The shape of the human heart bears little resemblance to the one representing St. Valentine's Day. Instead, its shape is more conical with the narrow end pointing downward, to the left and slightly forward. Its location in the chest cavity is just to the left of the midline, behind the sternum and the second to sixth left ribs.

Despite its heavy work load, the human heart is not a large organ; it is about the size of a clenched fist and weighs 10 to 12 ounces. It is surrounded by a membranous sac, the pericardial sac, (peri means around; cardia is from the Greek word for heart, kardia). This sac contains a small amount of watery fluid that bathes the heart and protects it from contact with adjacent organs during its contractions.

The wall of the heart consists of three layers of tissue: the pericardium, a thin, transparent layer covering the outside of the heart; a similar thin layer, the endocardium (endo means inner), lining the heart cavity; and a thick layer of cardiac muscle, the myocardium (myo means muscle), that separates the two linings. The myocardium is a specialized type of muscle that is unique to the heart and responsible for its contractions.

The heart has two thin-walled receiving chambers: the left and right atria, and two thick-walled pumping chambers, the ventricles. Actually, the heart consists of two parallel pumps that work simultaneously -- the right-side pump receives blood from the veins and pumps it to the lungs where it is resupplied with oxygen, and the left-side pump receives the freshly reoxygenated blood from the lungs and sends it through the arteries to the rest of the body. The heart also has four valves. There are two valves between the atria and ventricles to prevent backflow or regurgitation of blood resulting from the high pressure of the ventricular pump action, and two valves to prevent backflow into the ventricles after they have finished their pumping action.

Let's trace the one-way flow of blood through the heart. Blood enters the right atrium from the veins and passes through the tricuspid valve into the right ventricle. The right ventricle contracts, expelling blood through the pulmonic valve and sending it to the lungs for a fresh supply of oxygen. As the right ventricle contracts, the tricuspid valve snaps shut, preventing regurgitation of blood into the right atrium. As the right ventricle relaxes, the pulmonic valve closes, preventing regurgitation of blood back into the ventricle. The fresh blood from the lungs returns to the heart into the left atrium that passes it into the powerful left ventricle through the mitral or bicuspid valve. The left ventricle now contracts, sending blood out of the heart through the aortic valve into the largest artery in the body, the aorta. As the left ventricle contracts, the mitral valve closes, preventing regurgitation of blood from the left ventricle into the left atrium. As the left ventricle relaxes, the aortic valve closes, preventing regurgitation from the aorta back into the left ventricle. This is the way that both sides of the heart, the two atria and the two ventricles, work simultaneously.

Although the heart can operate on its own, it is supplied with two sets of nerves to augment the work. The sympathetic nerves stimulate the heart and the parasympathetic nerves (mainly the vagus nerve) act to calm the heart down. These nervous systems carry signals from the brain and elsewhere in the body that help the heart respond and adjust to internal and external factors. They act chiefly by adjusting the rate at which the heart beats. They are also useful as the mechanism by which many drugs exert their therapeutic effects on the heart by stimulating or blocking the sympathetic or parasympathetic nerves.

The heart also has its own important blood supply, the coronary circulation. The two main coronary arteries, left and right, branch from the aorta just as it leaves the heart. They, in turn, give rise to numerous branches, guaranteeing the heart a rich supply of blood and oxygen. A decrease in the oxygen supply to the heart usually occurs as a result of narrowing or complete obstruction of one or more coronary arteries. When a portion of the myocardium is deprived of oxygen, that portion of the heart muscle may die, a condition known as a myocardial infarction.