A medical instrument including a first fluid line having an end with an edge, a second fluid line, and a cavity in which the end of the first fluid line is arranged, and which is connected to the second fluid line. A wall region of the cavity is designed to be flexibly movable. In a first configuration, the flexibly movable wall region bears on the edge of the end of the first fluid line and thus closes the end of the first fluid line. In a second configuration, the flexibly movable wall region does not bear, or bears only partially, on the edge of the end of the first fluid line, such that the first fluid line and the second fluid line are fluidically connected.