The veins of the lower limbs are subdivided into three sets:

  • superficial veins
  • deep and communicating veins
  • perforator veins.

These three hydraulic systems are engaged in interactive balancing.

The superficial main veins are the great saphenous and small saphenous veins. All the other superficial veins are tributaries of saphenous veins. The deep veins accompany the arteries, the communicating or perforator veins allow communication between the superficial venous system and the deep venous system of limbs.

The average number of perforator veins per extremity is highly variable and reported to be as many as 155 perforator veins per lower limb. All sets of veins are provided with valves, which permit unidirectional flow: from the superficial to the deep vessels.

These communicating channels allow the superficial veins to empty some of their blood into the deep veins during rythmic muscular contractions in the legs. In normal veins, blood flows from the lower limbs to the right side of the heart because of an energy gradient.

Venous functions are:

  • Draining
  • Thermoregulation
  • Transport, Filling right hearth
  • Reservoir

One of the main function of veins is drainage: to carry blood from the body back to the heart.

This blood is:

  • low in oxygen,
  • depleted of nutrients,
  • loaded with waste products.


At any given time, about 75% of circulating blood in our body is moving through the venous system. The venous system is phasic which refers to the ebb and flow that occurs in normal veins in response to respiration.

The way in which the blood moves in phase with respiration differs according to the part of the body affected and the position in which the body is placed. During inspiration the chest cavity expands, the diaphragm lowers and the abdomen becomes smaller. The volume of the veins of the thorax increases and the pressure decreases in response to the reduced intrathoracic pressure.

In the abdomen, because of the descent of the diaphragm, the pressure increases. Increased abdominal pressure decreases pressure gradients between peripheral veins in the lower extremities and the abdomen, thus reducing flow in the vessels.

During expiration the thoracic cavity decreases, the diaphragm elevates and the abdomen becomes larger. The volume of the veins of the thorax decreases and the pressure increases.

In the abdomen, because of the elevation of the diaphragm, the pressure decreases.

Decreased abdominal pressure increases pressure gradients between the peripheral veins in the lower extremities and the abdomen, thus increasing flow to the vessels.

Hydrostatic pressure is the pressure due to the weight of a fluid. The height of the column of blood is always referenced back to the level of the heart.

Hydrostatic pressure varies with position. When supine, there is virtually no hydrostatic pressure in the legs, as they are at the same level as the right atrium, which has a pressure of zero. Gravity exerts significant effects on venous return because of our upright posture. As body posture is changed from supine to standing, gravity acts upon the vascular volume, so that blood accumulates in the lower extremities. Because venous compliance is high, most of the blood volume shift occurs in the veins. Therefore, when standing, venous volume and pressure becomes very high in the feet and lower limbs. When venous valves are working correctly, every movement of the leg causes blood to be pumped inward and upward past a series of valves.

During walking, the normal pressure in the venous system of the lower leg is low. Immediately after walking, the early standing pressure in the normal leg remains low.

Arterial inflow fills the leg veins slowly and the only source of venous pressure is the hydrostatic pressure of a column of blood as high as the nearest competent valve.

After prolonged standing, the veins are completely filled and all the venous valves float open. At this time, high hydrostatic venous pressure results from the unbroken column of fluid that extends from the head to the foot.

Failed valves cause the column of standing blood in the vein to remain high even when walking. The hydrostatic pressure increases during and immediately after ambulation.

High venous pressure is directly responsible for many aspects of the venous insufficiency syndrome, including edema, tissue protein deposition, perivascular fibrin cuffing, red cell extravasation, impaired arterial inflow, and other locally mediated disturbances.

Venous valves play a very important role in the function of venous return, especially in the lower extremities. They are irregularly located along the veins, but are always found at the junctions of tributaries with the main venous channels or where two large veins join.

Venous valves are usually bicuspid and occasionally tricuspid. In the leg, some veins have fewer valves than others. The deep system has more valves than the superficial.

Venous valves direct flow, they keep the blood moving back toward the heart in both the deep and the superficial veins. In the perforating veins, the valves direct the flow from the superficial to the deep veins.

If the venous valves are functioning normally, they prevent reflux. Venous valves open and close in conjunction with the action of the muscles: the valvulo-muscular pump. In the extremities, the deep veins are surrounded by muscles.

As these muscles contract, they squeeze the veins within them. To function, muscles do not remain permanently contracted. They must alternately contract and relax, thus propelling the blood up the leg. In the venous system, it is the interaction of the venous valves and the muscular pump that keep venous blood moving and moving in the right direction. When these two elements are working properly, the veins in the lower extremities can empty so effectively that emptying is actually complete.(from ASUM Ultrasound Bulletin 2004 August 7:4: 14–21).

The return of blood to the heart is assisted by the action of the skeletal-muscle pump, and by the thoracic pump action of breathing during respiration.