By far the most common type of heart disease occurs when one (or more) of the coronary arteries becomes partly or almost completely blocked. This is called coronary heart disease. A good way to treat coronary heart disease is to by-pass the blocked bit of coronary artery. Usually, pieces of one of the veins in the leg are used to make the by-pass. The leg does not suffer, because there are plenty of other veins there to share the job of carrying the blood.

For this operation to be successful, the heart must be stopped and by-passed completely. However, the heart is really two pumps. One delivers blood to the lungs. The other delivers blood to the rest of the body. Therefore, in order to make the operation simpler, the lungs are by-passed as well. This means that only one artificial pump is needed and an artificial oxygenator is used instead of the lungs.

The circulation diagram can be modified to show what is needed. Normal body temperature is 37 °C. Cooling the body to about 30 °C reduces the activity in all parts. Therefore less oxygen is needed. The heart is kept much cooler and treated with various chemicals which stop its muscular activity. Therefore it is not damaged when its blood supply is cut off. There is no great leakage of blood when the coronary artery is cut, because there is very little blood in it. Also, the heart has stopped beating and is perfectly still. This enables the surgeon to put the stitches in exactly the right places.

This kind of arterial disease is very serious if it occurs in the arteries leading to the brain. In this case it can cause a stroke because the brain gets insufficient oxygen. The person collapses. They might recover, but if they do the brain is often permanently damaged. Arterial disease in the coronary arteries is also serious.

The coronary arteries carry blood to the muscles in the walls of the heart. If these arteries become narrower, it may lead to a heart attack. When the coronary arteries become narrower this is called coronary heart disease or ischaemic heart disease. Coronary heart disease kills more people than anything else in Britain today. Death is caused, in this case, when the heart stops beating properly (a heart attack) and the blood supply to the brain stops. It is often very sudden and very unexpected. Some people survive one heart attack only to be killed later by another one.

Coronary heart disease is much more common in smokers than in non-smokers. Smokers who give up smoking after surviving a heart attack are less likely to have another heart attack than those who continue to smoke. Nearly all people with arterial disease of the legs are smokers. For these people, walking becomes painful and many of them get gangrene so that one or both legs have to be amputated (cut off). Gangrene is caused by lack of food and oxygen. In this case it is caused by the blocked leg arteries.

No one knows why the fatty material collects in the walls of the arteries. There seem to be several things which increase the risk of this type of arterial disease. They are: smoking, high blood pressure, obesity, high levels of a fatty substance (cholesterol) in the blood, lack of exercise, stress (worry), and old age. The disease is also more common in some families than it is in others. It is more common in men than in women up to the age of 50. After this age, it is equally common in men and women.