heart

Muscular organ that rhythmically contracts to force blood around the body of an animal with a circulatory system. Annelid worms and some other invertebrates have simple hearts consisting of thickened sections of main blood vessels that pulse regularly. An earthworm has ten such hearts. Vertebrates have one heart. A fish heart has two chambers – the thin-walled atrium (once called the auricle) that expands to receive blood, and the thick-walled ventricle that pumps it out. Amphibians and most reptiles have two atria and one ventricle; birds and mammals have two atria and two ventricles. The beating of the heart is controlled by the autonomic nervous system and an internal control centre or pacemaker, the sinoatrial node.

The human heart is more or less conical in shape and is positioned within the chest, behind the breast bone, above the diaphragm, and between the two lungs. It has flattened back and front surfaces and is, in health, the size of a person's closed fist. However, it varies in size with the person's weight, age, sex, and state of health. Its capacity is about 20 cm³ in the newborn, reaching 150–160 cm³ in the mid-teens. The female heart has a smaller capacity and is lighter than the male. Mammals have a double circulatory system (see double circulation). In this kind of system the heart is divided into two halves, each half working as a separate pump. One side of the heart pumps blood to the lungs and back to the heart. The other side of the heart pumps blood to all other parts of the body and back to the heart.