The heart is made up of a number of components: cardiac muscle, valves, great vessels, pacemaker and electrical conducting system, coronary vessels, autonomic innervation, and pericardium (Table 1).

The heart's major function is that of a pump, imparting kinetic and potential energy to the blood so that tissues can be adequately perfused (Table 2). It also acts as a sensor of blood pressure and volume and is an endocrine organ secreting atrial natriuretic hormone.

The heart is really two pumps joined by a common septum, and driven by a common pacemaker (Table 3). Each pump has a thin-walled antichamber, the atrium, gathering venous blood from the periphery, and a thick-walled structure, the ventricle, which propels blood into the arteries. A valve between the atria and the ventricle, and one between the ventricle and the arteries guarantee unidirectional blood flow, most of which goes to the body, while a little perfuses the heart muscle itself.

Like an internal combustion engine, there are four phases to the cardiac cycle: a pressure increase, a volume decrease, a pressure decrease, and a volume increase. Adequate filling must occur for adequate pumping to occur.

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