artery

Blood flows through 96,500 km/60,000 mi of arteries and veins, supplying oxygen and nutrients to organs and limbs. Oxygen-poor blood (blue) circulates from the heart to the lungs where oxygen is absorbed. Oxygen-rich blood (red) flows back to the heart and is then pumped round the body through the aorta, the largest artery, to smaller arteries and capillaries. Here oxygen and nutrients are exchanged with carbon dioxide and waste products and the blood returns to the heart via the veins. Waste products are filtered by the liver, spleen, and kidneys, and nutrients are absorbed from the stomach and small intestine.

Cross sections of an artery and a vein. Arteries have thicker walls than veins as they have to withstand a higher blood pressure than do veins. Veins have valves to prevent blood from flowing backwards.

Blood vessel that carries blood from the heart to any part of the body. It is built to withstand considerable pressure, having thick walls that contain muscle and elastic fibres. As blood pulses out of the heart, arteries expand to allow for the increase in pressure - this elasticity helps the blood to flow evenly. The pulse or pressure wave generated can be felt at the wrist. Not all arteries carry oxygenated (oxygen-rich) blood - the pulmonary arteries convey deoxygenated (oxygen-poor) blood from the heart to the lungs.

Arteries are flexible, elastic tubes, consisting of three layers. The middle layer is muscular and its rhythmic contraction aids the pumping of blood around the body. In middle and old age, artery walls become damaged. Some of this damage is caused by the build-up of fatty deposits (see fats). This reduces elasticity and narrows the space (or bore) through which the blood can flow. This condition is known as hardening of the arteries or atherosclerosis and can lead to high blood pressure, loss of circulation, and death. When it affects the arteries supplying blood to the muscle of the heart wall it is called heart disease. People with heart disease run an increased risk of having a heart attack. In a heart attack blood vessels supplying the heart muscles are blocked by a blood clot and this can result in sudden death. Research indicates that for some people a typical Western diet, high in fat from animals, increases the chances of heart disease developing.