|1.||any of the membranous air-filled extensions of the lungs of birds, which increase the efficiency of gaseous exchange in the lungs|
|2.||any of the thin-walled extensions of the tracheae of insects having a similar function|
air sac n.
A tiny, thin-walled, capillary-rich sac in the lungs where the exchange of oxygen and carbon dioxide takes place. Also called alveolar sac.