Veins

Veins are thin walled blood vessels. They are also composed of the same three layers as that arteries, but possess less muscles and less elastic fibers. Their lumen is large. Semi lunar valves are present which prevent the back flow of blood. The pressure of blood flowing through them is low and are non-pulsatile. The blood flows slowly and smoothly. The veins carry de-oxygenated blood except the pulmonary veins which transport oxygenated blood from the lungs to the heart. The veins are super-facial and collapse when empty.