air sacs

名词 air sac:

1. a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways
   同义词：alveolus, air sac, air cell
2. any of the thin-walled extensions of the tracheae of insects
   同义词：air sac
3. any of the membranous air-filled extensions of the lungs of birds
   同义词：air sac