- alveolus, air sac, air cell - a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways
--1 is a kind of sac
--1 is a part of lung - air sac - any of the thin-walled extensions of the tracheae of insects
--2 is a kind of sac
--2 is a part of insect- air sac - any of the membranous air-filled extensions of the lungs of birds
--3 is a kind of sac
--3 is a part of bird - air sac - any of the membranous air-filled extensions of the lungs of birds