Veins have valves to prevent the backflow of blood, according to Professor Tamarkin at Springfield Technical Community College. Veins carry blood from the body back to the heart, so they need these one-way valves to prevent blood from flowing backward.
The valves of veins consist of endothelial tissue, states Professor Tamarkin. The valves are just the bending back of the veins' endothelium, the innermost cell layer of blood vessels.
In addition to keeping blood from flowing backward, valves help overcome the force of gravity, says the Franklin Institute. Blood from the extremities, especially those below the level of the heart such as the legs, must be able to flow upward toward the heart; the valves help blood "climb" up the veins toward the heart.