How the heart works

To understand heart disease, it helps to know how the heart works. 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. Your heart is divided into the right and the left side. The division protects oxygen-rich blood from mixing with oxygen-poor blood. Oxygen-poor blood returns to the heart after circulating through your body.

The right side of the heart, composed of the right atrium and ventricle, collects and pumps blood to the lungs through the pulmonary arteries. The lungs refresh the blood with a new supply of oxygen, making it turn red. Oxygen-rich blood then enters the left side of the heart, composed of the left atrium and ventricle, and is pumped through the aorta to supply tissues throughout the body with oxygen and nutrients.

Four valves within your heart keep your blood moving the right way. The tricuspid, mitral, pulmonary and aortic valves open only one way and only when pushed on. Each valve opens and closes once per heartbeat — or about once every second while you're at rest.

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 — much like ketchup being forced out of a squeeze bottle. The right ventricle contracts a little bit before the left ventricle does. Your ventricles then relax during diastole and are filled with blood coming from the upper chambers, the left and right atria. The cycle then starts over again.

Your heart also has electrical wiring, which keeps it beating. Electrical impulses begin high in the right atrium and travel through specialized pathways to the ventricles, delivering the signal to pump. The conduction system keeps your heart beating in a coordinated and normal rhythm, which in turn keeps blood circulating. The continuous exchange of oxygen-rich blood with oxygen-poor blood is what keeps you alive.The causes of heart disease vary by type of heart disease.