alveolus vs sac

alveolus and sac both are nouns.

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

As nouns, sac is a hypernym of alveolus; that is, sac is a word with a broader meaning than alveolus:

alveolus: a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways
sac: a structure resembling a bag in an animal

alveolus (noun)	sac (noun)
a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways	a structure resembling a bag in an animal
a bony socket in the alveolar ridge that holds a tooth	a case or sheath especially a pollen sac or moss capsule
	an enclosed space