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.

Hospitals are always prepared for cardiac arrests. In a large hospital, there will usually be three or four doctors called the crash team. Their job is to get to any patient with a cardiac arrest as soon as possible. The problem is that hospitals are large, busy places. These doctors may be a long way from the patient. To overcome this, each member of the crash team carries a special bleep for emergencies. The bleeps are small radios which are tuned to a special transmitter at the hospital switchboard. There are telephones all over the hospital. If a patient has a cardiac arrest someone, usually a nurse, calls the switchboard. The switchboard operator presses a button and all the emergency bleeps make a loud bleeping noise. This noise can easily be heard by those who carry the bleeps. At the same time, the operator can tell the doctors, over the radio, where the patient is. The members of the crash team stop what they are doing. They go as quickly as possible to the cardiac arrest patient. When they arrive, they try to get the heart beating again. They may use drugs or a special electric shock machine. If these fail, an operation may be needed.

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.

Doctors in Harvey’s day were taught the Greek ideas about blood. They were not expected to study the heart and blood vessels for themselves. It was enough to hear about the Greek ideas and to glance at a few dissected bodies, Harvey was the first doctor to make a really careful study of the heart and blood vessels for himself. He was able to show that the Greeks were wrong. The blood moves in one direction only through the heart and blood vessels. The heart acts as a pump to keep the blood moving.

Harvey discovered the circulation of the blood. He worked out how much blood was pumped out of the heart each minute. He also showed that this was far more than could be made from the food and water taken in. This helped to support his idea that blood moves round the body continuously, and that it is not used up. Harvey was unable to see the smallest blood vessels, which we now call capillaries, but he knew that they must be there. However, soon afterwards in 1661, an Italian scientist named Malpighi used a microscope to examine the lungs of a frog. He was able to see the capillaries. Blood passes from the arteries through the capillaries to the veins. The last link in Harvey’s discoveries about blood circulation was now in place.

Blood is a liquid with some small solid objects in it. The red blood cells carry oxygen. The white blood cells help the body to fight against infection and diseases. When the skin is cut, the platelets help to form a clot. This stops further bleeding.

The liquid part of the blood is called plasma. Plasma is water with lots of things dissolved in it, for example glucose.

The pump of you heart makes blood run in your blood vessels. One of the most important things that blood does is to carry things around the body. For example, it carries oxygen and glucose to the parts where they are needed. Blood also carries waste products away from the places where they are produced. This is why the blood and the blood vessels are called a transport system.