bladder vs sac

bladder and sac both are nouns.

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

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

bladder (noun)	sac (noun)
a bag that fills with air	a structure resembling a bag in an animal
a distensible membranous sac (usually containing liquid or gas)	a case or sheath especially a pollen sac or moss capsule
	an enclosed space