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. The walls can be operably connected by movable structures configured such that two adjacent walls are configured to rotate around substantially orthogonal axes with respect to each other when the reservoir moves from an uncompressed to a compressed state. In some embodiments, the movable structures can be hinges, such as snap-fit assembly hinges. Methods of ventilating a patient that involve the device are also disclosed.