In one embodiment, a volumetric pump in a medical fluid delivery system comprises at least one piston in a hollow cylinder, the pump having at least one inlet port through which a medical fluid can be sucked into a pump chamber during an instroke of said piston, and at least one outlet port through which the medical fluid can be expelled during an outstroke of the piston. The piston or the hollow cylinder can be actuated directly or indirectly by a rotor. This rotor transmits on the one hand a bi-directional linear movement to the piston or to the cylinder and on the other hand, a bi-directional angular movement either to the piston or to another ratable element in order to open and close alternately the inlet and outlet ports.