Abstract:
In general, this disclosure relates to extracorporeal fluid circuits. In some aspects, an air-release device for allowing air to be released from a liquid in extracorporeal circuitry includes an elongate chamber having a bottom region and a top region and a fluid entry port and fluid exit port at or near the bottom region. The air release device also includes a vent structure at or near the top region of the elongate chamber that includes a porous material capable of swelling when moistened such that the vent structure can inhibit liquid from escaping the air-release device during use.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application and claims priority to U.S. application Ser. No. 12/233,111, filed Sep. 18, 2008, which claims the benefit of the filing date of U.S. Provisional Application No. 60/973,730, filed Sep. 19, 2007. The contents of U.S. application Ser. Nos. 12/233,111 and 60/973,730 are incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to extracorporeal fluid circuits. 
       BACKGROUND 
       [0003]    Hemodialysis removes toxic substances and metabolic waste from the bloodstream using an extracorporeal circuit with components designed to perform ultrafiltration and diffusion on the blood. Before the blood is returned to the body, air bubbles are removed from the blood to inhibit embolisms. 
       SUMMARY 
       [0004]    In one aspect, an extracorporeal medical fluid circuit component is described. The component includes a vent assembly. A vent structure adjacent to a micro-porous membrane forms the assembly. The vent structure is porous, but expands when the vent structure becomes wet, thereby closing off the pores and inhibiting (e.g., preventing) fluid from flowing through the vent structure. The vent structure also protects the membrane from becoming wet, such as from condensation. The component is capable of being used in an extracorporeal medical fluid circuit. 
         [0005]    In another aspect, a transducer protector includes a body that defines a fluid pathway. A vent assembly is disposed in the fluid pathway. The vent assembly includes a vent structure and a micro-porous membrane. The vent structure is porous, but expands when the vent structure becomes wet, thereby closing off the pores and inhibiting (e.g., preventing) fluid from flowing through the vent structure. The vent structure also protects the membrane from becoming wet, such as from condensation. The transducer protector is capable of being connected in fluid communication with a fluid circuit and a pressure transducer such that the vent assembly inhibits liquid flowing in the fluid circuit from contacting the pressure transducer. 
         [0006]    In a further aspect, an extracorporeal medical fluid circuit apparatus, e.g., for removing air from a bodily liquid in extracorporeal circuitry used in a hemodialysis machine, is described. The apparatus includes a chamber having a fluid entry port, a fluid exit port, and a vent assembly. The vent assembly includes a micro-porous membrane and a vent structure adjacent to the micro-porous membrane. The vent structure includes a porous material that is capable of swelling when moistened. The fluid entry port and the fluid exit port are arranged to allow liquid to pass through the chamber from the entry port to the exit port so as to fill the chamber with the liquid when back pressure is applied, and the vent assembly is arranged to allow gas to exit the chamber as the liquid passes through the chamber. 
         [0007]    In yet another aspect, an integrated fluid circuit component adapted to removably seat in a dialysis machine is described. The component includes a rigid body having a substantially flat main portion and a plurality of recessed portions extending from the flat main portion and a flexible backing covering at least one of the of recessed portions. A first recessed portion of the plurality of recessed portions forms a chamber. A second recessed portion of the plurality of recessed portions forms a first channel that is in fluid communication with the chamber, and a third recessed portion of the plurality of recessed portions forms a second channel in fluid communication with the chamber. The component also includes a vent assembly that is in fluid communication with the chamber. The vent assembly includes a micro-porous membrane and a vent structure. 
         [0008]    In yet another aspect, a dialysis system is described. The system includes a machine body, a pump on the machine body, and fluid circuitry (e.g., tubes) in fluid communication with the pump. The pump is configured to push fluid through the circuitry. The system also includes a vent assembly in fluid communication with the fluid circuitry. The vent assembly includes a micro-porous membrane and a vent structure adjacent to the micro-porous membrane. The vent structure includes a porous material that is capable of swelling when moistened. 
         [0009]    In another aspect, a method of removing air from a bodily liquid in dialysis circuitry is described. A chamber with an entry port, an exit port, a micro-porous membrane and a vent structure is provided. A first liquid is passed through the entry port, filling the chamber so that substantially no air remains in the chamber. A second liquid is passed through the entry port, forcing a portion of the first liquid out of an exit port of the chamber and forming a liquid-liquid interface between the first and second liquids. Any gas bubbles contained in the second liquid can be forced out of the chamber through the micro-porous membrane and the vent structure. 
         [0010]    Embodiments of the disclosed methods, systems and devices may include one or more of the following features. 
         [0011]    The vent structure can have an average pore size of about 15 microns to about 45 microns. 
         [0012]    The vent structure can include a polymer such as polyethylene (e.g., high density polyethylene (HDPE)), polypropylene, or polystyrene. 
         [0013]    The vent structure can include a swelling agent such as carboxymethylcellulose (CMC), methyl-ethyl-cellulose or other similar swelling agents. 
         [0014]    The vent structure can include a blend of a polymer and a swelling agent. 
         [0015]    The vent structure can have a thickness greater than a thickness of the micro-porous membrane. 
         [0016]    The micro-porous membrane can have an average pore size of about 0.05 microns to about 0.45 microns (e.g., about 0.22 microns or about 0.2 microns). 
         [0017]    The micro-porous membrane can be held by a plastic ring (e.g., by insert molding, heat welding, ultrasonic welding, adhesive, clamping, etc.) and the assembly can also include an insert for holding the micro-porous membrane adjacent to the vent structure. 
         [0018]    In some embodiments, a structure or assembly can include a plastic ring into which the micro-porous membrane is press-fit, wherein the ring surrounds the vent structure and retains the vent structure adjacent to the micro-porous membrane. 
         [0019]    The vent structure can include a first porous layer adjacent to the micro-porous membrane, and a second porous layer adjacent to the first porous layer. 
         [0020]    The second porous layer can include a porous material that is capable of swelling when moistened. 
         [0021]    The first porous layer can also include a porous material that is capable of swelling when moistened. 
         [0022]    The second porous layer can have a greater propensity to swell in the presence of moisture than the first porous layer. 
         [0023]    The second porous layer can have an average pore size that is greater than an average pore size of the first porous layer. For example, the first porous layer can have an average pore size of about 10 microns, and the second porous layer can have an average pore size of about 30 microns. 
