It is intended for varying the volume of a chamber of a perfusion pump by alternatively inflating and deflating the same by means of a movement transmitting liquid fluid. It is characterized in that it comprises a lung body (18) provided with an elastic and deformable frontal membrane (24) over which an external piston (16), which can be moved alternatively in both directions, enters into contact, and said piston originates from one of the sides of a control console (13). This console also has an enveloping ring head (17) that surrounds the external piston (16), while constituting the fixation means of the lung body (18), which has a terminal narrowing (21) connected to an intermediate tubular duct (22) culminating in the chamber (12) of the perfusion pump (23), and the movement transmitting liquid fluid is located in a space comprising a front part of the lung body (18), the intermediate tubular duct (22) and the chamber (12) of the perfusion pump (23).
