Anatomy of a Vein

Veins are the blood vessels that return blood to the heart. With the exception of the pulmonary veins, the blood that is returned is deoxygenated (that is, all of the oxygen has been used in the body and must be replaced in the lungs).

Veins, like arteries, have three layers: the intima, media, and adventitia. The intima is lined by a single layer of cells that allows for the flow of blood. Vein layers are much thinner than those in arteries and contain less muscle. In addition, veins collapse when empty. Flow in the veins depends upon whether the walls are collapsed, changes in breathing (respirations), the effects of gravity, and muscle activity around the veins themselves. One major difference between veins and arteries is that veins have valves. These are tissue-paper-thin leaflets within many veins that prevent the backflow of blood. Gravity tends to pull the blood back towards the lowest point, and valves stop this from happening. Valves are found most often in the extremities; the largest number are in the legs. Once the blood gets up to the veins in the abdomen, natural pressure changes and muscle action are enough to keep the blood flowing toward the heart. If the valves in the extremities do not work properly, individuals may develop varicose veins, swelling, skin changes, and ulcers.

Veins begin at the capillaries, which are the ends of the arterial system. They gradually become larger and larger. Branches combine and flow into the vena cava (the large vessel in the abdomen and the upper chest) to return the blood to the heart.

Damage to the intima of the vein can result in the formation of a blood clot. Other things that may cause or contribute to clotting are stagnant flow in the veins (stasis) and increased amounts of factors for blood clotting (a hypercoagulable state, also known as thrombophilia). As long as the venous system works, blood should return easily to the heart, where it is then sent to the lungs for oxygen and back out into the arteries by way of the heart.