Connection device for connecting, in particular coupling, at least one external functional means to an arrangement being a blood treatment apparatus by pressing the external functional means, wherein the connection device comprises: - at least one reception means having at least one first contact portion for receiving the external functional means between the first contact portion and one or several further contact portions (A, 201), wherein the first contact portion is arranged at a variable spacing d from the further contact portion (A, 201); - at least one pressing means for reducing the variable spacing d for pressing the external functional means between the first contact portion and the further contact portion (A, 201) by transferring at least the first contact portion from a first position, in particular a set-up position, of the connection device, in which a first spacing between the first contact portion and the further contact portion (A, 201) is present, into a second position in which a second spacing between the first contact portion and the further contact portion (A, 201) is present, wherein the second spacing is smaller than the first spacing; - wherein the pressing means is configured for transferring at least the first contact portion from the first position into the second position by applying a first spring force and a second spring force, wherein said second spring force is larger than the first spring force; characterized in that: - the pressing means comprises at least two spring elements (12, 13) for transferring at least the first contact portion from the first position into the second position, wherein a stepped or abrupt change of force application in the force-path-diagram as a function of of the mutual spacing of the contact portions (111, A, 201) is achievable by applying the two different spring forces (F1, F2) during operation of the pressing means. - at least one reception means having at least one first contact portion for receiving the external functional means between the first contact portion and one or several further contact portions (A, 201), wherein the first contact portion is arranged at a variable spacing d from the further contact portion (A, 201); - at least one pressing means for reducing the variable spacing d for pressing the external functional means between the first contact portion and the further contact portion (A, 201) by transferring at least the first contact portion from a first position, in particular a set-up position, of the connection device, in which a first spacing between the first contact portion and the further contact portion (A, 201) is present, into a second position in which a second spacing between the first contact portion and the further contact portion (A, 201) is present, wherein the second spacing is smaller than the first spacing; - wherein the pressing means is configured for transferring at least the first contact portion from the first position into the second position by applying a first spring force and a second spring force, wherein said second spring force is larger than the first spring force; - the pressing means comprises at least two spring elements (12, 13) for transferring at least the first contact portion from the first position into the second position, wherein a stepped or abrupt change of force application in the force-path-diagram as a function of of the mutual spacing of the contact portions (111, A, 201) is achievable by applying the two different spring forces (F1, F2) during operation of the pressing means.