Every organ in the body needs to be supplied with blood for it to function properly. Fresh blood brings oxygen and food to the tissues and carries away unwanted carbon dioxide and other waste products. The blood circulates around the body through a closed system of blood vessels. The heart is the pump which pumps it around. It is a 4-chambered pump with a one-way valve system. Each contraction or heartbeat ejects blood forward into the arteries. The arteries divide into progressively smaller branches to supply a microscopic network of capillaries, so distributing the blood to every part of the body. Blood is collected from the capillaries by the veins whose branches join to form progressively larger veins delivering circulated blood back to the heart. Venous blood fills the heart as it relaxes during the interval (diastole) between each contraction (systole). This total circulation system is called the cardiovascular system. It contains about 8 pints of blood continuously re-circulated by the heart, which beats some 100,000 times and pumps some 5,000 gallons of blood per day.
The heart is in fact two separate pumps which works synchronously. The right heart receives dark blood which has circulated around the body and pumps this to the lungs, to pick up a fresh supply of oxygen and become bright red again.
The left heart receives fresh blood from the lungs and pumps it into the arterial system which supplies the rest of the body. Each side of the heart has a thin-walled collecting reservoir (atrium) which helps fill the thick-walled major pump (ventricle). The heart wall is made of a special sort of muscle called the myocardium.
Like every other living tissue, the myocardium itself needs to be continuously supplied with fresh blood, through arteries that are in this case called the coronary arteries.