What is the Venous System?

The venous system in the body returns deoxygenated blood from all parts of the body, including the organs, to the right side of the heart, and then on to the lungs, to be oxygenated. From the lungs, the oxygenated blood passes to the left part of the heart, to be pumped to all the tissues and organs of the body.

The venous system consists of large and small veins; the large veins tend to lie alongside arteries. Veins are more thin-walled than arteries, they act as a reservoir for blood, and about 75% of the body’s blood is in the venous system.

Many veins have one-way valves to facilitate the flow of blood back to the heart against the force of gravity. This is especially applicable to the veins in the legs, and to a lesser extent, in the arms. The valves work in the same way as one- way swing doors, the blood pushing the valves open as it travels toward the heart; the valves close as blood fills that part of the vein, and prevents backward flow.

Though the legs have superficial veins (under the skin) and deep veins (in the muscles) which carry blood back to the heart, as well as connecting veins known as “perforating veins” (which connect the superficial and deep veins), most of the blood is propelled back to the heart by the deep veins. Blood from the superficial veins mostly travels through the connecting veins to the deep veins and is returned to the heart this way, this is also a one-way system, as the connecting veins have valves as well.

The deep veins are surrounded by muscles enclosed in a fascial sheath. Walking compresses the veins and thrusts the blood back towards the heart.

This is why exercise, especially walking, is so useful in preventing stasis of the blood in the legs, and consequently venous thrombosis.

The one-way valves also help in this process of return of blood, as well as allowing the deep veins to refill from the superficial veins.

What is Deep Venous Thrombosis (DVT)?

Deep venous thrombosis (DVT) and pulmonary thromboembolism, also known as pulmonary embolism (PE), are sometimes described together as Thromboembolic disease.

DVT occurs when there is thrombosis or clot formation in the deep veins (often of the leg) but it may occur higher up, such as the femoral (upper leg) and iliac veins in the pelvis.

There are many causes of DVT, and the most important factor which increases the risk of developing a DVT is the history of previous DVT or PE.

The true incidence and prevalence of thromboembolic disease is greatly underestimated and autopsy studies have shown that around 80% of these conditions may remain undiagnosed. The annual incidence is around 1 per 1000 persons per year.

Click on the links for details of:

• How Valves Work in Veins (Diagram)
• Pulmonary Embolism
• Varicose Veins
• Venous Ulcers
• Superficial Thrombophlebitis
• Chronic Venous Insufficiency

Amended April 2008