The coronary arteries supply the heart muscle with blood, which carries oxygen and nutrients to the heart muscle.

The three main coronary arteries are:

• Right coronary artery
• Left coronary artery
• Posterior circumflex artery

The coronary arteries become blocked by a process known as atherosclerosis, which is also called hardening of the arteries.

The process of atherosclerosis includes:

• The inner wall of the artery becomes damaged by changes in blood flow, infection, smoking, diabetes, or high blood pressure.
• As the wall or the artery attempts to repair itself, cholesterol builds up on the inside surface of the artery, forming a plaque.
• Plaques narrow the inside of the arteries, which reduces blood flow.

The heart muscle can tolerate narrowed coronary arteries, as long as the blood supply meets the demands of the heart muscle.

Blood flow may not meet demand under two circumstances:

• The artery narrows to the point that it does not supply enough blood when demand is high, such as during exercise: this causes angina.
• A blood clot forms near a plaque, blocking the flow of blood completely. The area of heart muscle supplied by the artery does not receive oxygen, and the muscle starts to die. If the supply of oxygen is not restored quickly, the muscle dies: this causes a heart attack.