It first appears in "Squid on Strike" when SpongeBob's body is attached to the Krusty Krab doors, not wanting to quit his job. Squidward tries to get him to knock it off by attempting to remove him, which at one point removes his face and reveals his skeleton and internal organs, including his heart.
In "Have You Seen This Snail?," SpongeBob's heart is shown beating like a metronome during the song montage of "Gary Come Home," matching with the lyric "Now my heart is beating like the saddest metronome."
In "Grooming Gary," SpongeBob briefly transports his head inside his body and sees a Gary-themed trophy next to his beating heart.
In "The Inside Job," Plankton, inside SpongeBob's body to easily get the formula, goes to his heart and connects the plungers on his head to it to get a better chance at hearing the formula be revealed.
In "InSPONGEiac," SpongeBob's heart is shown beating fastly as he develops a serious case of imsomnia.