There are two sets of veins in the legs. The important veins located within the leg muscles called deep veins carry nearly all the venous return to the heart.
The main deep veins in the leg include the common femoral vein, the femoral vein, the profunda femoris vein, the popliteal vein, and the calf veins. For the blood to travel up to the heart against gravity, the blood is squeezed forward with muscle contractions, while one-way valves help prevent retrograde flow.
As these deep veins return nearly all the blood from the lower extremities, a clot in the deep venous system called a deep vein thrombosis (DVT) usually causes pain and swelling in the leg, especially if the thrombus is at or above the knee. This is also worrisome as it can propagate secondary to stasis, break off and travel to the lungs causing a pulmonary embolus (PE), or cause chronic leg issues like post-thrombotic syndrome. In severe cases, circulation of the legs can be compromised.
In contrast, the second set of veins are called superficial veins, so named because they are closer to the skin in the subcutaneous tissue. Because of their superficial location outside of the musculature, there is minimal assistance from muscle contraction in moving blood upwards against gravity. Rather, the superficial veins rely mainly on their one-way valves to return the blood.
The two main superficial veins in the leg are the great saphenous vein (GSV) coursing medially up the calf and thigh to the groin, where it confluences with the common femoral vein of the deep system, as well as the small saphenous vein (SSV) located in the posterior calf, where it typically dumps blood into the popliteal vein of the deep system.
There are other veins in the leg called perforator veins, which connect the deep and superficial venous systems. Also, intersaphenous veins connect the GSV and the SSV and can play an important role in hemodynamic flow.