Your heart is a pump. It's a muscular organ about the size of your fist and located slightly left of center in your chest.

Together, your heart and blood vessels make up your cardiovascular system, which circulates blood and oxygen around your body.

Your heart is divided into four chambers. These include two on the right, called the right atrium and right ventricle, and two on the left, called the left atrium and left ventricle. The division protects oxygen-rich blood from mixing with oxygen-poor blood.

Your heart has four valves that keep your blood moving in the correct direction by opening only one way and only when they need to. These valves include the tricuspid, mitral, pulmonary and aortic valves. Each valve has flaps, called leaflets or cusps, that open and close once during each heartbeat.

In the beginning of a pumping cycle, oxygen-poor blood, shown here as blue, returns to the heart after circulating through your body.

The oxygen-poor blood fills the right atrium and then flows to the right ventricle, where it is pumped to the lungs through the pulmonary arteries. The lungs refresh the blood with a new supply of oxygen, which comes from the air that you breathe in.

The now oxygen-rich blood, shown in red, then returns from the lungs and enters the left atrium. The oxygen-rich blood then flows from the left atrium to the left ventricle. The blood is then pumped through the main artery that supplies blood to the body, called the aorta, to supply tissues throughout your body with oxygen.

Your heart is nourished by blood, too. Oxygen-rich blood is delivered by coronary arteries that extend over the surface of your heart.

A beating heart contracts and relaxes. Contraction is called systole, and relaxation is called diastole.

During systole, your ventricles contract, forcing blood into the vessels going to your lungs and body.

Your ventricles then relax during diastole and are filled with blood coming from the upper chambers, the left and right atria. Then the cycle starts over again.

This cycle is driven by your heart's electrical wiring, called the conduction system. Electrical impulses begin high in the right atrium, in the sinus node, and travel through specialized pathways to the ventricles, delivering the signal for the heart to pump.

The conduction system keeps your heart beating in a coordinated and normal rhythm, which in turn keeps blood circulating. This results in the continuous exchange of oxygen-rich blood with oxygen-poor blood that is necessary to keep you alive.