A hollow muscular organ that, by means of regular contractions, pumps blood through the circulatory system (see circulation). The vertebrate heart has a thick wall (see myocardium) composed of a specialized muscle (see cardiac muscle); it is surrounded by the pericardium. Mammals have a four-chambered heart consisting of two atria and two ventricles; the right and left sides are completely separate from each other so there is no mixing of oxygenated and deoxygenated blood (see illustration). Oxygenated blood from the pulmonary veins enters the heart through the left atrium, passes to the left ventricle, and leaves the heart through the aorta. Deoxygenated blood from the venae cavae enters the right atrium and is pumped through the right ventricle to the pulmonary artery, which conveys it to the lungs for oxygenation. The tricuspid and bicuspid valves ensure that there is no backflow of blood. The contractions of the heart are initiated and controlled by the sinoatrial node (see pacemaker); in an average adult human the heart contracts about 70 times per minute. See also cardiac cycle; cardiac output.
The hearts of other vertebrates are similar except in the number of atria and ventricles (there may be one or two) and in the degree of separation of oxygenated and deoxygenated blood. Invertebrates, however, show great variation in the form and functioning of the heart.
http://www.bartleby.com/107/illus490.html Sequence of diagrams of the human heart from Gray's Anatomy
Structure of the mammalian heart
Subjects: Medicine and Health — Biological Sciences.