alveolus vs air sac

alveolus and air sac both are nouns.

Word	Noun	Adjective	Verb	Adverb
alveolus	Yes	No	No	No
air sac	Yes	No	No	No

As nouns, alveolus and air sac are synonyms defined as:

alveolus and air sac: a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways

Other synonyms of alveolus include air cell.

alveolus (noun)	air sac (noun)
a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways	any of the membranous air-filled extensions of the lungs of birds
a bony socket in the alveolar ridge that holds a tooth	any of the thin-walled extensions of the tracheae of insects
	a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways