Apparatus for the respiration of patients with means for the circulation in one direction of respiratory gas, with a connection for the patient and with further connections for the supply and discharge of the various components of the gas, whereby the device is provided with means by which the pressure in the line system can be varied according to a certain respiratory pattern, with a primary part that is connected to means for the generating of a work fluidum, the varying pressure of which can establish the varying of the pressure in the line system, and with a secondary part that is part of or is connected to the line system, whereby a cylinder is provided which is divided in a part that connects to the primary part and a part that connects to the secondary part by a disc that is movable relative to the inner wall thereof.