A valve arrangement (222,300) comprising: a valve body of elastomeric material, and a valve housing defined in a medical device within which the valve body is seated, wherein the valve housing includes an interior wall surface, and wherein the valve body includes: a proximal cylindrical end, a distal portion and a central passage extending from the proximal end to the distal end, the distal portion including a pair of distally extending opposed flaps converging at flap ends forming a distal tip having a slit therethrough, and the valve body further including a pair of tensioners integrally formed therewith and projecting radially outwardly from side sections of the respective distally extending opposed flaps to exterior tensioner faces for engaging an interior wall of the valve housing, characterised in that each tensioner comprises two lateral side walls joined at an exterior end and at the distally extending opposed flaps, defining a hollow center opening therebetween, and that an angle α of less than 180° and more than 90° is formed into an inner surface of the side walls, whereby the valve body has a plane of separation aligned with the slit and defined by lines of separation on each side of the valve body, whereby the length of the lateral side walls of each tensioner are so dimensioned to be larger than the distance between the flaps and the valve housing wall as to the tensioners bear against and be pre-stressed against the interior valve housing wall upon assembly to press the distally extending flaps toward and against each other at the distal tip to close the slit either completely in a sealing fashion or sealingly against outer surfaces of another medical device inserted through the valve's central passage and slit. a valve body of elastomeric material, and a valve housing defined in a medical device within which the valve body is seated, wherein the valve housing includes an interior wall surface, and wherein the valve body includes: a proximal cylindrical end, a distal portion and a central passage extending from the proximal end to the distal end, the distal portion including a pair of distally extending opposed flaps converging at flap ends forming a distal tip having a slit therethrough, and the valve body further including a pair of tensioners integrally formed therewith and projecting radially outwardly from side sections of the respective distally extending opposed flaps to exterior tensioner faces for engaging an interior wall of the valve housing, a proximal cylindrical end, a distal portion and a central passage extending from the proximal end to the distal end, the distal portion including a pair of distally extending opposed flaps converging at flap ends forming a distal tip having a slit therethrough, and the valve body further including a pair of tensioners integrally formed therewith and projecting radially outwardly from side sections of the respective distally extending opposed flaps to exterior tensioner faces for engaging an interior wall of the valve housing, characterised in that each tensioner comprises two lateral side walls joined at an exterior end and at the distally extending opposed flaps, defining a hollow center opening therebetween, and that an angle α of less than 180° and more than 90° is formed into an inner surface of the side walls, whereby the valve body has a plane of separation aligned with the slit and defined by lines of separation on each side of the valve body, whereby the length of the lateral side walls of each tensioner are so dimensioned to be larger than the distance between the flaps and the valve housing wall as to the tensioners bear against and be pre-stressed against the interior valve housing wall upon assembly to press the distally extending flaps toward and against each other at the distal tip to close the slit either completely in a sealing fashion or sealingly against outer surfaces of another medical device inserted through the valve's central passage and slit.