setpoints instead of the above formulas for the priming fluid mixtures. But in any case, the formulation of CO 2  gas generation fluid will be different in at least one respect than the formulation of fresh dialysis fluid prepared for treatment. 
     Referring now to  FIG. 1B  an alternative system for making CO 2  gas in dialysis fluid circuit  30  and delivering such gas to blood circuit  100  is illustrated by system  10   b . System  10   b  is the same in many respect to system  10   a  (including all structure, functionality and alternatives discussed above). System  10   b  adds a recirculation line  138  extending in the illustrated embodiment from a bottom portion of gas generation chamber  90  back to a third three-way valve  192   c  under control of control unit  20 , and located upstream of metering pump  82 . Bicarbonate and acid solutions are delivered to gas generation chamber  90  as described above, but here with the path through third three-way valve  192   c  to recirculation line  138  closed. Once the desired amount of bicarbonate and acid solutions are delivered to gas generation chamber  90 , the state of third three-way valve  192   c  is switched such that the path through third three-way valve  192   c  to three-way valve  192   a  is closed, while the path through third three-way valve  192   c  to recirculation line  138  is opened. 
     During and after formation of the CO 2  gas, metering pump  82  recirculates the CO 2  gas generation fluid down recirculation line  138  and back up through third three-way valve  192   c  to gas generation chamber  90 . The recirculation stirs and agitates the CO 2  gas generation fluid, causing CO 2  gas to be released from the fluid and to flow out of gas generation chamber  90  through gas line  86 . In an embodiment, recirculation line  138  may be fitted with a spiraled member, a staggered flanged member, or other type of turbulator to further disrupt and turbulate the CO 2  gas generation fluid and release the CO 2  gas. 
     Referring now to  FIG. 1C , an alternative system for delivering CO 2  gas from dialysis fluid circuit  30  to blood circuit  100  is illustrated by system  10   c . System  10   c  is the same in many respect to system  10   a  (including all structure, functionality and alternatives discussed above). System  10   c  may also include recirculation line  138 , third three-way valve  192   c , and the associated structure and functionality discussed in connection with system  10   b . System  10   c  adds a pressure measurement line  120  leading from a top of drip chamber  114  to a machine connector  122 . In the illustrated embodiment, CO 2  gas is produced within machine  12  and is delivered to blood set  100  as described above. Control unit  20  causes blood pump  112  to run slowly to remove air from arterial line  104  and venous line  106  (connected together in  FIG. 1C ) and to deliver the air into venous drip chamber  114 . Air in chamber  114  is in turn led out of blood set  100  via venous pressure line  120  into the interior of machine  12  via valve  92 Pv under control of control unit  20 . 
     Referring now to  FIG. 1D , one embodiment for a priming fluid portion of the priming of blood circuit  100  of system  10   c  is illustrated. Here, the patient ends of arterial line  104  and venous line  106  are disconnected from each other (connected in  FIG. 1C ). After CO 2  gas priming has been completed, a saline bag or container  124  is connected to the distal end of arterial line  104 , while an empty drain bag or container  126  is connected to the distal end of venous line  106 . Blood pump  112  under control of control unit  20  is operated to pull saline from saline bag or container  124  through the portion of arterial line  104  upstream of blood pump  112 , and to push saline through the portion of arterial line  104  downstream from blood pump  112 , dialyzer  102 , drip chamber  114  and venous line  106  to drain bag or container  126 . The saline prime pushes CO 2  gas both to drain bag or container  126  and through venous pressure line  120  into the interior of machine  12  via valve  92 Pv under control of control unit  20  until a level detector operable with drip chamber  114  detects a high fluid level, after which valve  92 Pv is closed. Additionally, as discussed above, saline especially under positive pressure will also absorb some of the CO 2  gas back into solution. Moreover as discussed above, small amounts of CO 2  gas are not harmful to the patient. 
     In each of the previous examples, CO 2  gas is created in dialysis fluid circuit  30  and delivered in gaseous form to blood circuit  100 . In an alternative primary embodiment illustrated in  FIGS. 2A to 2D , system  110   a  instead causes CO 2  gas to be carried within the formulated priming fluid into blood circuit  100 , where the CO 2  gas is then degassed from the priming fluid to help rid blood circuit  100  of small air pockets. System  110   a  may include any of the structure, functionality and alternatives discussed above for systems  10   a ,  10   b  and  10   c  in combination with the additional structure, functionality and alternatives described below for  FIGS. 2A to 2D . 
