The mechanism has a closing device provided for connection of an inner lumen of an inner tube (1) with a space, which is provided between the inner tube and an outer tube. A sealing device is arranged adjacent to an expansion unit in a usage condition of the mechanism. A pre-centering device is provided for horizontal and/or lateral alignment of tubular pieces e.g. tubular channels, of medical technical systems, where the pieces are connected with each other. The pre-centering device comprises two springy and hingable tongues. Independent claims are also included for the following: (1) a method for connecting two fluid guiding medical technical systems (2) a closing sleeve that is provided with a drop protection device.
