Vein Anatomy

Veins are blood vessels that carry blood from the body back to the heart. Most of our blood volume is carried in the veins (64%). 

More than 80 million Americans are living with venous disease.



Veins and Valves

Blood return from the legs occurs mainly through the deep veins. Within the veins, especially those of the legs, are valves. Venous valves are bicuspid (two) flap like structures made of elastic tissue. The valves function to keep blood moving in one direction.

Venous Blood Flow

The flow of blood in the venous system is complex for several reasons:

  • Low pressure within the veins
  • Flow rates that vary from high (during muscle contraction) to almost no flow during quiet standing or sitting positions
  • Gravity
  • Collapsible nature of the venous wall
  • Presence of valves
  • Large volume of blood carried in the veins

Once the blood has passed from the arteries through the capillaries, it is flowing at a slower rate because little pressure remains to move the blood along. Blood flow in the veins below the heart is helped back up to the heart by the muscle pump. The walls of the veins are thin and somewhat floppy. To compensate for this, many veins are located in the muscles. Movement of the leg squeezes the veins, which pushes the blood toward the heart. When the muscles contract, the blood within the veins is squeezed up the vein and the valves open. When the muscle is at rest, the valves close helping to prevent the backward flow of blood. 

Open valve

Open valve

closed valve

Closed valve

Damaged valve

Healthy vs. Damaged Valves

Healthy legs have veins with smooth, elastic walls that are perfectly designed to adapt to the changes in pressure within a vein. Veins have valves that keep blood moving in one direction: back toward the heart. As the leg muscles are activated, the venous valves open to allow one-way flow in the direction of the heart. When the muscles relax, the valves close to stop any back-flow.

But if the walls of a vein have been damaged by varicosis or thrombosis, the vein may dilate and the valves fail to close properly. When valves fail to work properly, blood flows backward into the veins. This results in blood pooling, putting pressure on lower leg veins, which may cause even more valves to fail over time.

So when the body is upright, the blood being transported back to the heart may stagnate in the legs. The pressure in the superficial veins directly under the skin rises and the veins become swollen. Tired, aching legs are the most common early symptoms – particularly after prolonged standing. Later, fluid may collect in the feet and ankles causing them to swell. The skin above the ankles may become thin and discolored or even break to form a venous stasis ulcer.