air sac

Noun
  1. A tiny sac for holding air in the lungs;
    Formed by the terminal dilation of tiny air passageways (synset 105536028)
  2. Any of the thin-walled extensions of the tracheae of insects (synset 102470515)
  3. Any of the membranous air-filled extensions of the lungs of birds (synset 102470377)

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