         [0024]    The second porous layer can include about 5% to about 50% by weight carboxymethylcellulose (e.g., about 10% by weight carboxymethylcellulose). 
         [0025]    The first porous layer can include 0% to about 10% by weight carboxymethylcellulose (e.g., less than 5% by weight carboxymethylcellulose). 
         [0026]    The extracorporeal medical fluid circuit component can be configured for use in an air release chamber. 
         [0027]    The extracorporeal medical fluid circuit component can be configured for use in a transducer protector. 
         [0028]    The extracorporeal medical fluid circuit component can be configured for use in a blood circuit. The blood circuit can be capable of being used with a dialysis machine. 
         [0029]    The micro-porous membrane can be between the vent structure and the chamber. 
         [0030]    The vent structure can include porous material that is capable of swelling when moistened. 
         [0031]    The dialysis system can include a pressure transducer and a transducer protector that includes the vent assembly. 
         [0032]    The transducer protector can be disposed between, and in fluid communication with, the circuitry and the pressure transducer. 
         [0033]    Passing the second liquid through the entry port can include passing moisture from the second liquid through the micro-porous membrane and allowing the moisture to pass through the vent structure, causing the vent structure to swell. 
         [0034]    Other aspects, features, and advantages are in the description, drawings, and claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0035]      FIG. 1  is a schematic diagram of an extracorporeal fluid circuit for hemodialysis system. 
           [0036]      FIG. 2  is schematic view of a pressure sensor assembly. 
           [0037]      FIG. 3A  is side view of a transducer protector. 
           [0038]      FIG. 3B  is a cross-sectional side view of the transducer protector of  FIG. 3A . 
           [0039]      FIG. 4A  is a cross-sectional side view of a first part of the transducer protector of  FIG. 3A . 
           [0040]      FIG. 4B  is a cross-sectional side view of a second part of the transducer protector of  FIG. 3A . 
           [0041]      FIGS. 5A-5C  are cross-sectional views illustrating the assembly of the transducer protector of  FIG. 3A . 
           [0042]      FIGS. 6A and 6B  illustrate a dialysis machine measuring pressure patterns of an extracorporeal blood circuit. 
           [0043]      FIG. 7  is a schematic cross-sectional view of an air release chamber with a vent assembly. 
           [0044]      FIG. 7A  is a schematic top view of the air release chamber of  FIG. 7 . 
           [0045]      FIG. 7B  is a schematic bottom view of the air release chamber of  FIG. 7 . 
           [0046]      FIG. 7C  is a schematic perspective view of the air release chamber of  FIG. 7 . 
           [0047]      FIGS. 7D ,  7 E, and  7 F are each schematic cross-sectional views of air release chambers. 
           [0048]      FIG. 8  is a schematic top view of a hydrophobic filter assembly. 
           [0049]      FIG. 8A  is a schematic cross-sectional view of the hydrophobic filter assembly of  FIG. 8 , taken along line  8 A- 8 A. 
           [0050]      FIG. 9  is a schematic top view of a vent structure. 
           [0051]      FIG. 9A  is a cross-sectional view of the vent structure of  FIG. 9 , taken along line  9 A. 
           [0052]      FIG. 10  is a schematic top view of an insert. 
           [0053]      FIG. 10A  is a cross-sectional view of the insert of  FIG. 10 , taken along line  10 A- 10 A. 
           [0054]      FIG. 10B  is a side view of the insert of  FIG. 10 . 
           [0055]      FIG. 11  is a schematic top view of a retainer. 
           [0056]      FIG. 11A  is a cross-sectional view of the retainer of  FIG. 11 , taken along line  11 A- 11 A. 
           [0057]      FIG. 12  is a schematic cross-sectional view of a vent assembly. 
           [0058]      FIG. 13  is a schematic side view of a chamber and port cap than can be assembled to form a bottom entry/bottom exit chamber. 
           [0059]      FIG. 14  is a schematic side view of a bottom entry/bottom exit chamber. 
           [0060]      FIG. 15  is a schematic cross-sectional view of a bottom entry/bottom exit chamber and a vent assembly. 
           [0061]      FIG. 16  is a schematic side view of a bottom entry/bottom exit chamber with a vent assembly. 
           [0062]      FIG. 17  is a schematic top view of a filter assembly. 
           [0063]      FIG. 17A  is a cross-sectional view of the filter assembly of  FIG. 17 , taken along line  17 A- 17 A. 
           [0064]      FIG. 17B  is a side view of the filter assembly of  FIG. 17 . 
           [0065]      FIG. 18  is a schematic cross-sectional view of a vent assembly. 
           [0066]      FIG. 19  is a schematic side view of a bottom entry/bottom exit chamber and a vent assembly. 
           [0067]      FIG. 20  is a schematic cross-sectional view of a bottom entry/bottom exit chamber and a vent assembly. 
           [0068]      FIG. 21  is a schematic side view of a bottom entry/bottom exit chamber and a vent assembly. 
           [0069]      FIG. 22  is a flow diagram for using an air release chamber in an extracorporeal circuit. 
           [0070]      FIG. 23  is a schematic diagram of the blood flow path through an air release chamber. 
           [0071]      FIG. 24  is a schematic diagram of an extracorporeal circuit for a hemodialysis system including pre-pump and post-pump arterial pressure sensor assemblies and a venous pressure sensor assembly. 
           [0072]      FIG. 25  is a schematic cross-sectional view of an air release chamber with a vent assembly having a multilayer vent structure. 
           [0073]      FIG. 26  is a plan view of an integrated extracorporeal circuit. 
           [0074]      FIG. 26A  is a cross sectional view of the integrated extracorporeal circuit of  FIG. 26 , take along line  26 A- 26 A. 
           [0075]      FIG. 27  is a perspective view of the integrated extracorporeal circuit of  FIG. 26 . 
       
    
    
     DETAILED DESCRIPTION 
       [0076]    A fluid circuit, such as an extracorporeal fluid circuit used in filtering blood from a patient during hemodialysis, can be provided with one or more self-sealing vent assemblies to inhibit (e.g., prevent) fluids flowing within the circuit from coming into contact with the surrounding, external atmosphere and/or coming into contact with, and possibly contaminating, neighboring devices. The self-sealing vent assemblies can also inhibit (e.g., prevent) foreign particles and organisms from the external atmosphere from coming into contact with liquid flowing within the fluid circuit. 
