sac vs air bladder

sac and air bladder both are nouns.

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

As nouns, air bladder is a hyponym of sac; that is, air bladder is a word with a more specific, narrower meaning than sac:

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

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