Apparatus and method for controlling ultrafiltration during hemodialysis

Hemodialysis ultrafiltration apparatus comprises a dialyzer. A closed circuit is provided for supplying fresh dialysate to and for removing spent dialysate from the dialyzer. Blood is supplied to and removed from the dialyzer. The closed circuit for supplying fresh dialysate and removing spent dialysate is apportioned into below mean dialyzer blood pressure and above atmospheric pressure sections, with the dialyzer being positioned in the below mean dialyzer blood pressure section. A mechanism is provided for controlling the pressure of fresh dialysate introduced into the below mean dialyzer blood pressure section to substantially match the pressure on the dialysate side of the dialyzer. A mechanism is provided for controlling the above atmospheric pressure portion of the spent dialysate circuit. A pump removes spent dialysate of equivalent ultrafiltrate volume from the closed circuit to control the rate of ultrafiltrate removal from blood in the dialyzer, thus affecting the pressure on the dialysate side of the dialyzer required to sustain that rate. A method of hemodialytic ultrafiltration is carried out substantially in accordance with the above-described apparatus.

BACKGROUND OF THE INVENTION 
1. Field of the Invention. 
The present invention relates to a volumetric ultrafiltration system and 
its method of use, and more particularly, concerns an apparatus and method 
for controlling ultrafiltration during hemodialysis. 
2. Description of the Prior Art. 
Ultrafiltration is the procedure during hemodialysis wherein excess water 
is removed from the blood. It is well-known to achieve satisfactory 
ultrafiltration by maintaining the dialysate pressure within the dialyzer 
lower than that of the blood pressure. During this procedure, while excess 
water in the blood is removable, the rate of ultrafiltration is a critical 
factor, since rapid removal of water from the blood may traumatically 
affect the patient. Various solutions to the control of the rate of 
ultrafiltration have been proposed, one of which is found in U.S. Pat. No. 
4,021,341. 
More recently, ultrafiltration control has been achieved by utilzation of a 
volumetric system relying upon a principle of volume conservation. One 
such system is described in U.S. Pat. No. 4,209,391. In the patented 
apparatus, a known and equal quantity of fluid is moved into and out of 
the dialyzer by two matched positive displacement pumps. As dialysate is 
moved to the dialyzer, a third pump extracts the programmed amount of 
dialysate from the fresh dialysate supply. The spent dialysate line 
demands a fixed quantity of fluid and the difference that is drawn off the 
fresh dialysate supply is made up by ultrafiltrate drawn across the 
dialyzer membrane. In U.S. Pat. No. 4,209,391, the patentees recognized 
that there are certain errors which arise in attempts to monitor dialysate 
volumes. These patentees contended that the major source of such errors is 
the inclusion of gases in the circulating dialysate. Based on that 
premise, the solution to that problem as proposed in U.S. Pat. No. 
4,209,391 was to remove all gases which may enter the closed circuit in 
the subatmospheric pressure portion from all of the spent dialysate in 
order to achieve actual liquid integrity in a closed circuit. 
However, in an article entitled "The Governing Equation Describing The 
Transient Characteristics Of Hemodialysis Ultrafiltration," by Massie, H. 
L., and Chen, P. I., Thirtieth ACEMB, Los Angeles, California, 5-9 Nov. 
1977, page 158, the authors analyzed the process of hemodialysis 
specifically with respect to machine disturbances and patient blood 
pressure and movement. The authors concluded that dialyzer compliance is 
the primary source of error in the measurement of ultrafiltration, and 
that air displacement of fluid volume is a secondary factor. While the 
above-identified authors suggested an equation useful for dialysis 
simulation, no tangible or physical adaptation of their theory was 
proposed at that time. Specifically, dialyzer compliance is a reference to 
dialyzer membrance compliance and is a function of mean transmembrane 
pressure (TMP) as well as the time function change in TMP. An overall 
characterization of the term "compliance" is the elasticity of the closed 
system. Accordingly, inasmuch as the major source of error in dialysate 
volumetric monitoring has been shown to be the compliance factor, the 
hemodialysis ultrafiltration control system should be designed to account 
for compliance in order to accurately monitor and/or control 
ultrafiltration. It is to such a system that the present invention is 
directed in order to solve the major errors in monitoring dialysate 
volumes. 