       System Overview 
       [0077]    Referring to  FIG. 1 , an extracorporeal circuit  100  includes tubing through which the blood flows and components for filtering and performing dialysis on the blood. Blood flows from a patient  105  through arterial tubing  110 . Blood drips into a drip chamber  115  where a connecting tube  116  from the drip chamber  115  attaches to an arterial pressure sensor assembly  120  on a hemodialysis machine  50  that determines the pressure of the blood on the arterial side of the circuit  100 . The arterial pressure sensor assembly  120  includes a pressure transducer  130 , which can be mounted within a dialysis machine  50 , so that the pressure of blood flowing through the circuit  100  on the arterial side can be monitored. The arterial pressure sensor assembly  120  also includes a transducer protector  140 , which carries a self-sealing vent assembly  141  ( FIG. 3B ) that includes a micro-porous membrane  144  ( FIG. 3B ) and a liquid activated self-sealing vent structure  146  ( FIG. 3B ). The vent assembly  141  helps to protect the pressure transducer  130 , and the dialysis machine  50  in which it is mounted, from direct contact with blood flowing within the extracorporeal circuit  100 . In the event that the micro-porous membrane  144  ruptures, blood will come into contact with the liquid activated self-sealing vent structure  146 . The vent structure  146  will seal, and, by sealing, will inhibit (e.g., prevent) the dialysis machine  50  from becoming contaminated, and will allow the machine  50  to detect a failure via analysis of pressure patterns. 
         [0078]    A pump  160 , such as a peristaltic pump, forces the blood to continue along the path through the circuit  100 . The blood then flows to a dialyzer  170 , which separates waste products from the blood. 
         [0079]    After passing through the dialyzer  170 , the blood flows through venous tubing  180  towards an air release chamber  230  in which gas (e.g., air) in the blood can escape before the blood continues to the patient  105 . During treatment, should air be present in the blood, the blood with air bubbles flows in through the bottom of the air release chamber  230 . The upper motion of the blood is impeded by gravity and becomes stagnant, while the air continues to the top of the chamber  230  where it is vented out to the atmosphere through another self-sealing vent assembly  270 . The vent assembly  270  of the chamber  230  includes a micro-porous membrane and a self-sealing vent. The micro-porous membrane normally operates to inhibit liquids within the chamber from coming into contact with the atmosphere. However, in the event that the micro-porous membrane ruptures, liquid will come into contact with the self-sealing vent, which will self seal and inhibit (e.g., prevent) the blood from coming into contact with the atmosphere. 
         [0080]    After leaving the chamber  230 , the blood travels through a venous line  190  and back to the patient  105 . 
       Pressure Transducer Assembly 
       [0081]    As shown in  FIG. 2 , the pressure transducer assembly  120  includes the pressure transducer  130  and the transducer protector  140 . Referring to  FIGS. 3A and 3B , the transducer protector  140  includes a body  143  that defines a fluid pathway. The body  143  includes a vent assembly compartment  142  in which the micro-porous membrane  144  and the self-sealing vent structure  146  are disposed. A first open end  148  can be connected to the dialysis machine  50 , e.g., via a machine fitment  52  ( FIG. 2 ) and tubing  117 , and provides for fluid communication between the pressure transducer  130  and the vent assembly compartment  142 . A second open end  149  can be connected to the tubing (e.g., connecting tube  116 ) of the extracorporeal circuit  100  ( FIG. 1 ) to provide for communication between the vent assembly compartment  142  and blood flowing within the circuit  100 . This arrangement allows gas (e.g., air) to pass through the vent assembly  141  from the second open end  149  toward the first open end  148 , while inhibiting the passage of blood, and thereby allows the pressure transducer  130  to measure changes in air pressure. 
         [0082]    The micro-porous membrane  144  allows gas (e.g., air) to pass through the vent assembly compartment  142 , but impedes the flow of liquid, thereby inhibiting or preventing the blood from directly contacting, and possible contaminating, the pressure transducer  130  on the opposite side of the vent assembly compartment  142 . The micro-porous membrane  144  can also help to inhibit (e.g., prevent) foreign particles and organisms from entering the extracorporeal circuit  100  from the transducer side of the vent assembly compartment  142 . 
         [0083]    The micro-porous membrane  144  includes a hydrophobic material, such as polytetrafluoroethylene (PTFE) (e.g., expanded polytetraflouroethylene (ePTFE)) backed by a mesh material. In some embodiments, the membrane  144  is a fibrous carrier with a matted and woven layer on top of which ePTFE or other micro-porous material is applied. A suitable membrane has an average pore size of about 0.05 to about 0.45 microns (e.g., about 0.22 microns or about 0.2 microns). Suitable membranes are available from Pall Corporation, East Hills, N.Y., under the Versapor® mark and from W. L. Gore &amp; Associates, Inc., Newark, Del. 
         [0084]    The self-sealing vent structure  146  is a solid porous block, having an average pore size of about 15 to about 45 microns, that allows air to pass through the vent assembly compartment  142 . In some embodiments, the self-sealing vent structure  146  is formed of a blend of polyethylene (e.g., high density polyethylene (HDPE)) and carboxymethylcellulose (CMC), a blend of polystyrene and methyl-ethyl-cellulose or of polypropylene- or polyethylene-based porous material. Such materials are available from Porex Corporation, Fairburn, Ga., such as EXP-816, which is a product containing 90% by weight polyethylene and 10% by weight carboxymethylcellulose with an average pore size of about 30 microns to about 40 microns. However, other percentages of the materials can be used, as well as other materials and other pore sizes. For example, the vent structure  146  can include about 80% to about 95% by weight high density polyethylene and about 5% to about 20% by weight carboxymethylcellulose. 
         [0085]    Referring to  FIGS. 4A and 4B  the body  143  of the transducer protector  140  can be formed from two parts. As shown in  FIG. 4A , a first part  150  defines the first open end  148  and a first portion  151  of the vent assembly compartment  142 . As shown in  FIG. 4B , a second part  152  defines the second open end  149  and a second portion  153  of the vent assembly compartment  142 . The first and second parts  150 ,  152  of the transducer protector  140  can be formed of one or more medical grade materials. Plastics, such as polyvinylchloride, polycarbonate, polyolefins, polypropylene, polyethylene or other suitable medical grade plastic can be used because of their ease of manufacturing, ready availability and disposable nature. The first and second parts  150 ,  152  of the transducer protector can be separately formed, such as by molding (e.g., extruding, blow molding or injection molding). 
