Veins are found all over the body and serve each organ. The primary function of all veins is to return the deoxygenated blood back to the lungs. Veins vary in size. The largest vein in the body is called the inferior vena cava. Unlike the arteries, veins have very thin walls and have poorly developed muscle. They only have a thin rim of elasticity and are not able to tolerate any high pressures.
Most veins have valves (one way doors) inside them to prevent the back flow of blood. Valves prevent the blood from pooling in one part of the body.
In most cases, veins do not actively pump blood out of their system; veins are surrounded by muscle which squeeze the veins and this causes them to empty.
Because veins carry deoxygenated blood, they appear blue in color. Oxygen rich blood is bright red and only seen in the arteries.
Because the veins have the capacity to become big without tension, they can hold enormous amounts of blood.
Veins can also become affected by some diseases. The most common ailment of veins is formation of blood clots and varicose veins.
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