     In  FIGS. 2A to 2D , blood circuit  100  operates with dialysis fluid circuit  30  as shown above, except that gas generation chamber  90  and associated plumbing is not needed or provided.  FIGS. 2A to 2D  also illustrate that machine  12  may further include a hemodiafiltration (“HDF”) port  94  and/or a waste handling option (“WHO”) port  98 . As mentioned above, a portion of fresh dialysis fluid flowing along fresh dialysis fluid line  76  may be diverted through one or more ultrafilter to purify the fresh dialysis fluid to such a level that it may be delivered as replacement fluid directly into arterial line  104  as illustrated to perform pre-dialyzer or pre-infusion HDF or into venous line  106  to perform post-dialyzer or post-infusion HDF.  FIGS. 2A to 2D  illustrate a replacement fluid pump  96 , which under control of control unit  20  delivers the replacement fluid to blood circuit  100  at a desired flowrate.  FIGS. 2A to 2D  illustrate that arterial line  104  and venous line  106  may be plugged together into WHO port  98  to form a loop for the priming fluid and CO 2  gas. 
     It should be appreciated that in any embodiment described herein, CO 2  gas, e.g., via gas generation chamber  90  of system  10   a , or via priming fluid in system  110   a , may be introduced into blood circuit  100  ( i ) via fresh dialysis fluid line  76  and tube  78  or (ii) via a pre-, post-, or pre- and post-HDF port  94 . In a further alternative embodiment, CO 2  gas may be introduced into blood circuit  100  via an air vent line running between drip chamber  114  and hydrophobic or hydrophilic vent  116 . 
     In  FIG. 2A , control unit  20  of system  110   a  causes replacement fluid pump  96  to pump priming fluid formulated to create CO 2  gas into blood circuit  100 . The priming fluid formulated to create CO 2  may be created using bicarbonate solution and any of the acid solutions described above, e.g., (i) a citric acid solution, (ii) an acetic acid solution (e.g., from A-concentrate), (iii) a lactic acid solution, (iv) a malic acid solution, (v) a hydrochloric (“HCl”) acid solution, (vi) a phosphoric acid solution, (vii) any other biocompatible acid solution, and (v) blends thereof. The flowrate of priming fluid formulated to create CO 2  gas may be a desired flowrate Q p  mL/minute (e.g., from 100 to 500 mL/minute) at replacement fluid pump  96  and below Q p  mL/minute at blood pump  112 . The introduction of the priming fluid into blood circuit  100  causes much of the air to be pushed out of blood circuit  100  via hydrophobic or hydrophilic vent  116 . 
     In  FIG. 2B , control unit  20  causes the speed of blood pump  112  to be increased, e.g., above Q p  mL/minute, so that blood pump  112  pumps faster than replacement fluid pump  96 , which creates a relatively lower pressure in replacement fluid line  118 . Blood pump  112  pumping towards dialyzer  102  also places venous line  106  and the portion of arterial line  104  upstream of blood pump  112  under negative pressure. Venous drip chamber  114  with a hydrophilic filter  116  is inverted, e.g., manually, or via an electromechanical actuator under control of control unit  20  in  FIG. 2B , so that air is not pulled into blood circuit  100 . The relatively low pressure in replacement fluid line  118  and the negative pressure in venous line  106  and in the portion of arterial line  104  upstream of blood pump  112  both cause the priming fluid to degas CO 2  into those portions of blood circuit  100 . The CO 2  gas dislodges air pockets in tight portions of blood circuit  100  as has been described above. 
     Control unit  20  may then cause blood pump  112  in  FIG. 2B  to move in reverse to place the priming fluid in the remaining portion of arterial line  104  and dialyzer  102  under negative pressure to cause the priming fluid to degas CO 2  into the remainder of arterial line  104  and dialyzer  102 . The CO 2  gas dislodges air pockets in the remainder of arterial line  104  and in the membranes and pores of the membranes of dialyzer  102  as has been described above. Control unit  20  may cause blood pump  112  to reverse back and forth multiple times to repeat the above-described sequence multiple times to ensure that blood circuit  100  has been subjected fully to CO 2  gas, so that virtually all air within blood circuit  100  has been dislodged for removal. 