         [0086]    The first and second parts  150 ,  152  of the transducer protector  140  each include an associated sidewall  154 ,  155 . The sidewalls  154 ,  155  of the respective first and second parts  150 ,  152  help to retain the micro-porous membrane  144  and the self-sealing vent structure  146  within the vent assembly compartment  142  following assembly. As illustrated in  FIGS. 5A-5C , the transducer protector  140  is assembled by first inserting the micro-porous membrane  144  into the second part  152  in a position in which the micro-porous membrane  144  is disposed within the second portion  153  of the vent assembly compartment  142 . In this position, as shown in  FIG. 5A , the micro-porous membrane  144  is seated against a ledge  156  that is defined by the second part  152 . The micro-porous membrane  144  can be dimensioned such that a press-fit is provided between the micro-porous membrane  144  and the sidewall  155  of the second part  152  of the transducer protector  140 . Next, as illustrated in  FIG. 5B , the self-sealing vent structure  146  is positioned adjacent the micro-porous membrane  144  in a position in which the self-sealing vent structure  146  is partially disposed within the second portion  153  of the vent assembly compartment  142 . The self-sealing vent structure  146  can also be dimensioned such that a press-fit is provided between the vent structure  146  and the sidewall  155  of the second part  152  of the transducer protector  140 . Then, as illustrated in  FIG. 5C , the first part  150  can be connected to the second part  152  of the transducer protector  140  such that the respective sidewalls  154 ,  155  of the first and second parts  150 ,  152  of the transducer protector  140  together define the vent assembly compartment  142 . The first and second parts  150 ,  152  of the transducer protector  140  can be bonded to each other, such as by welding, adhering (e.g., with epoxy), solvent bonding, mating threaded connections or other suitable method. 
         [0087]    Referring to  FIGS. 6A and 6B , pressure can be read out and displayed through the electronics of the dialysis machine  50 . Dynamic pressure pulse variations may take place, and will be transmitted through tubing sections  110 ,  140  to the pressure transducer  130 , for a continuous pressure measurement. The measured pressure pattern is compared to a machine pressure pattern, which is determined as a function of pump operation. If there is a variance between the measured pressure pattern and the machine pressure pattern automatic shut-off can occur and/or an alarm can be sounded. If, for example, the micro-porous membrane  144  ruptures, thereby allowing liquid (e.g., blood) to contact the self-sealing vent structure  146 , the vent structure  146  will self seal and inhibit (e.g., prevent) fluid, including gases, from passing. As a result, as illustrated in  FIG. 6B , the pressure transducer  130  will sense a change in the pressure pattern (e.g., a diminished pressure pulse), which the associated dialysis machine electronics will interpret as a possible membrane rupture. 
       Air Release Chamber 
       [0088]    Referring to  FIGS. 7 ,  7 A,  7 B and  7 C, the air release chamber  230  is substantially hollow for filling with a liquid. The chamber  230  can be used for removing gas (e.g., air bubbles) from blood. The chamber  230  has a bottom region  234  and a top region  236 , where the bottom and top are relative to the chamber&#39;s orientation during use. An entry port  240  and an exit port  242  are in the bottom region  234  of the chamber  230 . In some implementations, the ports  240 ,  242  are located in a bottom surface of the chamber  230 . In other implementations, as shown in  FIG. 7F , at least one of the ports  240 ,  242  is located in a side surface of the chamber  230 . In some implementations, a dam  248  is between the ports  240 ,  242 . The dam  248  extends at least part way from one side wall to an opposite side wall. In some implementations, the dam  268  contacts each side wall so that all fluid entering entry port  240  flows over the top of the dam  248  before flowing out the exit port  242 . In some implementations, a clot filter  254  is positioned adjacent to the exit port  242 . Fluid flows through the clot filter  254  prior to flowing out of the exit port  242 . In some implementations, the clot filter  245  has a porosity of about 50 microns to about 500 microns. 
         [0089]    The ports  240 ,  242  are holes in the chamber  230  which can be in fluid communication with tubular shaped extensions. The extensions are able to be connected to tubes, such as by pressure fitting or bonding. The extensions can be integrally formed with the chamber  230  or subsequently attached to the chamber  230 , such as by bonding or welding. 
         [0090]    At the top region  236  of the chamber  230  is a self-sealing vent assembly  270 . The self-sealing vent assembly  270  includes a micro-porous membrane  260  and a vent structure  264 . The assembly with the vent structure  264  and micro-porous membrane  260  may provide reduced condensation or minimize condensation on the micro-porous membrane  260 . The micro-porous membrane  260  allows gas (e.g., from air bubbles in the blood) to vent from the chamber  230 . Pores in the micro-porous membrane  260  are small enough to keep foreign particles and organisms from entering the chamber  230  from the outside air. 
         [0091]    In some implementations, the membrane  260  includes a hydrophobic material, such polytetrafluoroethylene (PTFE) (e.g., expanded polytetrafluoroethylene (ePTFE)) In some embodiments, the membrane  260  is a fibrous carrier with a matted and woven layer on top of which ePTFE or other micro-porous material is applied. The hydrophobic micro-porous membrane  260  keeps liquid from leaking out of the chamber  230  when the chamber  230  is substantially filled with liquid and allow air to pass through. A suitable membrane has an average pore size of about 0.05 microns to about 0.45 microns (e.g., about 0.22 microns, about 0.2 microns). Suitable membranes are available from Pall Corporation, East Hills, N.Y., under the Versapor® mark and from W. L. Gore &amp; Associates, Inc., Newark, Del. 
         [0092]    The vent structure  264  is a solid porous block, having an average pore size of about 15 micron to about 45 microns, that allows air to pass through and escape from the chamber. The vent structure  264  is also a self-sealing vent structure. In some implementations, the vent structure  264  is formed of a blend of polyethylene (e.g., high density polyethylene (HDPE)) and carboxymethylcellulose (CMC), a blend of polystyrene and methyl-ethyl-cellulose or of polypropylene- or polyethylene-based porous material. Such materials are available from Porex Corporation, Fairburn, Ga., such as EXP-816, which is a product containing 90% by weight polyethylene and 10% by weight carboxymethylcellulose with an average pore size of about 30 microns to about 40 microns. However, other percentages of the materials can be used, as well as other materials and other pore sizes. For example, the vent structure  264  can include about 80% to about 95% by weight high density polyethylene and about 5% to about 20% by weight carboxymethylcellulose. 
