Angina occurs when there is reduced blood flow to your heart muscle. Blood carries oxygen, which is essential to your heart muscle. The most common reason why reduced blood flow happens is coronary artery disease. It develops when fatty deposits, called plaque, build up in the blood vessels that supply blood and oxygen to the heart. This buildup is also known as atherosclerosis. When you are resting, your heart muscle may still be able to cope with reduced blood flow without triggering angina symptoms. However when you increase demand for oxygen, such as when you exercise or do a strenuous activity, angina can occur.