Disclosed is a manually operable volume-adjustable ventilation device. The device includes a reservoir with an inlet mechanism, an outlet mechanism, and a volume adjuster configured to move a volume adjustment limit of the reservoir and change an expressed maximum volume of the reservoir. The reservoir has a body having a plurality of movable walls defining an enclosed volume. The reservoir has an uncompressed state and a compressed state. The walls of the reservoir are movable with respect to each other, such that moving the walls expresses the volume adjustment limit of the reservoir.
