fig1 shows an access system with the very application device , different connections etc . eliminated for the sake of clarity . on an access device 2 is held a coupling device 3 of a first embodiment as a short - circuiting unit . sideward of this short - circuiting unit 3 is shown a coupling device 4 according to a second embodiment which is as a coupling unit having connections 22 to an external circuit . for better understanding of the general purpose of an access device according to the invention reference is made to the applicant &# 39 ; s own wo 99 / 20338 , wherein also the principle of the function of an application device is described . the access device 2 has first and second holding elements 5 for the co - operation with holding means 17 which are arranged transversally of the respective coupling devices 3 and 4 , and which extends over the width of the side of the respective coupling device 3 and 4 . the holding elements 5 form a guide for coupling devices that is open in both ends which is shown in more detail in said wo document . on the holding means 17 , there are arranged strips 17 ′ ( one shown in the figure ) for vertical ( as seen in the figure ) positioning of the coupling device against the holding surface of the holding elements 5 . at an angle of a coupling surface ( 10 in fig3 , 4 and 5 ), preferably at about right angle , extends on each side a positioning surface 23 , which is arranged to co - operate with a positioning edge ( 26 in fig2 and 3 ) on the holding element 5 in order to achieve an adequate positioning in horizontal direction transversal to a direction of inserting the coupling device . for positioning in a direction of insertion are arranged firstly snap lock arms 14 on the coupling device 4 , that co - operate with holding edges ( indicated at 14 ′) on the access device 2 . secondly , the coupling device 4 has stop lugs 24 , which cooperate with stop recesses 25 on the access device , at the ends of each holding element . this way , good precision during application is ensured , which increases the precision of the channel formed by the access device and the coupling device so that thereby protruding edges and recesses that negatively affect the flow because of erroneous positioning are avoided . fig2 shows the access device 2 in greater detail , whereby application directions for coupling devices are indicated with arrows r 1 and r 2 , respectively . with 6 is indicated a connecting surface and with 7 two mouths to two access channels for the connection to a patient . fig3 shows in a section an access device 2 with positioned coupling device 3 of the kind that includes a short - circuiting channel 9 . in the figure is shown that the holding elements 5 co - operate with the holding means 17 and in particular the strip 17 ′ for adequate positioning in a vertical direction as seen in the figure . with 6 is indicated the connecting surface of the access device and with 10 a coupling surface of the coupling device 3 . outward of the short - circuiting channel 9 is positioned a surrounding groove 12 for receiving a strip - shaped means comprising a flexible portion 11 , which is arranged with one part to protrude past , or outside the plane of said coupling surface 10 . the flexible portion 11 comprises a sealing lip 13 , which has a more limited , in the protrusion direction narrowing extension , which sealing lip functions well in order to receive possible tolerance deviations of the parts involved . fig4 shows the underside of the coupling device 3 , wherein is shown the short - circuiting channel 9 which is positioned inside the surrounding strip - shaped flexible portion 11 with its sealing lip 13 . upwardly in the figure is directed a sideward directed sealing portion 15 with a sideward directed sealing lip 16 , which has the purpose of sealingly contact against another coupling device during the process of exchanging coupling devices on an access device 2 . said sideward directed sealing portion 15 with the sealing lip 16 guarantees that leakage in the area of the arrow 19 in fig1 , i . e . between two contacting coupling devices , is avoided or at least reduced in relation to what is the case with the background art . fig5 differs from fig4 only by the fact that this figure concerns a coupling device 4 of the kind which is a coupling unit as shown in fig1 . all details relating to the present invention are the same as is shown in fig4 . with 18 are indicated mouths of the coupling channels of the coupling device 4 . fig6 shows a diagrammatical section through a flexible portion 11 which is positioned in a groove 12 of a coupling device 3 . contact between the coupling surface 10 and the connecting surface 6 results in an inward pressing of the flexible portion . this in turn results in that a part 21 of the flexible portion will be pressed out into a space 20 which is arranged in the vicinity thereof , since the flexible portion in principle is not compressible and an inward pressed volume must find space somewhere . the invention can be modified within the scope of the following claims . thus , the flexible portion can be shaped otherwise , and it is not excluded , for example , that as much as the entire coupling surface is covered with a flexible material even if such a solution is not preferred . in particular it is preferred that a strip - shaped portion is formed such that the material of the flexible portion , which extends outside the coupling surface , can be received in an adjacent space , either at the side of for example a groove for receiving the flexible portion in the form of a strip , or in such a way that inside the strip there is , on the one hand a protruding sealing lip , on the other hand a recess , with respect to the level of the coupling surface for receiving pressed - in flexible material . the sideward directed sealing portion is preferably formed such that the space between the two contacting coupling devices is small , such that under the process of exchanging coupling devices on an access device , the amount of blood being present here is limited . the term “ coupling channel ” should be interpreted to include all channels or parts thereof of a coupling device , firstly such having a mouth in a coupling surface , secondly short - circuiting channels leading a liquid from an access channel to another . coupling devices according to the invention can be of different kinds besides the ones mentioned above , thus a coupling unit to be connected to an external circuit can be used for extracorporeal blood treatment such as for example cavh . a coupling unit for connecting to a fluid circuit source can be used for medication , supply of nutrition etc . further , a coupling device according to the invention can include sensor elements , for example for temperature control , which can be used on the one hand for monitoring the function of the access device , on the other hand show certain conditions of the patient . other parameters in the blood can be monitored , for example the concentration of different substances . this can be indicated with the aid of different kinds of signal systems . the flexible portion and the sideward directed sealing portion can be produced from any material from the group silicon rubber or any other synthetic rubber or natural rubber , thermoplastic elastomeric material or any other suitable flexible plastic . the primary material of the coupling device can be any suitable relatively hard , per se known plastic .