The heart is a pump. It is divided into the right and the left side. The division protects oxygen-rich blood from mixing with oxygen-poor blood.
A heart pumps about 5 quarts of blood every minute. It beats about 100,000 times in one day.
Oxygen-poor blood ("blue blood") returns to the heart after circulating through the body.The right side of the heart, 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 ("red blood") then enters the left side of the heart, composed of the left atrium and ventricle, and is pumped through the aorta to the body to supply tissues with oxygen.
Four valves within your heart keep the blood moving the right way. The tricuspid, mitral, pulmonary and aortic valves work like gates on a fence. They open only one way and only when pushed on. Each valve opens and closes once per heartbeat — or about once every second.
A beating heart contracts and relaxes. Contraction is called systole, and relaxing is called diastole.
The 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.
The heart is nourished by blood, too. Blood vessels called coronary arteries extend over the surface of the heart and branch into smaller capillaries.
The 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 the heart beating in a coordinated and normal rhythm, which in turn keeps blood circulating.
source: Mayo Clinic