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 resting heart rate normally ranges from about 60 to 100 beats per minute (bpm), although elite athletes might be in the range 40 to 60 bpm. The rate of contractions (i.e. the heartbeat) and contractility (i.e. strength of the heartbeat) are regulated by the nerves of the autonomic nervous system that supply the heart: the sympathetic nerve fibres of the cardiac nerves release noradrenaline, accelerating heartbeat and increasing contractility, whereas the parasympathetic fibres of the vagus nerve release acetylcholine, slowing the heartbeat and decreasing its strength. These two neurotransmitters thus have antagonistic effects on the pacemaker cells. 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.

Structure of the mammalian heart

http://www.bartleby.com/107/illus490.html Sequence of diagrams of the human heart from Gray’s Anatomy