         [0093]    When the vent structure  264  comes into contact with liquid, e.g., humidity or moisture, the swelling agent (e.g., cellulose component, e.g., carboxymethylcellulose) of the vent structure expands, thereby closing off the pores in the polymer component (e.g., high density polyethylene) of the vent structure  264 . The vent structure  264  is mounted adjacent to and just above the membrane  260  so that the hydrophobic membrane  260  is located between the vent structure  264  and the chamber  230 . The vent structure  264  inhibits (e.g., prevents) condensation from accumulating on and contacting the membrane  260 . In some embodiments, the vent structure  264  directly contacts the membrane  260 . The vent structure  264  can be substantially disc shaped or can be another shape that is compatible with a chamber on which the vent structure  264  is mounted. In embodiments, the vent structure  264  is about 0.1 mm to about 10 mm thick. 
         [0094]    When the chamber  230  is filled with blood, inhibiting (e.g., preventing) the protein in the blood from accumulating on the membrane  260  can maintain the hydrophobic characteristic of the membrane  260 . Whole blood can be kept from the membrane  260  by providing a barrier between the blood and the membrane  260 , such as a liquid barrier  268 , as described further below. The height of the chamber  230  is sufficient to maintain this barrier  268  and inhibits (e.g., prevents) the liquid above the barrier  268  from substantially mixing with liquid below the barrier  268 . 
         [0095]    The shape of the chamber is approximately elongate. In some implementations, such as those shown in  FIGS. 7 and 7D , the bottom region  234  of the chamber  230 ,  230 ′ is wider than the top region  236 , such that the chamber  230 ,  230 ′ has a quasi-conical shape or a flare at the bottom. In some implementations, such as those shown in  FIG. 7E , the top and bottom dimensions of the chamber  230 ″ are approximately equal so that the chamber  230 ″ has a rectangular or cylindrical shape. The bottom region  234  can also be narrower than the top region  236 . If the ports  240 ,  242  are in the bottom surface of the chamber, the bottom surface has a sufficiently large dimension to accommodate the ports  240 ,  242  as well as any tubes coupled to the ports for directing fluid into and out of the chamber. For example, if the tubing has an outer diameter of 6.25 mm, the bottom surface is at least 12.5 mm wide. The chamber  230  is sized to maintain the liquid barrier  268 . In some implementations, the chamber  230  is at least about two inches in height, (e.g., about three to about four inches). 
         [0096]    The chamber is formed of a material suitable for medical devices, that is, a medical grade material. Plastics, such as polyvinylchloride, polycarbonate, polyolefins, polypropylene, polyethylene or other suitable medical grade plastic can be used because of their ease of manufacturing, ready availability and disposable nature. The chamber is formed, such as by molding, for example, extruding, blow molding or injection molding. The chamber can be formed of a transparent or clear material so that the liquid flowing through the chamber can be observed. 
         [0097]    The construction of the vent assembly  270  is described with respect to the following figures. Referring to  FIGS. 8 and 8A , a ring  302  holds the micro-porous membrane  260  within its inner diameter. The ring can be formed of plastic, such as one of the plastics described herein. The micro-porous membrane  260  can be insert-molded into the ring  302 . That is, the micro-porous membrane  260  can be placed into a mold and held in place. The plastic for the ring  302 , which can be polyethylene, polystyrene or another other suitable material, is then injected into a mold to form the ring  302 . The ring  302  has an inner diameter z and an outer diameter y. Referring to  FIGS. 9 and 9A , the vent structure  264  has a diameter of z. 
         [0098]    Referring to  FIGS. 10 and 10A , an insert  312  is configured to hold the ring  302  and the vent structure  264 . The insert  312  has a first portion  314  and a second portion  316 . The first portion  314  has a greater outer diameter and greater inner diameter than the outer diameter and inner diameter of the second portion  316 . In some embodiments, the inner diameter of the first portion  314  is y and the outer diameter of the second portion  316  is x. The transition between the first portion  314  and the second portion  316  forms a ledge. The insert  312  can be formed of the same plastic or a different material from the plastic ring. 
         [0099]    Referring to  FIGS. 11 and 11A , a retainer  318  is configured to hold the ring  302 , the vent structure  264  and the insert  312 . In some embodiments, the retainer  318  has a constant outer diameter, that is, the outer diameter does not change from one end of the retainer  318  to the other. In some embodiments, the retainer  318  has three unique inner diameters. Near the top (as shown in the figure) of the retainer  318 , the inner diameter is the greatest and in some embodiments, the inner diameter is equal to or just slightly greater than the outer diameter of the first portion  314  of the insert  312 . Near the bottom of the retainer  318 , the retainer  318  can have an inner diameter that is less than z, or less than the diameter of the vent structure  264 . Between the bottom and the top of the retainer  318 , the inner diameter can be about equal to x, that is, about equal to or slightly greater than the outer diameter of the second portion  316  of insert  312 . 
         [0100]    Referring to  FIG. 12 , an assembly  300  can be formed from the ring  302 , vent structure  264 , insert  312  and retainer  318 . The retainer  318  holds the vent structure  264  so that the portion of the retainer with an the inner diameter that is less than the vent structure&#39;s diameter inhibits (e.g., prevents) the vent structure  306  from escaping. The ring  302  is within the inner diameter of the retainer  318  and adjacent to the vent structure  264 . In some embodiments, the ring  302  has sufficient height that the vent structure  264  can be seated within the inner diameter of the ring  302 . The first portion  314  of the insert  312  fits between the outer diameter of the ring  302  and the inner diameter of the retainer  318 . The retainer  318  can be bonded to the insert  312 , such as by welding, adhering, solvent-bonding or other suitable method. The second portion  316  of the insert  312  forms a shank that is sized to fit into a chamber, as described further herein. 
         [0101]    Referring to  FIG. 13 , the chamber can be formed from two parts. A two port cap  322  can form a bottom of the chamber. A gravity chamber  324  can form the top of the chamber. Referring to  FIG. 14 , when the cap  322  and gravity chamber  324  are brought together, they form a chamber body  326 . The top of the chamber body  326  is sized so that the shank of the assembly  300  can be fit into the chamber body  326 , as shown in  FIGS. 15 and 16 . The chamber body  326  and the assembly  300  can be sealed together, such as by welding, adhering, solvent-bonding or other suitable method. 
