vein

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.

The structure of a vein, showing how the valves prevent blood from flowing backwards.

Vessel that carries blood from the body to the heart in animals with a circulatory system. Veins contain valves that prevent the blood from running back when moving against gravity. They carry blood at low pressure, so their walls are thinner than those of arteries. They always carry deoxygenated blood, with the exception of the pulmonary vein, leading from the lungs to the heart in birds and mammals, which carries newly oxygenated blood.

The term is also used more loosely for any system of channels that strengthens living tissues and supplies them with nutrients – for example, leaf veins (see vascular bundle), and the veins in insects' wings. In leaves they make up the network that can normally be seen, especially from the underside of the leaf. These veins are made up of the two transport tissues of a plant, xylem and phloem.