SUMMARY OF THE INVENTION 
The hemodialysis ultrafiltration apparatus of the present invention 
comprises hemodialysis means. Means supplies fresh dialysate to and 
removes spent dialysate from the hemodialysis means. Means further 
supplies and removes blood from the hemodialysis means. The dialysate 
supplying and removing means is apportioned into below mean dialyzer blood 
pressure and above atmospheric pressure sections, with the hemodialysis 
means located in the below mean dialyzer pressure section. Means controls 
the pressure of fresh dialysate introduced into the below atmospheric 
pressure section to substantially match the pressure on the dialysate side 
of the hemodialysis means. Ultrafiltrate is removed from the spent 
dialysate means to control the rate of liquid removal from blood in the 
hemodialysis means. 
In a preferred embodiment of this aspect of the invention, the below mean 
dialyzer blood pressure section includes a fresh dialysate supply line and 
a first portion of a spent dialysate removal line. The first positive 
displacement unit is in fluid communication with the fresh dialysate 
supply line. Each of the units includes valves and interrelated switching 
means. The above atmospheric pressure section of the preferred 
ultrafiltration apparatus includes a dialysate access line interconnected 
to the fresh dialysate supply line by pressure reducing means. A second 
portion of the spent dialysate removal line is in the above atmospheric 
pressure section and is interconnected to the first portion of the spent 
dialysate removal line by pressure increasing means. The second positive 
displacement unit is in fluid communication with the second portion of the 
spent dialysate removal line. In addition, the above atmospheric pressure 
section includes a dialysate drain line in fluid communication with the 
second portion of the dialysate removal line. Actuatable ultrafiltrate 
removal means is connected between the second portion of the spent 
dialysate line and the drain line, and is adapted to withdraw dialysate 
from the removal line and transfer same to the drain line. The 
aforementioned valves and switching means associated with the first and 
second positive displacement units are adapted to cause concurrently one 
of the units to fill with fresh dialysate as fresh dialysate is supplied 
to the dialyzer while the other unit fills with spent dialysate as spent 
dialysate is drained, and to alternate functions after the valves are 
reversed. Another embodiment of this aspect of the invention is to plumb 
the units so that concurrently one of the units fills with fresh dialysate 
as spent dialysate is drained while the other unit fills with spent 
dialysate as fresh dialysate is supplied to the dialyzer, and to alternate 
functions after the valves are reversed. 
In a further aspect of the present invention, a method of hemodialytic 
ultrafiltration comprises the steps of supplying fresh dialysate to and 
removing spent dialysate from hemodialysis means by means apportioned into 
below means dialyzer blood pressure and above atmospheric pressure 
sections. The hemodialysis means is located in the below mean dialyzer 
blood pressure section. Blood is supplied to and removed from the 
hemodialysis means. Further, the method includes controlling the pressure 
of the fresh dialysate introduced into the below mean dialyzer blood 
pressure section to substantially match the pressure on the dialysate side 
of the hemodialysis means. A controlled volume of spent dialysate is 
removed from the fluid-tight portion of the spent dialysate circuit for 
controlling the rate of ultrafiltrate removal from blood in the 
hemodialysis means. 
In accordance with the principles of the present invention, a volumetric 
ultrafiltration system for hemodialysis is provided in which errors due to 
compliance of the closed circuit system are eliminated or minimized. 
Stated another way, in order to achieve actual volumetric liquid integrity 
in a closed circuit, it is desirable to control system compliance. As 
alluded to above, the most significant factors in the volumetric 
ultrafiltration system contributing to compliant error are the dialyzer 
and entrained air. Accordingly, the present design, in contrast to 
previous ultrafiltration systems known in the prior art, emphasizes 
controlling the pressure of the fresh dialysate introduced into the closed 
circuit to match the pressure on the dialysate side of the dialyzer, 
thereby substantially eliminating compliance error. In addition, an 
embodiment of the present invention is adapted to remove gases from the 
incoming dialysate in order to further minimize or reduce errors in 
monitoring dialysate volume. Further still, the atmospheric pressure pump 
preferably positioned in the spent dialysate line effectively reduces the 
volume that the gases occupy in the closed circuit as well as place the 
ultrafiltrate removal means (the positive displacement pump) into a 
position where its performance is most accurate. 
