air sac

(noun) a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways. Synonyms: alveolus, air sac, air cell.

(noun) any of the thin-walled extensions of the tracheae of insects. Synonyms: air sac.

(noun) any of the membranous air-filled extensions of the lungs of birds. Synonyms: air sac.