Medical Definition of heart
1: a hollow muscular organ of vertebrate animals that by its rhythmic contraction acts as a force pump maintaining the circulation of the blood and that in the human adult is about five inches (13 centimeters) long and three and one half inches (9 centimeters) broad, is of conical form, is placed obliquely in the chest with the broad end upward and to the right and the apex opposite the interval between the cartilages of the fifth and sixth ribs on the left side, is enclosed in a serous pericardium, and consists as in other mammals and in birds of four chambers divided into an upper pair of rather thin-walled atria which receive blood from the veins and a lower pair of thick-walled ventricles into which the blood is forced and which in turn pump it into the arteries
2: a structure in an invertebrate animal functionally analogous to the vertebrate heart
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