         [0102]    In other implementations, a different type of assembly can be formed. Referring to  FIGS. 17 ,  17 A and  17 B, for example, a support  328  can have an inner diameter in which the micro-porous membrane  260  is held. The inner diameter of the support  328  can be x. The support  328  can have a flange that extends outwardly from the outer diameter at a top of the support  328 . As shown in  FIG. 18 , the vent structure  264  can fit within the support  328  and on the micro-porous membrane  260 . The micro-porous membrane  264  can be insert-molded into the support  328 . The vent structure  264  can be press-fit into the support  328 . Referring to  FIGS. 19 ,  20  and  21 , the support  328  is sized so that the support  328  fits into a chamber body  326  with the flange extending beyond the inner diameter of the chamber body  326  to inhibit (e.g., (prevent) the support  328  from being pressed in or falling into the chamber body  326 . 
         [0103]    Although the vent assemblies described herein are shown as cylindrical, the assembly can have other shapes as well, such as rectangular, polygon, triangular or other suitable cross sectional shapes. Also, the vent assembly can have a threaded portion so that the assembly can be, for example, screwed into the air release chamber. Alternatively, the vent assembly can be welded, adhered with epoxy or otherwise fastened to the top of the chamber. 
       Methods of Operation 
       [0104]    Referring to  FIGS. 1 and 22 , the air release chamber  230  is in line in the extracorporeal fluid circuit of a system for fluid filtration and air removal. A first liquid that is compatible with the liquid to be filtered (the second liquid) is introduced into the system to prime the system (step  404 ). In hemodialysis, the first liquid is a blood compatible solution, such as saline. The saline flows through the arterial tubing  110  to the arterial pressure sensor assembly 120  so that the pressure of the liquid flowing through the circuit  100  on the arterial side can be monitored, as described above. The saline then flows through a portion of the channel that abuts the pump  160 . The pump  160  forces the saline through the circuit  100 . The saline then flows to the dialyzer  170 . Next, the saline, or the first liquid, flows through the entry port  242  of the chamber  230  and fills the chamber (step  412 ). To fill the chamber completely, venous line  190  can be clamped to create a positive pressure once the saline is introduced into the chamber  230 . Air is forced out the top of the chamber  230  and through the micro-porous membrane  260  and vent structure  264  as saline fills the chamber  230 . The saline contacts the membrane  260  and the chamber  230  is substantially free of air once the chamber  230  is completely filled. However, the saline does not exit through the membrane  260 , because the membrane  260  is hydrophobic. After the venous line  190  is unclamped, the saline exits through the exit port of the chamber and out the venous line  190 . 
         [0105]    The second liquid, such as a bodily fluid, for example, blood, is then introduced into the system (step  418 ). The blood follows the same route as the saline and, for the most part, pushes the saline through the circuit  100 . When the blood enters the chamber  230 , the blood forces the saline at the bottom of the chamber  230  through the exit port (step  422 ). However, the blood does not displace all of the saline within the chamber  230 . Because of the height of the chamber  230 , the blood enters the chamber  230  and only traverses part of the height of the chamber  230  before flowing back down along flow path  274  to the exit port (as shown in the air release chamber formed of transparent material in  FIG. 23 ). An interface  268  between the saline and the blood delineates the furthest extent of most of the blood within the chamber  230 . The interface  268  between the blood and saline can visually be observed and stretches across the entire width of the chamber. Because blood and saline are not immiscible, there is some amount of mixing between the two fluids around the interface  268 . 
         [0106]    The saline keeps the blood from contacting the filter  260 . However, a percentage of blood can be present in the saline without hindering the operation of the circuit  100 . That is, the saline need not be completely free from blood for the air release chamber  230  to both allow gas (e.g., from air bubbles in the blood) to vent from the circuit  100  and retain the liquid in the circuit  100 . The solution that is mostly saline substantially protects the membrane  260  from becoming coated with protein. If the chamber  230  is sufficiently elongated, the blood does not mix with the saline at the top portion of the chamber  230  because the saline remains relatively stagnant as the blood flows through the chamber  230 . 
         [0107]    Any unbound gas, or air, that is in the blood, such as air that is introduced by the dialyzer  170  or air that comes out of solution from the blood, rises as tiny air bubbles within the blood and saline until the air eventually vents out through the vent assembly  270 , including the micro-porous filter  260  and the vent structure (step  430 ). With a dam  248  inside of the chamber  230 , the blood travels up and over the dam  248  rather than straight across the bottom of the chamber  230  out the exit port  242 . By directing the flow of blood upwards, the blood with air is not able to flow in and directly back out of the chamber  230  without flowing upwards to at least a height greater then the height of the dam  248 . The surface area of the dam  248  and the inner walls of the chamber  230  enables air, including microbubbles, to separate from the blood and exit the circuit  100  through the micro-porous membrane  260 . 
         [0108]    Throughout the circuit, the blood flows without there being a substantial air-blood interface. Although the blood does not come into contact with air and thus clotting is less likely to occur, the blood can pass through an optional filter in the chamber. In some implementations, after exiting the chamber, the blood passes by or through one or more sensors, such as temperature or air detecting sensors. 
       Other Embodiments 
       [0109]    While certain embodiments have been described above, other embodiments are possible. 
         [0110]    As an example, although an embodiment of a extracorporeal circuit has been described in which an arterial pressure sensor assembly is arranged to measure a pre-pump arterial pressure, in some embodiments, as illustrated in  FIG. 24 , an arterial pressure assembly  120 ′ can, alternatively or additionally, be positioned downstream of the pump  160  for post pump arterial pressure measurement. In some embodiments, the circuit  100  can also include a venous pressure sensor assembly  182  in communication with the venous tubing  180 , for monitoring the pressure of liquid (e.g., blood) flowing through the circuit  100  on the venous side. The venous pressure sensor assembly  182  can have the same construction as the arterial pressure sensor assembly  120  described above with regard to  FIGS. 3A-5C . 
