Veins (anatomy)

In biology, a vein is a blood vessel which carries blood toward the heart. Veins form part of the circulatory system. The vessels that carry blood away from the heart are known as arteries.



Function
In systemic circulation de-oxygenated blood from the capillary blood vessels is taken by veins to the right part of the heart. As an exception, in the pulmonary circulation oxygenated blood from the lungs is taken to the left part of the heart by pulmonary veins.

Another special case is portal circulation where the portal vein transports blood rich in products of digestion from the intestines to the liver.

Anatomy
Veins have one-way valves called venous valve to prevent backflow caused by gravity. They also have a thick collagen outer layer, which helps maintain blood pressure and stop blood pooling. The hollow internal cavity in which the blood flows is called the lumen.

Notable veins and vein systems
Names of important veins:
 * Pulmonary veins
 * Portal vein
 * Superior vena cava
 * Inferior vena cava
 * Femoral vein
 * Great saphenous vein

Names of important venule systems:
 * Portal venous system
 * Pulmonary venous system
 * Systemic venous system

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. The latter is called intravenous delivery. It can be done by an injection with a syringe, or by inserting a catheter (a flexible tube).

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. Catheters can be threaded into the superior vena cava for these uses: if long term use is thought to be needed, a more permanent access point can be inserted surgically.

The precise location of veins is much more variable from person to person than that of arteries.

Why are veins green?
In the light, blood appears red because most colors are absorbed except for red, which bounces back from the blood. Every colour but red is absorbed by the oxygen-carrying pigment hemoglobin (Hb). If a filter that blocks the reflected color is positioned between the blood and the eyes of the person watching, the perceived color changes. In the case of humans, the skin serves as a filter for the color red, and the remaining color ends up being green. The exact color spectra is determined by the relative levels of oxygenated iron (HbO) and CO2 in the blood. High oxygen reflects red and high CO2 reflects blue, which mixed with the yellowish color of the fat and/or skin ends up showing as green.