Arteries, Veins and Capillaries

Blood never leaves its tubes on the way round the body. It travels through three main kinds of blood vessel, linked in a fixed order:

heart → arteries → arterioles → capillaries → venules → veins → heart

Arteries branch into smaller and smaller vessels. The smallest, the arterioles, lead into capillaries, where materials are exchanged with the body's cells. Capillaries join up into small veins called venules, which merge into larger and larger veins that carry the blood back to the heart. Each kind of vessel has a structure that suits its job.

Arteries carry blood away from the heart. Blood leaves the ventricles in powerful surges, so arteries have thick walls with a middle layer of smooth muscle and elastic fibres. The elastic tissue stretches as each surge arrives and then springs back, which helps keep blood moving forward between beats. The muscle can tighten or relax to change the width of the vessel and control how much blood reaches each organ.

Compared with the thickness of the wall, the space inside an artery, called the lumen, is fairly narrow.

Capillaries are the smallest blood vessels. Their lumen is only about 5 to 10 micrometres across (a micrometre is a thousandth of a millimetre), so the narrowest are barely wide enough for a single red blood cell to squeeze through.

The wall of a capillary is just one layer of thin cells. That short distance lets substances pass easily between the blood and the cells around it:

• oxygen and glucose move out of the blood to the cells;
• carbon dioxide and other waste move from the cells into the blood.

Capillaries form dense networks called capillary beds, so almost every cell in the body is close to one.

Veins carry blood back to the heart. By the time blood has passed through the capillaries, its pressure is low. Veins therefore have thinner walls with less muscle and elastic tissue, and a wider lumen that lets blood flow back with little resistance.

Many veins, especially in the arms and legs, contain valves. These stop blood from slipping backwards, which matters because the pressure is low and the blood in your legs has to travel upwards against gravity.

Veins also act as the body's blood store: at any moment the veins of the body hold about 64 per cent of all your blood.

A common slip is to say that arteries always carry oxygen-rich blood. The rule is about direction, not oxygen. The pulmonary artery carries oxygen-poor blood away from the heart to the lungs, and the pulmonary veins bring oxygen-rich blood back. Everywhere else in the body, arteries carry oxygen-rich blood and veins carry oxygen-poor blood.