sac vs swim bladder

sac and swim bladder both are nouns.

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

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

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

sac (noun)	swim 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