Bellows are classified according to their movement during EXPIRATION.
Ascending bellows rise during expiration and descending bellows fall during expiration. Most modern anesthesia machines have ascending bellows because they are thought to be safer. Ascending bellows are safer because if a circuit disconnection occurs, the bellows will not fill. If there is a complete circuit disconnect with descending bellows, the bellows will continue to rise and fall and the disconnection can go unrecognized.