Arteries and veins carry blood around the body. All the tissues and organs that make up the body must be continuously supplied with food and oxygen. The job of transporting these essentials is done by the blood. Pumped by the heart, blood picks up dissolved food from the intestines and oxygen from the lungs, then delivers them to all parts via the arteries. Veins carry blood containing waste carbon dioxide from the tissues back to the heart to be pumped to the lungs. With about 5 litres (9 pints) flowing around an adult’s body, blood also clears away waste, clots to prevent leaking where blood vessels are damaged, protects against invading bacteria and viruses, and helps keep the body at a steady 37°C (98.6°F).
What blood is made of
Seen under a microscope, blood is made up of millions of tiny cells floating in a yellowish, watery fluid called plasma. Nutrients and other substances needed by the body’s cells are dissolved in the plasma. The plasma picks up waste carbon dioxide and returns it to the lungs. There are red cells, used for carrying oxygen; white cells, which fight any infection by invading bacteria or viruses; and platelets, which make the blood clot when a vessel is damaged, so sealing the wound.
One tiny drop of blood the size of a pinhead contains about 5 million red blood cells, 9000 white cells and about 250,000 fragments of cells, called platelets. Nutrients are dissolved in the plasma. About 90% of blood is water.
Red and white cells
In the UK and USA, the commonest blood group is O (about 44% of people); the rarest is AB (less than 10%).
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