         [0111]    In some implementations, the vent assembly can include a multilayer self-sealing vent structure, where different layers of the vent structure have differing self-sealing (e.g., swelling) characteristics. For example,  FIG. 25  shows (in cross-section) a vent assembly  270 ′ including a multilayer self-sealing vent structure  264 ′. The multilayer self-sealing vent structure  264 ′ includes a first porous layer  265  disposed adjacent the micro-porous membrane  260 , and a second porous layer  266  disposed adjacent to the first porous layer  265 . The first porous layer  265  is a solid porous block, having an average pore size of about 5 microns to about 45 microns, e.g., about 10 microns. In some embodiments, the first porous layer  265  can be formed of polyethylene (e.g., high density polyethylene (HDPE)), polystyrene, or of polypropylene- or polyethylene-based porous material. Such materials are available from Porex Corporation, Fairburn, Ga. The first porous layer  265  is about 3 mm to about 5 mm thick, e.g., about 4 mm thick. In some embodiments, the first porous layer  265  can be self-sealing. In some embodiments, for example, the first porous layer  265  may include a relatively small amount of carboxymethylcellulose, e.g., about 0% to about 10% by weight carboxymethylcellulose. 
         [0112]    The second porous layer  266  is a solid porous block, having an average pore size of about 15 to about 45 microns, e.g., about 30 microns. The second porous layer  266  is about 3 mm to about 5 mm thick, e.g., about 0.4 mm thick. The second porous layer  266  is self-sealing, and is relatively more responsive to the presence of moisture that the first porous layer  265 ; i.e., the second porous layer  266  has a greater propensity to self-seal (e.g., swell) in the presence of moisture than the first porous layer  265 . In some embodiments, the second porous layer  266  is formed of a blend of polyethylene (e.g., high density polyethylene (HDPE)) and carboxymethylcellulose (CMC), a blend of polystyrene and methyl-ethyl-cellulose or of polypropylene- or polyethylene-based porous material. Such materials are available from Porex Corporation, Fairburn, Ga., such as EXP-816, which is a product containing 90% by weight polyethylene and 10% by weight carboxymethylcellulose with an average pore size of about 30 microns to about 40 microns. However, other percentages of the materials can be used, as well as other materials and other pore sizes. 
         [0113]    During use, condensation can, for example, form within the vent assembly. The first porous layer  265  allows for a small amount of condensation to be compensated for without activation of the self-sealing property of the second porous layer  266 . The first porous layer  265 , being relatively less responsive to the presence of moisture (i.e., as compared to the second porous layer  266 ) slows the progression of moisture from within the chamber  230  toward the second porous layer  266 . The first porous layer  265  provides additional surface area (e.g., within pores) where condensation can be pulled out of the air exiting the vent assembly  270 ′ before it reaches self-sealing, second porous layer  266 . Thus, small amounts of humidity and moisture (e.g., condensation) from within the air release chamber  230  can be compensated for without triggering closure of the self-sealing vent. 
         [0114]    In some embodiments, the air release chamber and one or more other components can be incorporated into an integrated fluid circuit. The integrated fluid circuit has the components described above, such as the air release chamber, formed together in one assembly or integrated molding rather than discrete separate or modular devices. The integrated fluid circuit is adapted to removably seat into a machine, such as a blood purification machine, like a hemodialysis machine The integrated fluid circuit is similar to a cassette or cartridge, where an operator merely snaps the integrated fluid circuit into the machine and after just a few additional connections, begins operation. 
         [0115]    Referring to  FIG. 26 , the integrated fluid circuit  512  has a rigid body  518  and a flexible backing (not shown). The rigid body has a substantially flat surface  520  with one or more concave (when viewed from the backside) portions or recessed portions protruding from a front surface of the body  518 . The flexible backing can be applied so that the backing covers only the recessed portions or so that the backing covers more than just the recessed portions, up to all of the back surface of the rigid body. 
         [0116]    The integrated fluid circuit has a recessed portion that serves as the air release chamber  526 . As with the chamber described above, the air release chamber  526  includes a self-sealing vent assembly  570  at a top region and optionally includes a dam  560  and a clot filter  568 . The vent assembly  570  can be formed separately from the body  518  and fit into the top of the air release chamber  526 , similar to the method described with respect to forming the devices shown in  FIGS. 16 and 21 . Alternatively, a micro-porous membrane and vent structure can be fit into the integrated fluid circuit after the body  518  has been formed, without a support or retainer. 
         [0117]    A first channel  534  in rigid body  518  leads from an edge of the rigid body  518  to a bottom region of the air release chamber  526 . Over one portion of the channel  534 , a venous recess or pocket  548  is formed. The flexible backing backs the venous pocket  548 . The venous pocket  548  is sized so that a transducer in the machine can measure the venous fluid pressure through the flexible backing. A second channel  578  extends from the outlet of the air release chamber  526  to an edge of the rigid body  518 . The first and second channels extend to the same or different edges of the rigid body  518 . The first channel  534  and second channel  578  are in fluid communication with the air release chamber  526 . 
         [0118]    In some implementations, a third channel  584  is formed in the rigid body  518 . The third channel  584  is not in fluid communication with the first or second channels when the integrated fluid circuit is not in the machine or connected to a dialyzer. In some implementations, an arterial pocket  588  is formed along the third channel  584 . The arterial fluid pressure can be measured through the flexible backing of the arterial pocket  588 . One end of the third channel  584  extends to one edge of the rigid body  518  and the other end extends to the same or a different edge, as shown in  FIG. 26 . 
         [0119]    Optionally, a fourth channel  592  extends across the rigid body  518 . A post-pump arterial pocket  562  overlaps the fourth channel  592 . In some implementations, additional recesses and channels are formed in the rigid body. 
         [0120]    In some implementations, tubes  594   a ,  594   b ,  594   c ,  594   d  and  594   e  are connected to the rigid body  518 , such as at the locations where the first, second, third and fourth channels extend to the edges. The tubes are connected to the rigid body using techniques known in the art. In some embodiments, the tubes fit into a pre-formed grooves in the rigid body  518 . The tubes can be pressure fitted into the grooves. In other implementations, the tubes are clipped onto the rigid body  518 . Optionally, at the end of the tubes  594   a ,  594   b ,  594   c  and  594   e  are fasteners for connecting the tubes to components of the machine, such as the dialyzer or to a patient. Tube  594   d  wraps around a peristaltic pump in the machine Tubes  594   a  and  594   e  connect to a dialyzer. Tubes  594   b  and  594   c  connect to a patient. 
         [0121]    Each of the recesses can protrude from the flat surface  520  to approximately the same distance. Alternatively, some of the recesses, such as the channels, may be shallower than other recesses, such as the air release chamber  526 . Referring to  FIG. 26A , a cross section of the integrated circuit  512  shows an outline of the part of chamber  526 , the clot filter  568 , the side of second channel  578 , the membrane  564  and a cross section of the vent assembly  570 . The rigid body  520  can have an overall thickness of less than about 2 mm, such as less than about 1 mm. Flexible membrane  564  covers the back of the rigid body  520 . 
