air bladder vs sac

air bladder and sac both are nouns.

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

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

air bladder: an air-filled sac near the spinal column in many fishes that helps maintain buoyancy
sac: a structure resembling a bag in an animal

air bladder (noun)	sac (noun)
an air-filled sac near the spinal column in many fishes that helps maintain buoyancy	a structure resembling a bag in an animal
	a case or sheath especially a pollen sac or moss capsule
	an enclosed space