Veins return blood to the heart. Their walls are thinner than those of the arteries and can expand to increase their volume, to a large extent. Blood pressure in the veins is very low compared with that of the arteries. Thus, blood flow in the venous circulation needs some help to ensure that the blood returns to the heart.
Venous valves within the major veins of the appendages prevent backflow of blood in this low pressure system. They trap blood that might flow backward, ensuring it goes in one direction, much like the veins of the heart. This reduces the work of venous return of blood to the heart. It only needs to be pumped from valve to valve and not all the way from the extremities to the heart in one go.
Aiding venous return are the skeletal muscle and respiratory pumps. The skeletal muscle pump uses normal muscle contraction to assist in pumping blood (especially from the lower extremities) from valve to valve. For example, as the calf muscles contract while we move or wiggle our toes, they push blood upward toward the heart. In the thorax, inspiration lowers thoracic pressure, drawing blood the into heart. This is the respiratory pump.
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