Veins are blood vessels that carry blood to the heart, while arteries carry it from the heart. The heart acts as a pump, pushing blood to every part of the body. Adults have between five and six litres (between nine and ten pints) of blood. As well as containing red cells to carry oxygen to the body’s organs, blood also plays an important part in fighting infection. White blood cells attack and digest harmful bacteria, while platelets in the blood form clots so that wounds can heal and no further infection can enter the body.
Arteries and veins both carry blood around the body, and they each have three main layers of tissue (a ring of endothelial tissue at the centre of the blood vessel surrounded by a layer of muscle and elastic fibres, which is surrounded by a layer of connective tissue). However, there are several differences between them:
- Arteries carry blood from the heart to the rest of the body, whereas veins carry blood from the rest of the body back to the heart.
- Almost all arteries carry oxygenated blood and almost all veins carry deoxygenated blood. The only exceptions are the pulmonary artery, which carries deoxygenated blood from the heart to the lungs, and the pulmonary vein, which carries oxygenated blood from the lungs to the heart.
- Arteries have a thick elastic muscle layer, whereas the muscle layer for veins is much thinner. This is because the heart pumps blood into the arteries at high pressures, so the walls of the arteries must be able to cope with the changes in pressure during a heartbeat. Veins carry blood at much lower pressures so do not need such a thick wall.
- Arteries have a much narrower lumen (the hole at the centre that the blood flows through) than veins. This helps keep higher blood pressures in the arteries, which is needed to keep blood flowing quickly to body tissues.
- Veins have valves and arteries do not. In arteries, blood flows in the right direction because of the heart pumping it forwards at high pressures. The lower blood pressure in veins means that valves are needed to stop blood flowing backwards (for example, in veins in the legs, blood needs to flow upwards against the pull of gravity).
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