turning now to fig1 there is shown a closure element 1 constructed in accordance with the invention , which is inserted in place of the natural mitral valve in a human heart 2 . the mitral valve plane is identified in fig1 with the numeral 6 . the closure element 1 is shown in the open condition and is formed of a valve housing 3 , a suture ring 4 , which serves to fasten the valve housing in the tissue , and a membrane 5 movable back and forth between an open position and a closed position . in the open condition the membrane takes a position approaching parallelism to the vertical middle axis of the left ventricle 7 and therewith also , in general , to the wall of the chamber 8 . the inclination of the mitral valve plane 6 relative to the longitudinal axis of the base of the aorta amounts on the average to 45 °. the base of the aorta , in turn , has , on the average , an inclination of 20 ° relative to the vertical middle axis of the left ventricle and therewith also the the chamber wall . thus , the angle between the valve plane 6 and the vertical heart axis 7 amounts to 65 °. the closure element according to the invention is shown in a position in fig1 by which , as a result of the atrium 9 pressure exceeding the pressure in the ventricle , the membrane has been widely opened and blood flows in the ventricle 8 parallel to the chamber walls . this blood forms an eddy behind the membrane sail . in the systole , when the ventricle pressure exceeds the atrium pressure , the membrane is rapidly transferred to the closed position which is exemplarily shown in fig2 b . the membrane is in this position fully seated in housing 3 . there thus results an undisturbed flow parallel to the membrane and therewith to the mitral valve plane 6 . in fig2 - 4 the closure element according to the invention is shown both in the open , as well as in the closed , position in perspective view and in the xz -, yz -, and xy - planes . it will be appreciated that the gate is formed in the form of a thin flexible membrane 5 . this membrane generally represents that part of the surface of an elliptical cylinder ## equ3 ## which is cut out by a circular cylinder x 2 + y 2 = r i 2 , whereby r i is the radius of the circular cylinder and α is half the valve opening angle and whereby the angle between the central axes of the two cylinders in the closed condition of the membrane amounts to 90 ° and in the open condition 90 °- 2 α . as will be seen from fig1 almost ideal operation is obtained when the valve opening angle 2α amounts to 65 °, that is , the membrane , in this case , lies , in the open condition , substantially parallel to the ventricle wall . when the angle α lies in a range of 20 °- 45 ° can also , however , good results be obtained . the valve housing 3 represents generally a part of the surface of a circular tube with an inner radius r i and an outer radius r a which is cut by the elliptical cylinder under an angle of 90 °. the membrane 5 is fastened along the half periphery of the line of intersection 13 between this pair of cylinders between the pair of extreme points 14 of the line of intersection . with the herein exemplarily disclosed embodiment , the membrane is not entirely symmetrical ; particularly in the x - direction it is formed slightly longer than 2r i , so that in the closed condition , it extends in the x - direction from the housing inner edge to the oppositely lying housing outer edge . the membrane can thus support itself in this manner in the closed condition on the edge 12 of the housing wall . that means that the surface of the membrane in its closed condition is bordered through two edges running parallel to the y - axis which , respectively , under an angle α and β are oppositely inclined to the x - axis , whereby the angle β equals arc tan ( r i / r a tan α ). the angle β , which is inserted in fig5 is thus slightly smaller than angle α . as can be particularly appreciated from fig3 and 4 , the chamber housing does not fully correspond to the form of a circular tube . rather , the outside of the valve housing 3 is formed tapered in the direction of its extreme points 14 . these tapered regions 11 are not of essential significance for the functioning of the closure element according to the invention . the height of the valve housing corresponds to the minor axis of the ellipse of the membrane generating cylinder part plus the surrounding rim 10 , which serves essentially for the mounting of suture ring 4 . the height s of this surrounding rim 10 amounts to approximately ( 0 . 1 - 0 . 3 ) 2r i . in fig5 the membrane and the valve housing are shown in projection view in the perspective planes , whereby additionally , the development of the membrane and the construction of the ellipse are shown . referring now to fig6 it will be appreciated that fig6 a is a representation of the valve housing 3 in section . fig6 b shows a view of the valve housing from below . the peripheral rim 10 of the valve housing 3 has on its inner side a recess 15 suitable for the accommodation of affixing ring 17 for the membrane . this recess extends on the inside of the housing over half the periphery of the housing between the pair of extreme points 14 and is variable in dimension as shown in fig6 a and 6c . . the membrane is fastened in this recess by means of an affixing ring , which , for example , can be provided by means of a thin - walled plastic or metal ring with the aid of a shrink fit . naturally , other fastening techniques for these affixing rings will also occur , for example , with the aid of screws . it is of significance that the affixing ring occurs entirely in the recess 15 so that a smooth housing inner surface emerges which presents no resistance to blood passing through . on the outer side of housing periphery 10 is likewise a recess 16 arranged which serves for the accommodation of the suture ring which is required for the fastening of the valve housing in the tissue . fig6 b shows clearly how the membrane 5 is fastened in a peripheral half of the valve housing and supports itself on the other half against the rim 12 of the housing wall , so that it extends from the intersection point of the housing wall with the x - axis to the outer edge of the housing wall . the outer radius r a of the valve housing ( 3 ) is ( 1 . 1 - 1 . 3 ) times the inner radius r i . as concerns the production of the individual parts of the closure element according to the present invention , it should be noted , that the membrane for example , can be produced by a dipping process . thus , a preformed master pattern is dipped in a fluid solution of a plastic compatible with the blood , for example , a block copolymer based on a polyurethane or silicone rubber . the number of dipping operations determines the membrane thickness which should amount to approximately 0 . 2 - 0 . 4 mm . the valve housing 10 can be produced from a metal compatible with the blood , for example , titanium alloy or stainless steel . alternatively , the use of pyrolytic carbon or a plastic compatible with the blood would be possible . the suture ring is preferably produced out of knit or woven fabric . fig7 shows a closure element 1 in which membrane 5 has strengthening ribs 20 . fig8 shows a valve housing 3 having support 21 parallel to the x - axis extending transverse to the opening . support 21 serves as additional abutment for membrane 5 in the closed position .