The function of the valves in the peripheral veins is to ensure that the overall movement of blood in the veins is in the right direction, toward the heart. This is especially important in the legs, where the blood must flow a relatively long distance against gravity.
Unlike blood in the arteries, blood in the veins is under relatively low pressure, since the heart is not pushing it along. Instead, the blood is pushed along by the compression of the veins by skeletal muscles in the limbs, and the expansion of the lungs in the torso. The valves found in most veins ensure that the blood can only flow in one direction, making blood flow to the heart more efficiently.
Long periods of time with the leg muscles inactive, such as sitting or standing in place, causes blood to pool in the veins. The body has ways to compensate for this by releasing vasoconstrictor hormones that cause the veins to decrease in size, pushing the blood along. The veins are made of connective and muscular tissue which enables contraction. This tissue is thinner than that of the arteries because of the lower pressure. The muscle tissue is found in the middle of three layers, known as the tunica media.