pericardial sac vs sac

pericardial sac and sac both are nouns.

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

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

pericardial sac (noun)	sac (noun)
the membrane surrounding the heart	a structure resembling a bag in an animal
	a case or sheath especially a pollen sac or moss capsule
	an enclosed space