Preferably, the present invention differs from prior art ultrafiltration 
systems by arranging matched, commonly driven positive displacement units 
and a third independently driven positive displacement pump in an 
atmospheric pressure portion of a closed circuit which has as its boundry 
at each end one of the two matched positive displacement units. The 
positive displacement pump provides a mechanism for removing a controlled 
volume of spent dialysate from the fluid tight portion of the spent 
dialysate circuit and is positioned in the post-dialyzer region (spent 
dialysate) of the closed circuit in the above atmospheric pressure portion 
thereof. Preferably, the atmospheric pressure pump separates the positive 
displacement pump from the dialyzer dialysate outlet. 
There are also notable distinctions between the present invention and the 
apparatus and method described in U.S. Pat. No. 4,209,391. Although the 
apparatus in the just-mentioned patent and the present apparatus 
preferably employ dual, commonly driven positive displacement units for 
the dialysate proportioning system, as well as a third independently 
driven positive displacement pump, the pressure apportioning and controls 
are significantly different between the two systems. The apparatus of U.S. 
Pat. No. 4,209,391 isolates the two positive displacement units from the 
dialyzer by means of a pressure reducer prior to the dialyzer and a 
positive pressure pump after the dialyzer. Accordingly, the pressures of 
the two piston-cylinder units are always maintained at a positive 
pressure, or at an above atmospheric pressure. The major contribution 
according to the patentees of U.S. Pat. No. 4,209,391 is the inclusion of 
the degassifier in the spent dialysate delivery line adjacent to the 
chamber of the piston-cylinder unit to be filled with spent dialysate. 
Moreover, the aforementioned patentees regulate the positive pressure to 
the post dialyzer piston-cylinder in reference to a predetermined above 
atmospheric pressure piston-cylinder and do not control pressure changes 
or fluctuations which the dialyzer may be exposed to as a result of 
changes to the incoming pressures. On the other hand, the present 
invention demonstrates the recognition that compliance serves as the major 
contributor to ultrafiltration measurement error. To overcome such error 
due to compliance, the present invention controls the supply fresh 
dialysate pressure so that the dialyzer perceives as close to a zero 
pressure change as is possible when the supply fresh dialysate is 
introduced into the closed circuit. In accordance with the foregoing, the 
present invention affords ultrafiltration control during hemodialysis on 
an accurate basis.

DETAILED DESCRIPTION 
While this invention is satisfied by embodiments in many different forms, 
there is shown in the drawings and will herein be described in detail a 
preferred embodiment of the invention, with the understanding that the 
present disclosure is to be considered as exemplary of the principles of 
the invention and is not intended to limit the invention to the embodiment 
illustrated. The scope of the invention will be measured by the appended 
claims and their equivalents. 
Adverting to the drawings, and FIG. 1 in particular, there is illustrated a 
schematic representation of the preferred apparatus 10 for controlling 
ultrafiltration during hemodialysis, it being understood that only the 
major components are represented herein, with minor components being well 
within the purview of one skilled in the art to ascertain. Sufficient 
quantities of water for the dialysis procedure are available from water 
supply 11 which is controlled by an on/off valve 12 in accordance with the 
control of the apparatus. An appropriate water filter 14 purifies the 
water prior to subsequent dialysis. A pressure regulator 15 monitors and 
controls the water pressure so that uniformity can be achieved prior to 
the water entering the heater and heat exchanger 16. Water is heated in 
the heater/heat exchanger to a temperature of 38.degree. C. before 
entering the volumetric proportioning unit 18 through feedline 19. 
Volumetric proportioning unit 18 includes a series of valves 20, 21, 22, 
23 and 24. Valves 20 are associated with the post-dialysis function; 
valves 21 are associated with the pre-dialysis function; valves 22 are 
associated with the drive function for incoming water and outgoing 
dialysate; valves 23 are associated with the flow of acid concentrate; and 
valves 24 are associated with the flow of bicarbonate concentrate. The 
aforementioned valves control and regulate the flow of various liquids 
through manifold 26 which is in fluid communication with a series of 
piston-cylinder units 28, all of which are part of the volumetric 
proportioning unit whose specific function with respect to fresh and spent 
dialysate will be described more completely hereinafter. Acid concentrate 
29 is supplied to the volumetric proportioning unit through valves 23, 
whereas bicarbonate concentrate 30 is supplied to the volumetric 
proportioning unit through valves 24. Supply water leaves the volumetric 
proportioning unit through valves 22 into an air removal access line 31. 
Acid concentrate and bicarbonate concentrate are supplied to mix point 37 
where the mixture of concentrates and the supply water result in fresh 
dialysate supplied to pressure reducing means 71. 