       FIG. 2C  illustrates that after the blood circuit  100  has been subjected fully to CO 2  gas, venous drip chamber  114  may be flipped to its normal position, e.g., manually, or via an electromechanical actuator under control of control unit  20 . Control unit  20  may again reverse blood pump  112  multiple times to ensure that all parts of blood circuit  100  see positive pressure leading to venous drip chamber  114  and its vent  116 , so that virtually all air within blood circuit  100  has been dislodged for removal. 
       FIG. 2D  illustrates patient P connected to arterial line  104  and venous line  106  for treatment. Control unit  20  may be programmed such that after the CO 2  gas prime of  FIGS. 2A to 2C , CO 2  gas and any remaining CO 2  priming fluid for preparing the CO 2  gas may be absorbed and/or displaced using dialysis fluid prepared for treatment or saline. As discussed above, CO 2  is a physiologically compatible substance that is readily absorbed in the patient&#39;s bloodstream and removed from the body via breathing. Any trace amount of CO 2  remaining after the fluid prime of  FIGS. 2B and 2C  is not harmful to the patient and is instead handled naturally. 
     Referring now to  FIGS. 3A to 3D , system  110   b  illustrates an alternative arrangement for carrying CO 2  gas within the priming fluid formulated to produce CO 2  gas into blood circuit  100 , where the CO 2  gas is then degassed from the priming fluid to help rid blood circuit  100  of small air pockets. System  110   b  includes all structure, functionality and alternatives discussed above and incorporated into system  110   a . System  110   b  moves replacement fluid line  118  and replacement fluid pump  96  as shown in the illustrated embodiment, so that replacement fluid is delivered via pump  96  under control of control unit  20  to venous line  106  (post-dilution) instead of to arterial line  104  (pre-dilution) in  FIGS. 2A to 2D . Either system  110   a  and  110   b  may be configured to have pre- or post-dilution or pre- and post-dilution replacement fluid delivery. System  110   b  also moves hydrophilic vent  116  to be just in front of, or to be part of, machine connector  122 . Any of systems  10   a  to  10   c ,  110   a  or  110   b  may have hydrophilic vent  116  arranged as shown in  FIGS. 3A to 3D . 
     System  110   b  of  FIGS. 3A to 3D  in the illustrated embodiment also includes a WHO pump  130  located within machine  12  (WHO pump may be any of the types of pumps described for metering pump  82 ), which is under control of control unit  20  and is positioned and arranged to pull liquid from blood set  100  via WHO port  98 . A WHO valve  92   WHO  under control of control unit  20  is located within machine  12  between WHO port  98  and WHO pump  130 . Any of systems  10   a  to  10   c ,  110   a  or  110   b  may have WHO pump  130  and WHO valve  92   WHO  arranged as shown in  FIGS. 3A to 3D . System  110   b  of  FIGS. 3A to 3D  in the illustrated embodiment further includes an air pump  132  located within machine  12  (air pump  132  may be any of the types of pumps described for metering pump  82 ), which is under control of control unit  20  and is positioned and arranged to push or pull air to or from venous drip chamber  114  of blood set  100 . Air pump  132  is located between machine connector  122  and valve  92 Pv in the illustrated embodiment. Any of systems  10   a  to  10   c ,  110   a  or  110   b  may have air pump  132  arranged as shown in  FIGS. 3A to 3D . 
     System  110   b  of  FIGS. 3A to 3D  further includes a level sensor  142  operable with venous drip chamber  114  and located to sense a level of liquid within the chamber. Level sensor  142  sends a signal indicative of the level of liquid within chamber  114  to control unit  20 , which reacts accordingly. Any of systems  10   a  to  10   c ,  110   a  or  110   b  may have a level sensor  142  arranged as shown in  FIGS. 3A to 3D . System  110   b  of  FIGS. 3A to 3D  also includes a replacement fluid line clamp  140  under control of control unit  20 . Replacement fluid line clamp  140  allows or disallows replacement fluid and other fluids to flow to venous line  106  of blood set  100 . Any of systems  10   a  to  10   c ,  110   a  or  110   b  may have a replacement fluid line clamp  140  arranged as shown in  FIGS. 3A to 3D . Still further, System  110   b  of  FIGS. 3A to 3D  may include a gas/liquid sensor  144  under control of control unit  20 . Gas/liquid sensor  144  is configured to sense the difference between CO 2  gas generation fluid and CO 2  gas or air residing just below venous drip chamber  114 . Any of systems  10   a  to  10   c ,  110   a  or  110   c  may have a gas/liquid sensor  144  arranged as shown in  FIGS. 3A to 3D . 
