SA node vs cardiac muscle

SA node and cardiac muscle both are nouns.

Word	Noun	Adjective	Verb	Adverb
SA node	Yes	No	No	No
cardiac muscle	Yes	No	No	No

As nouns, cardiac muscle is a hypernym of SA node; that is, cardiac muscle is a word with a broader meaning than SA node:

SA node: a specialized bit of heart tissue that controls the heartbeat
cardiac muscle: the muscle tissue of the heart; adapted to continued rhythmic contraction

Other hypernyms of SA node include heart muscle.

SA node (noun)	cardiac muscle (noun)
a specialized bit of heart tissue that controls the heartbeat	the muscle tissue of the heart; adapted to continued rhythmic contraction