Blood looks red because of the red corpuscles which carry oxygen. It also contains white corpuscles which deal with disease. All blood cells are carried in plasma, a straw-coloured liquid. To reach every living cell, blood is carried round the body in tubes. The larger tubes are called arteries and veins, and the smallest are capillaries. Arteries carry blood from the heart to all parts of the body, and veins take it back to the heart. Capillaries take the blood to every single cell.
The blood is the body’s central heating system. The temperature is controlled by the brain at about 37 degree centigrade. On average , an adult has about 5 litres of blood in the body.
A single red cell lives for about four months. Red cells have to be replaced regularly and new ones are produced in the bone marrow. People who develop leukemia cannot produce new cells. They can have transplants of bone marrow from a donor whose marrow type matches their own.
The red blood vessels are arteries carrying blood with a lot of oxygen in them. The blue blood vessels are veins carrying less oxygen and more carbon dioxide and other wastes. The blood takes oxygen out of the air in the lungs. The heart pumps it round the body. The blood system consists of the heart, arteries, veins and capillaries. Blood leaves the heart through a large artery called aorta.
Various kinds of white cells in the blood help defend the body against attacks. Some white cells eat bacteria which get into the blood. Some make particles called antibodies, which attach themselves to harmful particles, or to poisons produced by bacteria, and stop them from doing any damage. White cells called platelets help the blood to clot.
Each person’s blood can be put into one of four groups. These are A, B, AB and O. For a transfusion, the blood has to be from a compatible group.
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