A displacement pump with a pump housing containing a pump chamber of varying volume, the limiting walls of which includes a moveable portion or diaphragm, the movement and/or deformation of which varies the pump chamber volume. The pump chamber has a fluid inlet on the suction side of the pump and a fluid outlet on the pressure side. Both the fluid inlet and the fluid outlet, or possibly only one of them, includes a flow controlling element having one diffuser and one diffuser inlet which, for the same flow, has a larger pressure drop in one flow direction (the nozzle direction) than in the opposite flow direction (the diffuser direction). A drive element is coupled to the diaphragm, whereby the diaphragm can be caused to oscillate, so that the fluid volume in the pump chamber is caused to pulsate and thereby produce a net flow of fluid through the pump.