vein - Function, Anatomy, Notable veins and vein systems, Medical interest, Color

A vessel usually conveying deoxygenated blood from tissues back to the heart. Deep veins accompany arteries; superficial veins lie in the subcutaneous tissue, and often appear as blue channels just below the skin (eg at the wrist and on the forearm). Blood flow in veins is slower and at a lower pressure than in arteries; consequently veins are usually larger than their corresponding arteries and their walls are thinner. When empty, veins collapse. In most veins valves are present, formed by endothelial folds strengthened by fibrous and elastic tissue. The valves are arranged to prevent flow back to the periphery, and so help to support a column of blood in veins where there is an upward flow (eg the veins of the legs). Incompetence of the valves in leg veins leads to varicose veins, which can become painful and require surgical removal.

In the circulatory system, a vein is a blood vessel which carries blood toward the heart.

Function

Veins serve to return blood from organs to the heart. In systemic circulation oxygenated blood is pumped by the left ventricle through the arteriesto the muscles and organs of the body, where it nutrients and gases are exchanged at capillaries, entering the veins filled with cellular waste and carbon dioxide. The de-oxygenated blood is taken by veins to the right atrium of the heart, which transfers the blood to the right ventricle, where it is then pumped to the pulmonary arteries and eventually lungs. In pulmonary circulation the pulmonary veins return oxygenated blood from the lungs to the left atrium, which empties into the left ventricle, completing the cycle of blood circulation.

The return of blood to the heart is assisted by the action of the skeletal-muscle pump which helps maintain the extremely low blood pressure of the venous system.

Anatomy

Most veins have one-way valves called venous valves to prevent backflow caused by gravity. They also have a thick collagen outer layer, which helps maintain blood pressure and stop blood pooling. Veins are surrounded by helical bands of smooth muscles which help maintain blood flow to the right atrium. The total capacity of the veins is more than sufficient to hold the entire blood volume of the body; Long periods of standing can result in blood pooling in the legs, with blood pressure too low to ascend to the heart. In these cases the smooth muscles surrounding the veins become slack and fill with blood, absorbing a large portion of the total blood volume, keeping blood away from the brain and causing unconsciousness.

Notable veins and vein systems

The pulmonary veins carry relatively oxygenated blood from the lungs to the heart.

A numbered vein is one of considerable importance that it is numerically classified by today's medical society, the most prominent of these being the 15 vein.

Medical interest

Veins are used medically as points of access to the blood stream, permitting the withdrawal of blood specimens (venipuncture) for testing purposes, and enabling the infusion of fluid, electrolytes, nutrition, and medications.

If an intravenous catheter has to be inserted, for most purposes this is done into a peripheral vein (a vein near the surface of the skin in the hand or arm, or less desirably, the leg.) Some highly concentrated fluids or irritating medications must flow into the large central veins, which are sometimes used when peripheral access cannot be obtained.

Color

In the light, blood appears red because most colors are absorbed except for red, which bounces back from the blood.