The heart's a muscular organ in all vertebrates responsible for pumping blood through the blood vessels by repeated, rhythmic contractions, or a similar structure in annelids, mollusks, and arthropods. The term cardiac (as in cardiology) means "related to the heart" and comes from the Greek καρδιά, kardia, for "heart." The heart of a vertebrate's composed of cardiac muscle, an involuntary striated muscle tissue which's found only within this organ. The average huma… (More on Heart)