Veins are responsible for transporting blood from parts of the body back to the heart. There are four types of veins; pulmonary, systemic, superficial and deep veins. The inner structure of veins consists of valves that keep the movement of blood from backing up.
Blood enters into veins through arterioles and capillaries. The blood transfers from arteries. Venules are the tiny version of veins that branch out from capillaries. There are three layers of a vein wall. These are the tunica adventitia, tunica media and the tunica intima.
Pulmonary veins transport oxygen filled blood from the lungs to the heart, specifically to the left atrium. There are four pulmonary veins extending from the heart to the lungs. These are the right superior, right inferior, left superior and left inferior. Systemic veins are responsible for returning the de oxygenated blood back to the right atrium of the heart from the rest of the body, and superficial veins are the type of veins that are located near the surface of the skin. This type of vein is not located near a corresponding artery. Deep veins are located, as their name indicates, deep in layers of muscle tissues. Deep veins do exist near a corresponding artery, sharing the same name for ease of reference.