vein

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.