Normal Venous System
After blood is pumped from the heart, it travels through specialized blood vessels called arteries to supply the legs. This same blood then needs to return to the heart and does this by way of the veins of the legs.
When the muscles of the thigh and calf contract, as with walking, blood is pumped in an upward direction through the veins, toward the heart. In order to allow efficient blood return, these veins have a series of one-way valves that typically only allow blood to flow up the vein, and they do not allow that blood to flow in a downward direction under the effects of gravity.
In a normal vein, blood is only allowed to move upward in the leg. The venous one-way valves prevent backward flow down the leg.
The venous system of the leg is comprised of two major sets of veins. The first of these, the deep venous system, is made up principally of the femoral vein and its feeding branches. These deep veins lie underneath the muscles of the leg and carry the majority of blood flow from the leg. When blood clots form in these deep veins, called deep vein thrombosis, severe leg swelling may occur, as it can become difficult for blood to return to the heart from the leg. This situation may also become quite dangerous if a clot in a deep vein should dislodge and travel to the lungs, blocking blood flow to the lungs.
Front (A) and back (B) views of the leg show greater and short saphenous veins of the leg along with deep (femoral) vein.
The second system of veins in the leg is the superficial venous system. The two main superficial veins are the greater saphenous vein, which runs along the inside of the thigh and calf, and the short saphenous vein, which runs along the back of the calf. Both of these veins have multiple feeding branches. The superficial veins tend to lie just below the skin surface and are by far most commonly involved in the development of varicose veins. If blood clots should develop in a superficial vein, the episode may cause pain and tenderness, but the risk of clot movement to the lungs is very small.
