"At the heart of the Matter," "She has such a big heart." English is full of such phrases that use the word "heart." All of them stress the importance of the heart, and rightfully so since it is the most important organ we have - without it nothing in the body will work. You Gotta Have Heart, in this Moment of Science.

The heart is a muscle the size of your fist that's located in the center of the chest behind the breast-bone, or sternum. We generally say the heart is on the left side of the chest because about two-thirds of its mass is to the left of the sternum. Although, it is often referred to as the seat of the emotions, its function really is to pump blood through the body.

The heart is two pumps in one: one on the right side, the other on the left side of the heart. The right side takes in blood from the body, and pumps it to the lungs. There the blood releases carbon dioxide, and picks up oxygen. Then the left side of the heart receives the oxygen-rich blood back from the lungs and pumps it out to the rest of the body.

The familiar "lubb-dupp" sound of the heart is caused by valves closing. There are four chambers in the heart, two atria which receive blood, and two ventricles which pump blood out of the heart. In these chambers there are four valves that regulate the flow of blood into and out of the heart.

When the ventricles contract to pump, two valves close; this makes the first sound. When the ventricles relax, the other two valves close; this makes the second sound. Repeating its "lubb-dupp" cycle over and over, the heart pumps about nineteen-hundred gallons a day, at the rate of about five quarts a minute, for eighty years or more.