Supply water passes through a flow valve 32 and also through an air removal 
pump 34 wherein air and other gases are driven from the water. Another 
pump 35 acts on the de-airified water to assist in the control of fresh 
dialysate pressure by the pressure reducing means 71. De-airified supply 
water then passes through an air trap 36 prior to entering the mix points 
and fresh dialysate supply line 38a. The air trap is intended to assure 
the removal of any entrained air in the supply water prior to mixing and 
the fresh dialysate re-entering the volumetric proportioning unit through 
valves 21. The specific proportioning and flow activities of the fresh 
dialysate in conjunction with the volumetric proportioning unit will be 
described in more detail hereinafter in conjunction with FIG. 2. 
As illustrated in FIG. 1, fresh dialysate exits the volumetric 
proportioning unit through valves 21 into a continuation of the fresh 
dialysate supply line 38b. Before fresh dialysate is delivered to the 
dialyzer, it passes through a conductivity probe 40, by-pass valve 41 for 
by-passing the dialyzer, flow/by-pass fail monitor 42, flow meter 44 and 
pressure relief valve 45, all of which assist in the control, regulation 
and safety procedures for the flow of fresh dialysate into the dialyzer. A 
dialyzer 48 is provided and may be any of the well-known dialyzers useful 
for hemodialysis and including a membrane 49 therein adapted to remove 
waste materials and ultrafiltrate from the blood. Fresh dialysate enters 
the dialyzer through an inlet port 50, and after collecting waste 
materials and ultrafiltrate from the blood, spent dialysate exits the 
dialyzer through outlet port 51 and enters spent dialysate removal line 
55. The hemodialyzer, of course, includes a blood inlet port 52 through 
which blood from a hemodialysis patient enters; a blood outlet port 54 is 
provided on the hemodialyzer to return blood, from which waste materials 
and ultrafiltrate have been removed, to the patient. 
Spent dialysate is driven by pump 56 through spent dialysate line 57a which 
includes a branch connection 58 therein. Spent dialysate passes through 
branch connection 58 and enters volumetric proportioning unit 18 through 
valves 20. In addition, spent dialysate is removed from the spent 
dialysate line through branch line 59. An independently driven, positive 
displacement pump 60 withdraws a volume of spent dialysate from the spent 
dialysate line at a pre-determined and controlled rate, equivalent to the 
volume of ultrafiltrate to be removed from the blood, preferably 
electrically and automatically. By controlling the rate of withdrawal of 
spent dialysate from the spent dialysate removal line, the rate of 
ultrafiltrate removal from the blood passing through the hemodialyzer can 
be controlled. Spent dialysate withdrawn by the positive displacement pump 
is passed through line 61 which joins with the continuation of the spent 
dialysate removal line 57b. After spent dialysate passes through the 
volumetric proportioning unit it exits through valves 20 into the 
continuation of spent dialysate removal line 57b. A pressure release valve 
62 is preferably provided in the spent dialysate removal line primarily 
for safety purposes; a blood leak detector 64 is also preferably provided 
in the spent dialysate removal line to monitor potential defects in the 
dailyzer membrane which would allow blood to pass into the spent dialysate 
rather than return to the patient. Pressure release valve 62 also provides 
the circuit for operation of the device in a conventional transmembrane 
pressure control hemodialysis procedure by control of atmospheric pressure 
pump 56. 
After spent dialysate passes through heat exchanger 16, it is delivered 
into a dialysate drain line 65 which leads the spent dialysate to a drain 
66, whereupon the cycle of the above described hemodialysis procedure with 
ultrafiltration control is completed. 
Referring now to FIG. 2, the simplified schematic therein illustrates the 
preferred closed circuit of fluid flow of the above-described apparatus. 
FIG. 2 depicts the fluid-tight area of the hemodialysis apparatus just 
described, and essentially includes those components within the dotted 
line area of FIG. 1. Supply water incoming through access line 33 is 
normally maintained at an atmospheric pressure level, due to air trap 36 
acting on same. For exemplary purposes only, the pressure of deairated 
water in the access line may range between atmospheric (0 mm H.sub.g) and 
positive 180 mm H.sub.g. After deairated water passes through air trap 36 
and the mix points 37, fluid-tightness of the closed circuit is desired. 
