A vein is a type of blood vessel in the body. All veins carry blood to the heart. Most veins carry blood that is low in oxygen, except for the pulmonary vein and the umbilical veins which carry blood that is high in oxygen.
A vein has a large lumen (width) and less pressure than an artery. There is smaller amounts of smooth muscle and elastic fibres in the vessel wall. Most veins have one-way valves that keep blood from going backwards.
Veins carry blood to the heart and enter the heart through the vena cava. Other important veins are the coronary veins and renal veins. Veins are mainly situated just below the skin, and could easily be seen from the outside, where they look blue because of the lack of oxygen. The blood carried by veins is dark red, but when a vein is cut or pierced, the dark blood immediately reacts with the oxygen in the air and becomes bright red colored. The vein appears blue because of the scattering of light through the skin and the way the eye perceives color and light.
Vein Facts for Kids. Kiddle Encyclopedia.