Unlike arteries, veins are thin-walled and highly distensible, with a capacity for containing up to two thirds of circulating blood flow. The venous intima consists of nonthrombogenic endothelium. Protruding into the lumen are unidirectional valves that promote venous return to the heart. The media contains circumferential rings of elastic tissue and smooth muscle that change vein caliber in response to even minor changes in venous pressure. The smallest veins, or venules, drain capillary beds and form interconnecting venous plexuses such as the prostatic and the rectal venous plexuses.

Veins from the arms, upper trunk, and head and neck drain into the superior vena cava, which empties into the right atrium. Veins from the abdominal viscera, lower trunk, and legs drain into the inferior vena cava, except for circulation through the liver. The portal vein, at the confluence of the nutrient-rich superior mesenteric and splenic veins, supplies ∼75% of the blood flow to the liver, supplemented by oxygenated blood from the hepatic artery. Blood from these vessels flows into the hepatic sinusoids, then drains into three large hepatic veins that empty into the inferior vena cava. Because of their weaker wall structure, the leg veins are susceptible to irregular dilatation, compression, ulceration, and invasion by tumors, and warrant special attention.

Deep and Superficial Venous System of the Legs. The deep veinsof the legs carry approximately 90% of the venous return from the lower extremities. They are well supported by surrounding tissues.

In contrast, the superficial veins are subcutaneous, with relatively poor tissue support (Fig. 1). They include:

■ The great saphenous vein, which originates on the dorsum of the foot, passes just anterior to the medial malleolus, continues up the medial aspect of the leg, and joins the femoral vein of the deep venous system below the inguinal ligament

■ The small saphenous vein, which begins on the lateral side of the foot, passes upward along the posterior calf, and joins the deep venous system in the popliteal fossa

FIGURE 1. Superficial veins of the leg.

Anastomotic veins connect the two saphenous veins and are readily visible when dilated. Bridging or perforating veins connect the superficial system with the deep system (Fig. 2).

When competent, the one-way valves of the deep, superficial, and perforating veins propel blood toward the heart, preventing pooling, venous stasis, and backward flow. Contraction of the calf muscles during walking serves as a venous pump, also propelling blood upward against gravity.

FIGURE 2. Deep, superficial, and perforating veins of the leg.

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