To this end, and for purposes of reducing the pressure of the fresh 
dialysate to that below mean dialyzer blood pressure resultant from the 
transmembrane pressure required to supply the volume of spent dialysate 
demanded by the UF pump 60, a pressure regulator 71 is provided. This 
pressure regulator may be nothing more than a controlled orifice or a 
controlled pump. This pressure regulator, while schematically illustrated 
adjacent air trap 36 and mix points 37, may be included anywhere in fresh 
dialysate supply line 38a between the air trap and valves 21 in the 
volumetric proportioning unit. Upon passing through pressure regulator 71, 
the pressure in fresh dialysate supply line is reduced to a range 
preferably within 0 to 10 mm H.sub.g of the required below mean dialyzer 
blood pressure resultant from the controlled rate of the ultrafiltration 
pump. 
It can be seen in FIG. 2 that fresh dialysate supply line 38a bifurcates in 
the below mean dialyzer blood pressure region, forming two fresh dialysate 
supply lines, 38a' and 38a". Fresh dialysate supply line 38a' and 38a" are 
directed to positive displacement unit 28a. Positive displacement units 
28a and b are preferably double acting piston-cylinder units which include 
a piston 72a and 72b, respectively, inside of each unit. These pistons are 
commonly driven by a linked-drive mechanism 74 and serve to separate each 
piston-cylinder unit into two chambers, i.e., chambers 75 and 76 in piston 
unit 28a, and chambers 78 and 79 in piston unit 28b. Each of the 
aforementioned chambers includes a valve associated therewith for 
regulating dialysate flow into and out of each chamber. These valves 20 
and 21 are illustrated in FIG. 2 as ports 20a through 20d and 21a through 
21d, respectively. While not illustrated in FIG. 2, both of the valves 20 
and 21 are controlled by switching means, preferably electrically, so that 
for each pair of ports for each chamber, one port is closed, while the 
other port of such pair is open. Further, the switching means is designed 
to alternate or reverse the open and closed conditions of the 
aforementioned pairs of ports. The alternating position of the respective 
valves occurs in conjunction with the reciprocating movement of the 
pistons inside of the piston-cylinder units which allows the cylinder 
units to alternately fill one of its chambers with dialysate while the 
other of such chambers is emptying. This arrangement provides the 
volumetric proportioning of dialysate which enables the integrity of the 
closed circuit to be maintained for accurate control of ultrafiltration. 
Though illustrated as double acting piston-cylinder units, any pair of 
matched positive displacement units will provide the volumetric 
proportioning of dialysate which enables the integrity of the closed 
circuit to be maintained for accurate control of ultrafiltration. 
Specifically, and referring to fresh dialysate supply line 38a', it can be 
seen that fresh dialysate enters chamber 75 through open port 21a, thereby 
filling chamber 75 since port 21b is closed. Piston 72a is moving upwardly 
as viewed in FIG. 2 so as to cause the fresh dialysate to enter chamber 
75. Each positive displacement unit may receive, for example, 
approximately 500 ml/min. of dialysate during the operation of the 
apparatus being described. Inasmuch as piston 72a is moving upwardly 
allowing chamber 75 to fill with fresh dialysate, chamber 76 (having been 
previously filled with fresh dialysate) is being compressed. With valve 
21d closed and valve 21c open, fresh dialysate is thus forced out of 
chamber 76 into fresh dialysate supply line 38b, as will be referred to 
hereinafter. While fresh dialysate from supply line 38a is entering piston 
unit 28a, the opposite is occurring in piston-cylinder 28b. Particularly, 
valve 20d is closed while valve 20c is open; with piston 72b moving 
upwardly, as viewed in FIG. 2, chamber 79 becomes compressed. Spent 
dialysate, previously supplied to chamber 79 when the valves were 
reversed, is thus forced out of that chamber into one of the spent 
dialysate removal lines 57b. With spent dialysate being forced out of 
piston unit 28b due to the upward movement of the piston therein, chamber 
78 fills with spent dialysate through open valve 20a, while valve 20b 
remains closed. Having just explained the operation of the piston-cylinder 
units, it is understood that the conditions of the aforementioned chambers 
alternate by virtue of the reciprocative motion of the respective pistons 
and the operation of switching the positions of each pair of valves 
associated with each chamber. 
