Abstract: An anesthesia ventilation system for controlling gas flow through a patient breathing circuit by the inflation and deflation of a bellows that is outwardly expandable in a predetermined direction comprises a relief valve mounted in an outwardly expandable portion of the bellows, with the valve being biased to a normally closed position but being openable by an external pressure applied against the valve. A valve actuating member mounted in the bellows canister top plate in an adjustable position relative to the bellows engages and opens the valve and relieves the pressure in the bellows after the bellows has expanded outwardly a predetermined distance. The engagement of the valve actuating member with the valve also serves to block further outward expansion of the bellows. This limits the volume of gas contained in the bellows to a predetermined maximum amount.