**2. Anatomy of the venous system**

The venous blood flow in the lower extremity is located at three positions, which are superficial, deep and perforating veins [9].

More than 80% of the blood flow in the lower extremity occurs through the deep veins. The deep veins accompany the arteries and run deep to the muscles and fascia [10, 11]. It includes the posterior tibial vein, anterior tibial vein, femoral vein, profunda femoris vein and common femoral vein in the leg. It also includes the popliteal vein and femoral vein in the thigh, and the external iliac vein, internal iliac vein, common iliac vein and inferior vena cava in the pelvis [12].

The major axial veins are accompanied by matching the venae comitantes in the calf. These veins that accompany the posterior tibial and peroneal veins are particularly large (up to 10 mm in diameter in male) and participate in the muscle pump mechanism for the return of blood from the legs to the heart. In addition, the medial and lateral heads of the gastrocnemius and soleus muscles include vein pairs in these muscles (the calf) that accompany the major arteries. These usually have a large diameter and participate in the calf muscle pump. The calf veins drain into the popliteal vein. Once the popliteal vein has entered the thigh, it is known as the femoral vein. The deep femoral veins which open into the femoral veins derived from the common femoral vein in the inguinal triangle drain the thigh muscles. This drains into the great saphenous vein and is the major point of the venous return in the lower extremity [12].

The superficial compartment lies between the dermis and the deep fascia [12]. Duplex scanning has resulted in recognition of a saphenous subcompartment and saphenous fascia. The vena saphena magna and vena saphena parva are intermediate superficial veins. The VSM is the longest vein in the human body [10, 11].

These veins connect the superficial venous system to the deep venous system. There are approximately 150 perforating veins in each lower extremity and a few of them have clinical significance. The medial calf perforators are very important at this point [12].

The perforating veins are divided into direct and indirect perforating veins. While direct perforating veins provide a connection between the superficial and deep axial veins, indirect perforating veins provide blood drainage from the superficial venous system and join other veins in the muscles [13].

Bicuspid valves in superficial veins provide one-way venous blood flow towards the heart. The frequency of valves is particularly higher below the knee. Valves are large (median 7–10, range 4–13) and placed closer in the small saphenous vein. The frequency of valves increases from proximal to distal in the deep veins. Deep leg veins, peroneal veins and posterior and anterior tibial veins contain plenty of valves which are arranged at approximately 2 cm intervals. However, the common femoral vein usually has one valve. In addition, it must be emphasized that there is only one valve (and sometimes, there is no valve in 37% of the reported cases) in external iliac and common femoral veins, which are proximal to the saphenofemoral junction. The common iliac and vena cava veins have no valves [14].
