What is Vein?
[[File:Ch20 veins.png|thumb|Major human veins]] 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 are smaller amounts of [[smooth muscle]] and elastic fibres in the vessel wall. Most veins have one-way [[valves]] that keep blood from going backwards. The [[vena cava]], the biggest vein in [[human body]], collects blood from the other veins and delivers it to the heart. Other important veins are the [[coronary]] veins (veins on the heart) and [[Kidney|renal]] veins (veins on the kidney). Veins are mainly seen just below the skin, and are easily 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. Veins contain blood even after death. This is why a dead body appears blue. {{Biology-stub}} [[Category:Blood vessels]]