heartbeat vs throbbing

heartbeat and throbbing both are nouns.

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

As nouns, throbbing is a hyponym of heartbeat; that is, throbbing 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
throbbing: an instance of rapid strong pulsation (of the heart)

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

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

heartbeat (adjective)	throbbing (adjective)
	pounding or beating strongly or violently