Veins are blood vessels that transport blood towards the heart. Almost all veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart. In comparison to veins, arteries carry blood away from the heart. Veins are less muscular than arteries and are usually closer to the skin. There are valves in the majority of veins to avoid backflow.
Veins also are subdivided into three classes:
- Large veins include some smooth muscle in the tunica media, but the thickest layer is the tunica externa. Examples of large veins are the superior vena cava, the inferior vena cava, as well as the portal vein.
- Small and medium veins consist of small amounts of smooth muscle, and the thickest layer is the tunica externa. Examples of small and medium veins are superficial veins in the upper and lower limbs and much deeper veins of the leg and forearm.
- Venules are the tiniest veins and drain the capillaries.
The walls of veins have the very same three layers as the arteries. Although all the layers are present, there is much less smooth muscle and connective tissue. This makes the walls of veins thinner than those of arteries, which is due to the fact that blood in the veins has less pressure than the arteries. Because the walls of the veins are thinner and less hard compared to arteries, veins can hold more blood. Almost 70 percent of the overall blood volume is in the veins at any given time. Medium and large veins have venous valves, much like the semilunar valves related to the heart that helps in keeping the blood flowing towards the heart. Venous valves are especially important in the arms and legs, where they avoid the backflow of blood in response to the pull of gravity.