in Body Fluids and Circulation (Cardiovascular System, Circulatory system) by

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The Blood is sent to the arteries under pressure. The blood is kept in one-way flow by the elastic recoil of the artery. The blood is carried out from the heart by the arteries at high pressure around the body to the organs, where as the blood flow in the veins is at the lower pressure compare to that of the arteries. The number of elastic fibers and the thick muscular walls are less in veins compared to that of the arteries. Even the pressure is very low in the veins to maintain the one-way flow. The contraction of the skeletal muscles against the veins is the reason to keep the blood flowing in one direction to the heart, but it may happen that when person is steady, that is not moving the blood may start to flow backwards. Therefore, the valves are very much necessary in the veins to prevent the back flow of the blood.
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