**3. Pathogenesis**

### **3.1. Normal venous anatomy and physiology**

The venous system of the lower extremities includes the superficial veins, perforator veins, and the deep veins according to their relationship to the muscular fascia. The superficial veins comprises the reticular veins, the large (larger) and small (smaller) saphenous veins, and their tributaries. The great saphenous vein originates from where the dorsal vein of the first digit merges with the dorsal venous arch of the foot. After passing in front of the medial malleolus, it ascends the medial side of the leg. It joins the femoral vein just below the inguinal ligament. The small saphenous vein arises from the dorsal venous arch of the foot and ascends poster‐ olaterally from behind the lateral malleolus. Usually, it drains into the popliteal vein near the popliteal fossa. The reticular veins, a network of veins parallel to the skin surface, communicate with either saphenous tributaries or the deep veins through perforators. The perforator veins connect the superficial and deep vein systems. The deep venous system is categorized as either intramuscular or intermuscular. Intermuscular veins are three paired tibial veins including, the posterior tibial vein, the anterior tibial vein, and the peroneal vein. These veins join to form the popliteal vein in the popliteal area. At the level of the adductor canal, the popliteal vein is renamed the superficial femoral vein. This vessel joins the deep femoral vein in the femoral triangle to form the common femoral vein. After passing beneath the inguinal ligament to enter the pelvis, the femoral vein is renamed the common iliac vein. The superficial veins are low‐ pressure systems, whereas the deep veins are high‐pressure systems. All three venous systems have one‐way bicuspid valves, which only open toward the deep venous system and, under normal conditions, prevent reflux of blood. Normally, ambulation and the pumping action of the calf muscles propel venous blood upward toward the heart, and the valves close when pressure rises in the deep venous system, which prevents retrograde flow [4, 14, 15].
