turning now to the drawings , in which like numerals refer to like portions thereof , in the block diagram shown in fig1 a patient is designated 1 . blood is withdrawn from patient 1 by means of a cannula 2 , and is then conducted through duct 3 into a blood treatment system . the blood is eventually returned to the patient through duct 4 and cannula 5 . for reasons of safety , these ducts 3 and 4 can be shut off and opened with the help of clamps 6 and 7 , respectively . after clamp 6 , the blood passes an arterial pressure gauge 8 , which is arranged upstream of a peristaltic pump 9 , by means of which circulation of the blood in the system is achieved . from the pump 9 the blood is pressed through inlet 10 into filter 11 , and then out of filter 11 through outlet 12 . the invention is particularly intended to be utilized in conjunction with hemofiltration or plasmapheresis , and the filter thus shown in fig1 is a hemofilter or a plasmapheresis filter , respectively . the scope of the present invention , however , is not intended to be limited to these particular examples . from outlet 12 , the blood passes through duct 13 to drip chamber 14 , which is coordinated with a venous pressure gauge 14a . a replacement fluid is also conducted to the drip chamber 14 , through a duct 15 . from the drip chamber , the blood mixed with replacement fluid is then conducted via duct 16 , clamp 7 and cannula 5 , back to the patient 1 . from the filter 11 , filtrate is withdrawn through a duct 17 , the pressure being measured by means of a pressure gauge 18 . in order to draw the filtrate out of the filter 11 , a pump 19 is utilized . this pump 19 is preferably a conventional peristaltic pump . a check valve 100 is inserted in accordance with the present invention which will be described in greater detail in connection with fig2 and which effectively prevents any back - flow into the peristaltic pump 19 . after the check valve 100 , the filtrate then passes into a pressure equalizing device 20 , which too will be described in greater detail in connection with fig2 . the inlet to the pressure equalizing device is designated 21 , and the outlet is designated 22 . the filtrate is then passed by means of a duct 23 , via a check valve 24 , and a suitable , controllable ultrafiltration valve 25 to a flow meter 26 . this may be designed , for example , in accordance with the european patent application no . ep 0 106940 , but other designs can , of course , be substituted therefor . from the meter 26 the filtrate passes through duct 27 either to a collecting point , if it is to be retained , or to a drain 40 . a replacement fluid prepared in a separate set - up is supplied to the meter 26 through duct 28 , conductivity meter 29 , and temperature measuring device 30 . the meter 26 preferably comprises a first part 26a , which is used to measure the difference between flows in the ducts 27 and 28 . moreover , as a check , measurement of the individual flows in parts 26b and 26c can also take place . furthermore , in order to effect possible adjustments in the measured flow on the basis of the temperature , meter 26 may be coordinated with a temperature measuring device 26d . from meter 26 , the replacement fluid is delivered to a valve 31 , which on the basis of the values measured on measuring devices 29 and 30 then conducts replacement fluid either to a by - pass duct 32 , via check valve 33 , and from there to discharge pipe 27 , or to a duct 34 , and via check valve 35 , to pressure equalizing device 36 , infusion pump 37 , and possibly to a filter system 38 , a duct 15 , and then to drip chamber 14 . finally , number 39 in fig1 designates a valve which allows flow only when pressure exists in the system , but which immediately terminates the connection to drain 40 if the pressure measured at that point is greater than that in the remainder of the system . in such a case , an air gap is then created , so that it becomes impossible for fluid to be conducted from the drain 40 back into the system . in fig2 are shown some preferred designs of components and , more particularly , the pump 19 , the check valve 100 and the pressure equalizing device 20 . the pump 19 is shown schematically as a conventional peristaltic pump comprising a rotor 19a with two rollers 19b which are adapted to press a pump segment 19c against a holding - up element not shown . with the help of the pump the filtrate is forced into the check valve 100 . the filtrate enters an inlet 100a of a first container 100g . an open end of a collapsible second container 100c is coupled to the inlet 100a . the filtrate flows into the second container 100c , and out through a hole 100e therein , preferably polygonal , particularly , triangular in shape . the filtrate then passes into a padlike outer part 100f of the first container 100g , and then leaves the check valve 100 via an outlet 100b of the first container 100g . in response to filtrate entering the outlet 100b , the second container 100c collapses , thereby preventing back - flow . preferably , the collapsible second container 100c comprises two thin , elongated plastic films extending from the inlet 100a . the plastic films face each other , and are joined along their peripheries 100d . the first container 100g is preferably tubular in shape . from the check valve the filtrate flows via the duct 21 to the pressure equalizing device 20 which has been given a modified design as compared with that shown in fig1 . more particularly , the pressure equalizing device 20 comprises an outer shell 20a with an inlet 20b and an outlet 20c . three annular defrothing elements 20f are suspended with the help of a rod 20d and a lock washer 20e . they may consist , for example , of a polyurethane foam coated with a special defrothing agent . as described in greater detail in the above - mentioned european patent application no . 85 . 106708 . 2 , the level in the device 20 is maintained constant with the help of a gas - permeable but liquid - tight membrane 20g which communicates via a duct 20h with an air outlet 20i . numeral 20j designates an extra outlet which may be used , for example , for sampling or the like . u . s . pat . no . 4 , 198 , 971 describes a drip chamber of similar design . it will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention . all such modifications and variations are intended to be included within the scope of the invention as defined in the appended claims .
a blood filtering system includes a check valve for preventing fluid back - flow , coupled between a peristaltic pump and a pressure equalizer , the valve including a tubular container having an inlet and an outlet , two elongated plastic films in the container extending from the inlet facing each other , joined at their peripheries , with a hole in an end of one film proximate the container outlet . fluid may flow from the inlet through the films to the outlet ; but , back - flow is prevented by collapse of the films .