Abstract: Inside a syringe (1) slides a plunger (2) which bears a joint (4) sliding sealingly over the internal wall of the syringe. The flexible membrane (6) of the joint (4) fixed to the end of the plunger (2) bears a closing component (25, 27) which follows the movement of deformation or displacement of the membrane through the effect of the pressure built up during the injection stage, but which is retained by immobilizing strips (17) in a position in which it opens the orifice, breaking the seal between the top and the bottom of the joint, as soon as the membrane returns to its initial position.