Sandra Giglietta, age 12, of Tacoma, Wash., for her question: .

HOW MANY TIMES DOES YOUR HEART BEAT IN AN HOUR?

Your heart is a muscular organ about the size of your fist. Its function, of course, is to pump blood through the body so that nourishment can be transported to all parts of the body, and that waste material can be carried off.

The heart is located behind the breastbone, or foremost flat bone of the chest. It is located in the middle of the chest with only a little more of its bulk to the left than to the right. It is not over on the left side, as many people say. In the average adult male, the heart weighs a bit less than a pound, while in the adult female it is a little over a half a pound

There are two sides to your heart, divided by a wall called the septum. Each side has two chambers  the upper called the atrium (plural is atria) and the lower called the ventricle.

When the heart muscles contract, narrowing the chambers, blood is forced throughout the body. The heart then relaxes, permitting the chambers to expand and take in more blood. Next there's a brief moment of rest before the heart muscles contract again.

The contraction period is called systole, and the relaxation period is called diastole.

This cycle is repeated from 70 to 80 times each minute, which adds up to between 4,200 and 4,800 times per hour. That's a lot of work for your heart, but it is designed to handle the job without too many problems.

You can hear a sound like "lubb dup " every time the heart beats if you put your ear against someone's chest. The first sound  " lubb "  which is rather dull, happens when the valves between the atria and ventricles shut at the beginning of systole. The sharper " dup " sound happens at the end of systole when the pulmonary and aortic valves snap shut under pressure in the big arteries. The valves keep the blood from flowing back into the heart.

Your blood, as it passes through the aortic valve and into the aorta, is then routed through various branches to the coronary system of the heart itself, to the head, body, abdominal area and to the lungs where waste carbon dioxide is eliminated and a fresh supply of oxygen is obtained for the next circuit.