     In  FIG. 3A  system  110   b  fills venous drip chamber  114  with CO 2  gas generation fluid. In one embodiment, control unit  20  causes replacement fluid pump  96  to run and fill venous drip chamber  114 . WHO pump  130  does not need to be operated. Here, in one embodiment, CO 2  gas generation fluid and any escaping CO 2  gas force air out of blood set  100  through drip chamber  114 , via venous pressure line  120  and into the interior of machine  12  via valve  92 Pv under control of control unit  20 . 
     In  FIG. 3B , when level sensor  142  at venous drip chamber  114  senses a high level of CO 2  gas generation fluid, CO 2  gas generation fluid is pumped or gravity flowed out the bottom of venous drip chamber  114  towards WHO port  98  via venous line  106 . Control unit  20  may be programmed to allow CO 2  gas generation fluid to flow out the bottom of venous drip chamber  114  for a preset period of time or until receiving a signal from a sensor, such as an ultrasonic or capacitive sensor (not illustrated), positioned at a desired location upstream of WHO port  98  to detect that no more air is coming down the line. 
     Control unit  20  then causes blood pump  112  to pull CO 2  gas generation fluid through connected venous line  106  and arterial line  104  to fill arterial line  104  up to dialyzer  102 . 
       FIG. 3C  illustrates one embodiment for filling arterial line  104  between blood pump  112  and dialyzer  102 , the dialyzer itself, and the remainder of venous line  106  with CO 2  gas generation fluid. Control unit  20  causes blood pump  112  to pump CO 2  gas generation fluid somewhat faster than the rate at which replacement fluid pump  96  introduces new CO 2  gas generation fluid into blood circuit  100 , so that blood pump  112 , with arterial and venous lines  104  and  106  connected together as illustrated and with WHO valve  92   WHO  closed, pulls CO 2  gas generation fluid out of venous drip chamber  114  more quickly than replacement fluid pump  96  refills the chamber, thereby lowering the liquid level in venous drip chamber  114 . The output of blood pump  112  fills the positive pressure portion of arterial line  104 , dialyzer  102  and the portion of venous line  106  up to the inlet of replacement fluid line  118  with CO 2  gas generation fluid. Air is again forced out of blood set  100  through drip chamber  114 , via venous pressure line  120  and into the interior of machine  12  via valve  92 Pv under control of control unit  20 . When all lines of blood set  100  are filled with CO 2  gas generation fluid, the liquid level within venous drip chamber  114  will rise, eventually tripping level sensor  142 , telling control unit  20  that blood set  100  is full. 
       FIG. 3D  illustrates one embodiment for creating a low pressure in blood set  100  to pull CO 2  gas out of the CO 2  gas generation fluid. Here, control unit  20  causes replacement fluid pump  96  to stop and closes replacement fluid line clamp  140 . Control unit  20  further causes (i) WHO pump  130  to create a suction through WHO valve  92   WHO  to the connection of arterial line  104  and venous line  106 , and (ii) blood pump  112  to run slowly to remove gas created inside of dialyzer  102  due to the degassing and to deliver CO 2  gas to all areas of blood set  100 . The suction pressure releases CO 2  gas from CO 2  gas generation fluid as has been discussed herein. If the level of CO 2  gas generation fluid in venous chamber  114  falls too low, control unit  20  may open replacement fluid line clamp  140  and cause replacement fluid pump  96  to restore the CO 2  gas generation fluid level. 
     When CO 2  gas priming is completed, a saline prime may be performed as illustrated in connection with  FIG. 1D , or dialysis fluid for treatment may be used to flush CO 2  gas through drip chamber  114 , via venous pressure line  120  and into the interior of machine  12  via valve  92 Pv under control of control unit  20 . CO 2  gas may additionally be absorbed into saline or dialysis fluid for treatment as described herein. 
     Referring now to  FIGS. 4A and 4B , system  110   c  illustrates an alternative arrangement for carrying CO 2  gas within the priming fluid formulated to produce CO 2  gas into blood circuit  100 , where the CO 2  gas is then degassed from the priming fluid to help rid blood circuit  100  of small air pockets. System  110   c  includes all structure, functionality and alternatives discussed above and incorporated into systems  110   a  and  110   b . System  110   c  moves replacement fluid line  118  as shown in the illustrated embodiment, so that replacement fluid is delivered via replacement fluid pump  96  under control of control unit  20  to arterial line  104  (pre-dilution) as with in  FIGS. 2A to 2D . 
