The heart is a hollow muscle that pumps blood throughout the blood vessels by repeated, rhythmic contractions. it is found in all animals with a circulatory system. the term cardiac means "related to the heart" and comes from the greek καρδιά, kardia, for "heart". the vertebrate heart is principally composed of cardiac muscle and connective tissue. cardiac muscle is an involuntary striated muscle tissue found only in this organ and responsible for the ability of the heart to pump blood.