         [0122]    In some implementations, instead of one or more of the channels being formed in the rigid body  518 , a tube is connected directly to a feature in the rigid body. For example, instead of forming second channel  578 , tube  594   b  can be connected directly to the air release chamber  526 . 
         [0123]    In some implementations, the integrated circuit  512  has two rigid sides. The first rigid side is as described above. The second rigid side is substantially flat with openings located adjacent to the pockets formed in the first side. The openings are covered with a flexible membrane. 
         [0124]    In some implementations, the integrated circuit  512  has posts that extend from one or more sides of the circuit. The posts can mate with recesses in the machine, ensuring correct registration of the integrated circuit  512  with components, such as sensors, in the machine. In some implementations, the integrated circuit  512  has latches, clips or other such device for registering the integrated circuit  512  with the machine and locking the integrated circuit  512  in place. 
         [0125]    The machine can have a mechanism that holds the integrated circuit in place. The mechanism can include a door, a locking device or a suction device for holding the integrated circuit in tight contact with the machine. When the integrated circuit is seated in the machine, pressure transducers interface with the flexible backing to directly measure the fluid pressure at each of the corresponding locations. Holding the integrated circuit in contact with the machine allows the pressure transducers to sense flow through the circuit. Once the integrated fluid circuit is plugged into the machine and connected with the machine&#39;s components, an operator uses the integrated fluid circuit in a manner similar to the method of using the circuit chamber  230  described above. 
         [0126]    As with the air release chamber  230 , the rigid body  518  is constructed of a medical grade material. The flexible backing is constructed from a polymer that is flexible and suitable for medical use, such as an elastomer, including silicon elastomers. Other suitable materials include, high and low density poly ethylene, high and low density poly propylene, separately co-extruded mono layers or multiple layers of polyamides, nylons, silicones or other materials commonly known in the art for flexible applications. The backing is attached to the back of the rigid body  518 , such as by laser, ultrasonic or RF welding or with an adhesive. In some implementations, the backing is attached so that the edge of each recess is sealed to the backing. Alternatively, the backing is attached only at the edge of the rigid body. If the backing does not seal the recesses from the flat portions, the machine into which the integrated fluid circuit seats is constructed to apply sufficient pressure to keep the fluid flowing through the circuit from leaking out of the recesses and between the backing and the flat surface  520 . In the back of the rigid portion  518 , ridges can be formed which surround the recesses. The ridges can aid in sealing the flexible membrane to the flat portion  518  when pressure is applied to the circuit. 
         [0127]    In some implementations, injection sites  598  are formed at one or more of the recesses. The injection sites  598  can be used to inject drugs or solutions into the fluid. Suitable injection sites  598  are formed of neoprene gaskets into which a needle can be introduced and removed so that the gaskets do not leak or weep after the needle is removed. 
         [0128]      FIG. 27  shows a perspective view of the integrated fluid circuit  512 . As in  FIG. 20 , the flexible membrane has been removed from the integrated fluid circuit  512  to show the recesses. 
         [0129]    Using the air release chambers described herein in an extracorporeal blood circuit inhibits (e.g., prevents) air from contacting blood flowing through the circuit Inhibiting air in the chamber can reduce the likelihood of forming clots in the blood. In the event that there is air in the blood before the blood exits the chamber, a hydrophobic micro-porous membrane and a self-sealing vent structure at the top of the chamber allows air that enters the chamber to escape. The membrane and vent structure are part of or connected directly to the air release chamber. This allows the air to easily escape from the liquid filled chamber. Thus, lines need not be connected to the top of the chamber for withdrawing air from the circuit. 
         [0130]    The self-sealing vent structure of the vent assembly inhibits (e.g., prevents) moisture or condensation from accumulating on the micro-porous membrane. The micro-porous membrane can lose its ability to vent efficiently if it gets wet. On occasion, the micro-porous membrane can leak due to becoming wet, which may allow blood to escape the chamber. The vent structure of the vent assembly can inhibit (e.g., prevent) the micro-porous membrane from getting wet and leaking blood to the outside of the chamber. In the even that the membrane fails, such as due to a puncture, and fluid passes through to the vent structure, the vent structure swells when it becomes wet. The swollen vent structure inhibits (e.g., prevents) blood from leaking outside of the circuit and into the atmosphere. 
         [0131]    The chamber is first filled with saline before being filled with blood. The chamber has a sufficient height so that after the saline and blood are introduced into the chamber, the saline is located near the top of the chamber and the blood is located near the bottom, and little mixing of the two liquids occurs. The saline inhibits (e.g., prevents) most of the proteins in the blood from contacting the micro-porous membrane of the vent assembly at the top of the chamber. If protein accumulates on the micro-porous membrane, the membrane&#39;s hydrophobic properties can be inhibited, that is, the membrane can wet, allowing liquid to leak from inside the chamber to outside the chamber. Also, if protein collects on the membrane, the membrane may become inefficient at allowing air to pass through. Thus, a sufficiently long chamber allows the saline to stagnate at the top, inhibiting (e.g., preventing) protein from contacting the membrane. 
         [0132]    A dam in the chamber between the entry and exit ports may provide a surface for microbubbles to accumulate. The microbubbles in the blood may then escape through the chamber rather than passing through the exit port. Reducing clot formation and reducing gas in the blood is safer for the patient undergoing hemodialysis. The dam also forces the liquids up into the chamber so that the liquids, and any gases traveling with the liquids, are not immediately pushed out of the chamber before the gas can escape out to the top of the chamber. 
         [0133]    Placing components, such as a pocket for taking pressure measurements, channels for fluid flow and the air release chamber, into a single integrated fluid circuit eliminates multiples separate components. Fewer components are easier for an operator to work with and reduce the risk of operator error. The integrated fluid circuit has a rigid side that maintains the integrity of the components, and flexible portions that allow for taking measurements, such as pressure or temperature measurements. Further, the pockets in the integrated circuit eliminate the need for pressure sensing lines in fluid communication with the top of the chamber. 
         [0134]    The components described herein can be used with other liquids, such as plasma, water, saline, and other medical fluids. Additionally, liquids other than saline can be used to prime the system. Accordingly, other embodiments are within the scope of the following claims.