     Pre-dilution system  110   c , unlike pre-dilution system  110   a  in  FIGS. 2A to 2D , uses valve  92 Pv inside machine  12  to close off the air path when needed. It is noted that filter  116  in system  110   c  is a hydrophilic filter in the illustrated embodiment. Filter  116  may be used to protect blood from entering and contacting the connection site at machine connector  122  and the inside of machine  12 . 
       FIG. 4A  illustrates one example embodiment for filling blood set  100 . Control unit  20  with replacement fluid line clamp  140  open causes blood pump  112  to run in reverse direction at a desired speed, pushing CO 2  gas generation fluid through connected lines  104  and  106  and backwards through venous line  106  to venous drip chamber  114 . Fluid/gas sensor  144  in communication with control unit  20  eventually sees the CO 2  gas generation fluid, which control unit  20  reads and causes blood pump  112  to stop. Control unit  20  then causes replacement fluid pump  96  to pump CO 2  gas generation fluid to blood set  100  including dialyzer  102  and the portion of venous line  106  to drip chamber  114  until level detector  142  operable with drip chamber  114  sees a high level of fluid. 
     In  FIG. 4B , system  110   c  with blood set  100  having been filled with CO 2  gas generation fluid and with replacement fluid line clamp  140  closed, may then create a low pressure in blood set  100  in the manner described above in  FIG. 3D , releasing the CO 2  gas from the CO 2  gas generation fluid to perform the CO 2  gas priming as described herein. After the CO 2  gas prime, blood set  100  may then be primed with dialysis fluid for treatment as has been described herein. 
     ELEMENT NUMBER LISTING 
     
         
         
           
               10   a ,  10   b ,  10   c —systems 
               12 —machine 
               14 —user interface  14   
               16 —processor 
               18 —memory 
               20 —control unit 
               22 —purified water source 
               24 —A-concentrate source 
               26 —B-concentrate source or bicarbonate cartridge 
               28 —citric acid source or source of acid solution 
               30 —dialysis fluid circuit 
               32 —purified water line 
               34 —A-concentrate line 
               36 —B-concentrate line 
               38 —A-concentrate pump 
               40 —conductivity cell 
               42 —temperature sensor 
               44 —B-concentrate pump 
               46 —conductivity cell 
               48 —temperature sensor 
               50 —degassing chamber 
               51 —degassing pump 
               52 —heater 
               54 —fresh dialysis fluid pump 
               56 —used dialysis fluid line 
               58 —used dialysis fluid pump 
               60 —drain 
               62 —air separator 
               64 —pressure sensor 
               66 —conductivity cell 
               68 —temperature sensor 
               70 —UF system 
               72 —heat exchanger 
               74 —fluid bypass line 
               76 —fresh dialysis fluid line 
               78 —fresh dialysis fluid tube 
               80 —used dialysis fluid tube 
               82 —metering pump 
               84 —return line 
               86 —CO 2  gas line 
               90 —gas generation chamber or CO 2  gas chamber 
               92 —plural valves 
               92   b   1  and  92   b   2 —priming sequence valves 
               92 Pv—air path valve 
               92   w —water line valve 
               92   WHO —waste handling option valve 
               94 —hemodiafiltration (“HDF”) port  94   
               96 —replacement fluid pump 
               98 —waste handling option (“WHO”) port 
               100 —blood set or blood circuit 
               102 —dialyzer or blood filter 
               104 —arterial line 
               106 —venous line 
               108   a —arterial pressure pod 
               108   v —venous pressure pod 
               110   a ,  110   b ,  110   c —systems 
               112 —blood pump 
               114 —venous drip chamber 
               116 —hydrophilic vent 
               118 —replacement fluid line 
               120 —pressure measurement line 
               122 —machine connector 
               124 —saline bag or container 
               126 —empty drain bag or container 
               128 —citric acid line 
               130 —WHO pump 
               132 —air pump 
               134 —A-concentrate priming line 
               136 —bicarbonate priming line 
               138 —recirculation line 
               140 —replacement fluid line clamp 
               142 —level sensor 
               144 —gas/liquid sensor 
               192   a —first three-way solenoid valve 
               192   b —second three-way solenoid valve 
           
         
       
    
     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 may be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.