heartbeat vs pounding

heartbeat and pounding both are nouns.

Word	Noun	Adjective	Verb	Adverb
heartbeat	Yes	No	No	No
pounding	Yes	No	No	No

As nouns, pounding is a hyponym of heartbeat; that is, pounding is a word with a more specific, narrower meaning than heartbeat:

heartbeat: the rhythmic contraction and expansion of the arteries with each beat of the heart
pounding: an instance of rapid strong pulsation (of the heart)

Other hyponyms of heartbeat include diastole, systole, throb, throbbing.

heartbeat (noun)	pounding (noun)
an animating or vital unifying force	the act of pounding (delivering repeated heavy blows)
the rhythmic contraction and expansion of the arteries with each beat of the heart	repeated heavy blows
a very short time (as the time it takes the eye to blink or the heart to beat)	an instance of rapid strong pulsation (of the heart)