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

In systemic circulation de-oxygenated blood from the capillary blood vessels is taken by veins to the right part of the heart. Differently, 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.

Names of important veins:

Veins are used medically as 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.