Heart quiz: Put your heart smarts to the test

The human heart is shaped like a cone. The narrower end of the heart points down and to the left side of the chest. In an adult, the heart weighs about three-fourths of a pound and is about the size of a fist. Major arteries, such as the aorta and pulmonary arteries, connect at the wider upper part of the heart. If arteries become clogged with cholesterol and other fatty deposits (atherosclerosis), you may have a heart attack or stroke.

An average adult heart beats about 70 times a minute. The hearts of superfit athletes, however, may beat as slowly as 40 times a minute. That's because their hearts are larger and can pump more blood with each heartbeat. You don't have to become an elite athlete to reap benefits from exercise, though. Moderate exercise most days of the week can tune up your heart, too, reducing the risk of such conditions as high blood pressure, stroke and heart attack.

If your heart muscle is weak, it can't pump as hard. To compensate for its inability to pump out the normal amount of blood, your heart may enlarge (dilate). It could eventually enlarge so much that it fails to function. Heart failure often results from the stress of a heart attack, high blood pressure or other forms of heart disease.

The "thump, thump" sounds heard through a stethoscope come from your heart valves closing after blood has passed through them. Valves are strong, thin leaflets of tissue attached to the heart muscle. The heart's four valves keep blood flowing in one direction. The valves normally open wide with each heartbeat and then close tightly to prevent any backward flow of blood.

Most currents in the heart measure less than 1-millionth of an ampere. In contrast, the current running through a 100-watt light bulb is about 1 ampere. Cardiac cells start their own electrical impulses and contractions. That means even a heart that has been removed from the body can continue to beat for a short time. Although tiny, this current is vital to the proper functioning of your heart. This electrical activity coordinates the contraction and relaxation of the heart's four chambers. It's what makes your heart beat in a coordinated and normal manner.

The heart propels blood by squeezing it out of the powerful ventricles into the arteries. The squeezing action is produced when the muscle fibers that make up the walls of the heart shorten (contract). With this contraction, the ventricles become smaller, forcing the blood out. After contraction, the heart muscle relaxes, the muscle fibers lengthen, the ventricles become more spacious, and blood flows into them from the atria.

The atria are small receiving chambers for blood returning from the body (right atrium) or from the lungs (left atrium). They push the blood into the ventricles with a rather weak contraction. The two ventricles are the main pumping chambers. The right ventricle forcefully pumps blood to the lungs while the left ventricle pumps blood to tissues and organs throughout the body.

Veins carry blood to the heart from the lungs or other parts of your body. Arteries carry blood away from the heart to supply oxygen and nutrients to the body. Capillaries act as the go-between, transporting blood from arteries to veins. The aorta, or main artery, is the largest blood vessel in your body. It has numerous branches to supply various parts of your body with blood. For instance, the aorta extends downward through your abdomen, branching into smaller vessels that supply your legs with blood.