Although in German spelling Pendelluft, like all nouns, begins with a capital letter, its English derivative is spelled with a small p. Transient movement of gas out of some alveoli and into others when flow has just stopped at the end of inspiration, or such movement in the opposite direction just at the end of expiration; occurs when regions of the lung differ in compliance, airway resistance, or inertance so that the time constants of their filling (or emptying) in response to a change of transpulmonary pressure are not the same. [Ger. Pendel, pendulum, + Luft, air]