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.

There are blood vessels all over the body. Some are easy to see, because they are close to the skin surface. Most blood vessels cannot be seen because they are buried deep among the muscles and other tissues. If you press your fingertips against some blood vessels you will feel a regular throbbing movement. This is called the pulse. Blood vessels which have a pulse are called arteries. They carry blood away from the heart. There are various places on the body where the pulse can be felt. The pulse is usually found on a person’s left wrist.

Each pulse throb is caused by a sudden rise in the pressure of the blood in the artery. What do you think causes this sudden rise in pressure of the blood in the artery? Okey, here’s another question: what makes the hot water move through the pipes and radiators of a modern central heating system?

Blood vessels are like pipes. The blood is like water. A pump is needed to move the blood through the blood vessels. This pump is the heart. Every time it beats, the heart forces blood out through the arteries. With each heartbeat there is a rise in pressure. This causes the arteries to swell, which is what you can feel at the pulse points. This explains why pulse rate is the same as heartbeat rate.

Your heart is the best pump ever made. In most people, the heart works all the time, without repair or servicing, for the whole of their lives. The speed at which it beats can be changed, in a few seconds, from about 70 to 140 beats per minute or more. Rates of 200 or more occur during violent exercise. Most important of all, the heart does not damage the blood as this passes through it. Blood is a delicate thing. No man has ever built a pump which is so gentle, so strong and so reliable. But the heart can go wrong if it is not looked after properly.