A manually operated ventilating element such as a bellows for administration of artificial respiration, for example, during anesthesia, has wall sections or pleats of different diameters. When the element is actuated through a small stroke length, the smaller section is actuated. A small volume of gas is delivered and the change in delivered volume per unit stroke length is small, so that the operator can precisely control the delivered volume. When the element is actuated through a larger stroke length, a larger volume per unit stroke length is delivered. The same element can be used to treat children and adults.