Following the course of the fresh dialysate from pressure regulator 71, 
such fresh dialysate remains at a below mean dialyzer blood pressure level 
through the piston unit as it travels toward the hemodialyzer. As seen in 
FIG. 2, fresh dialysate supply line 38b feeds into hemodialyzer 48 at 
inlet port 50. Spent dialysate exits hemodialyzer 48 through outlet port 
51 and enters spent dialysate removal line 55. Both the hemodialyzer and 
the portion of the spent dialysate removal line, segment 55, are 
maintained in the below mean dialyzer blood pressure section of the 
presently described closed fluid circuit. With the entire fresh dialysate 
supply line, through the respective chambers of the piston unit, the 
dialyzer and a segment of the spent dialysate removal line all being in 
the below mean dialyzer blood pressure section of the closed fluid 
circuit, control of system compliance is achieved. The pressure of the 
fresh dialysate introduced into this closed circuit is controlled to match 
the pressure on the dialysate side of the dialyzer (as opposed to the 
blood side of the dialyzer). This control and matching of the dialysate 
pressure permits the dialyzer to have as close to a zero change of 
pressure as is possible. 
Positive pressure pump 56 is placed in the spent dialysate removal line to 
act upon the spent dialysate to increase its pressure to a level at or 
above that of atmospheric pressure, in the range of about 0 (atmospheric) 
to positive 150 mm H.sub.g. The positive pressure pump also contributes to 
the removal of gases since it reduces the volume that the gases may occupy 
in the closed circuit. After passing through pump 56, the spent dialysate, 
now in the above atmospheric pressure section, enters the continuation of 
spent dialysate removal line 57a. As in the fresh dialysate line described 
above, spent dialysate removal line 57a bifurcates into two lines 57a' 
directed toward chamber 78 of piston unit 28b, and line 57a" directed 
toward chamber 79 of piston unit 28b. As explained above, spent dialysate 
enters, for example, chamber 78 in the one piston unit while being 
prevented from entering chamber 79. The opposite occurs when the 
respective valves are reversed in accordance with the switching mechanism 
built into the apparatus. Spent dialysate passing through the respective 
piston unit enters a continuation of spent dialysate removal line 57b 
which is in fluid communication with dialysate drain line 65 through which 
spent dialysate is drained. Spent dialysate removal line 57b, as well as 
drain line 65, is maintained at an above atmospheric pressure level 
consistent with the back pressure of the heat exchanger and drain line. 
Spent dialysate is withdrawn from spent dialysate removal line 57a through 
branching connection 58, whereupon the withdrawn spent dialysate travels 
through line 59. As described above, ultrafiltrate pump 60 operates to 
remove a measured volume of spent dialysate from the spent dialysate 
removal line. Pump 60 is preferably driven independently from driving 
mechanism 74 associated with the driving function of the piston units. 
Mbreover, it is preferred that pump 60 be a positive displacement pump. 
Anothger embodiment of this aspect of this invention may be a pump coupled 
with a flow measurement device. Independent control of the desired 
quantity of spent dialysate to be withdrawn demands an equal quantity of 
ultrafiltrate removal from the blood in the hemodialyzer due to the fluid 
tight integrity of the closed system. Due to the tight hydralic quality of 
the closed circuit and the volumetric accuracy of the matched positive 
displacement units 28a and 28b of the present invention, when pump 60 is 
not operating and valve 81 is closed, there is no resultant 
ultrafiltration. In this regard, ultrafiltration is controlled through the 
actuation of pump 60; further, operation of pump 60 and valves 81 and 82 
allow the quantity of dialysate removed from the spent dialysate removal 
line to be appropriately varied. Spent dialysate of equivalent 
ultrafiltrate volume withdrawn by virtue of pump 60 then travels into 
removal line 61 which communicates with the continuation of spent 
dialysate removal line 57b for eventual draining. 
Another branch of spent dialysate removal line 57a includes pressure relief 
valve 62 therein to safely regulate the eventuality of excess pressures. 
Valve 62 also provides the mechanism for operation of this invention in a 
conventional TMP control hemodialysis procedure by control of atmospheric 
pressure pump 56. 
In accordance with the foregoing apparatus and process, the withdrawal of 
spent dialysate from the fluid circuit controls the rate and quantity of 
water removal from blood in the hemodialyzer, thereby achieving 
ultrafiltration. Accuracies are also achieved and improved by virtue of 
the volumetric proportioning approach utilized by the present invention, 
and the matching of pressure on the fresh dialysate line prior to the 
inlet piston units with the pressure on the dialysate side of the 
hemodialyzer. This matching of pressures effectively controls and negates 
system compliance, attributed most significantly to the dialyzer and 
entrained gases, and therefore enhances accuracy since compliance has been 
deemed to be the primary source of error in the measurement of 
ultrafiltration.