How big is your heart?

From the time you are a child and throughout your life, your heart remains about the size of your balled-up fist. Inside your heart, there are four chambers. You have two atria stacked on the top of two ventricles.

Each of the atria is paired with a ventricle. A wall separates them into two different shafts.

On each side of the heart, blood enters the upper atrium and then goes through a valve into the ventricle, exiting through another valve on the way out of the heart.

When your heart beats, an electrical signal passes from the top of your heart, near the atria, down through your ventricles and chamber contracts, in that order.

When the upper atria contract, the atrioventricular valves - sandwiched between the atria and the ventricles - open, and the blood in each atrium flows through its valve down into a ventricle.

The valve to the left ventricle -- where oxygenated blood is coursing through - is called the mitral valve.

The valve on the right side - where oxygen-depleted blood is passing into the right ventricle -is known as the tricuspid valve.

Once both ventricles fill with blood, the atrioventricular valves shut, keeping the blood from flowing back into the atria. The shutting of the atrioventricular valves creates the first sound of your heartbeat -- "Lub!"

At this point, the heart's electrical signal has passed from the atria into the ventricles. While the atria relax, the ventricles contract. Now, a second set of valves open. These valves lead out of the ventricles - as exit doors from the heat. Together, they're known as semilunar valves.

The semilunar valves direct blood to its next destination. The oxygen-depleted blood in the right ventricle leaves the heart through the pulmonary valve. That valve connects to the pulmonary artery which leads to the lungs.

Oxygen-rich blood in the left ventricle, at the same time, departs through the aortic valve. That valve connects the heart to the aorta, which is the body's major expressway for freshly oxygenated blood.

Once the passing electrical current contracts the ventricles, the blood in the ventricles is forced out through the open semilunar valves. When they slam shut, you hear the second half of your heartbeat- "Dub!"