The heart is the "soul" of the body--and its effective functioning is required to keep the rest of the body functioning normally. The normal heart is a strong, muscular pump a little larger than a fist. It pumps blood continuously through the circulatory system. Each day the average heart "beats" (or expands and contracts) 70 times a minute, over 100,000 times and pumps about 2,000 gallons of blood. In a 70-year lifetime, an average human heart beats more than 2.5 billion times.

We are all familiar with the thump, thump, thump that means the heart is pumping blood from the veins returning from the body, then through the lungs to be replenished with oxygen, then back out to the body again. But how does the heart really work? The following information will help to answer this question.

What is the structure of the heart?

The heart has four chambers. The upper chambers, called atria, receive and collect blood. The lower chambers, called ventricles, pump blood. The heart also has four valves that open and close to allow blood to flow in only one direction when the heart contracts (beats), preventing it from backing up into the chamber from which it came.

The four heart valves are:

1. the tricuspid valve, located between the right atrium and right ventricle;

2. the pulmonary or pulmonic valve, between the right ventricle and the pulmonary artery;

3. the mitral valve, between the left atrium and left ventricle; and

4. the aortic valve, between the left ventricle and the aorta. Each valve has a set of flaps (also called leaflets or cusps).

The mitral valve has two flaps; the others have three. Under normal conditions, the valves permit blood to flow in only one direction. Blood flow occurs only when there's a difference in pressure across the valves that causes them to open.

How does the heart pump blood?

The heart pumps blood to the lungs and to all the body's tissues by a sequence of highly organized contractions of its four chambers. The oxygen-poor blood that returns from the body collects in the right side of the heart. It is then pumped into the lungs, where it picks up fresh oxygen. The oxygen-rich blood coming from the lungs flows into the left side of the heart, where it is then pumped to different parts of the body through the arteries.

The heart gets its own blood supply from three small arteries that originate from the aorta, the largest artery in the body located just beyond the aortic valve. These vessels, which are called the coronary arteries, wrap around the outside of the heart. In patients with coronary artery disease, angina or who experience heart attacks, the three coronary arteries become blocked with cholesterol--and the patients may require triple bypass surgery.

In the normal adult, the heart pumps five liters of blood per minute, which is recirculated continuously through the body. The blood moves from the heart into vessels called arteries, then into tiny tubes called capillaries and finally to vessels called veins that lead back to the heart.

What is the role of the circulatory system?

The circulatory system is the network of elastic tubes through which blood flows as it carries oxygen and nutrients to all parts of the body. It includes the heart, lungs, arteries, arterioles (small arteries), and capillaries. It also includes venules (small veins) and veins, the blood vessels through which blood flows as it returns to the heart. If all these vessels were laid end-to-end, they would extend for about 60,000 miles--far enough to encircle the earth more than twice.

The circulating blood brings oxygen and nutrients to all the organs and tissues of the body, including the heart itself. It also picks up waste products from the body's cells. These waste products are removed as they're filtered through the kidneys, liver and lungs.

How does the heart's electrical system work?

The heart also has an electrical system which includes a permanent pacemaker to regulate the heartbeat rate and specialized wiring to transmit the pacemaker beat to all parts of the heart. The pacemaker is located in the right atrium-- the sinus node. This pacemaker creates a miniature electrical impulse, on average 70 times a minute. This pulse of energy is then carried through the atria and into the ventricles by way of the AV node--which creates a brief delay in the impulse to allow the atria to contract and forces blood into the ventricles before they contract.

Some people have an auxiliary pacemaker located in the ventricle. Occasionally, this pacemaker can fire too early or prematurely, causing a brief flutter sensation or irregular pulse. These firings are termed PVC's (Premature Ventricular Contractions). Other people can have "bad wiring" between the upper and lower chambers of the heart. If only part of the wiring (or one of two "bundles") goes bad, called bundle branch block, the heart can still function normally. If both bundles go bad, impulses from the pacemaker never make it to the heart ventricles, causing a very slow heart rate of less than 40 beats per minute. This is called complete heart block--and these people generally need artificial pacemaker implants.

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