The common cause of angina is the development of one or more narrowings in the heart arteries. There are other causes of angina such as problems with one of the heart’s valves or severe thickening of the muscle.
The heart muscle needs a constant supply of blood in order to pump blood around the body. The blood is supplied to the heart though the coronary (heart) arteries. When you exercise the heart has to pump faster and harder to supply the rest of the body with blood and hence oxygen. In the normal heart on exercise the coronary arteries widen (dilate) to allow more blood to flow down the artery. When the artery has atheroma in the wall it is unable to widen so insufficient blood is supplied to the heart muscle. This results in the feeling of angina.