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

Derived forms: air sacs

Type of: sac

Part of: bird, insect, lung

Encyclopedia: Air sac

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