The heart is a hollow, muscular organ in 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 kardia, for "heart." The heart is composed of cardiac muscle, an involuntary muscle tissue which is found only within this organ.
(From Wikipedia, the free encyclopedia.)