All of the veins in the body are either part of the superficial venous system or the deep venous system. The superficial veins serve as a conduit to pass blood centrally and eventually into the deep venous system. The principal named superficial veins of the lower extremity are the small saphenous vein (SSV), which usually runs from ankle to knee, and the great saphenous vein (GSV), which usually runs from ankle to groin.
Many superficial collecting veins deliver their blood into the great and small saphenous veins, which deliver most of their blood into the deep system. Superficial veins are also connected to a variable number of perforating veins that pass through openings in the deep fascia to join directly with the deep veins of the calf or thigh.
All venous blood is eventually received by the deep venous system on its way back to the right atrium of the heart. The principal deep venous trunk of the leg is called the popliteal vein from below the knee until it passes upward into the distal thigh, where it is called the femoral vein (FV) for the remainder of its course in the thigh. They are one and the same, and this is the largest and longest deep vein of the lower extremity.
Veins returns blood from the periphery of the body back to the heart and lungs. Unlike arteries with thick walls, most veins are very thin and easily dilatable, so the peripheral venous system also functions as a reservoir to hold extra blood. You could say that the venous system is almost magical in its function when you are told that the entire cardiac output volume of 5–10 L/min is received into periphery venous system for eventual delivery back to the heart and lungs.
The veins have one-way valves to prevent them from backward flow. The correct functioning of the venous system depends on a complex series of valves. It has been known that varicose veins in the legs are caused by weakening of the veins and valves in the great saphenous veins and/or small saphenous veins. When the valves in these veins do not properly keep blood flowing from the legs up to the heart, blood begins to collect in the legs resulting in the buildup of pressure. The veins become enlarged and knotted and are visible near the surface of the skin.