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.