Veins are small blood vessels that generally carry deoxygenated blood to the heart from the tissues. Exceptions to this rule include the umbilical veins and the pulmonary veins which carry oxygenated blood.
Veins are known as return vessels because they return blood from the body to the heart. They are small tube-like structures that have a thick outer layer of connective tissue, a middle layer of smooth muscle, and an interior layer that is comprised of endothelial cells. They make up part of the human circulatory system and utilize gravity and inertia to pump blood through the entire human body.