Venous return

Explanation 

The role of venous return is to supply the heart with deoxygenated blood so it can be pumped back to the lungs for oxygenation. To improve this mechanism there are many different processes at play.

Oxygenated blood has traveled from the heart via the aorta to the tissues, for example the calf muscle (gastrocnemius). As the oxygen is used up the blood re enters the capillary system which slowly turn into venules and small veins. From small veins the veins increased in diameter until becoming medium / large veins. As the calf muscle in located in the lower section of your body and has to travel upward toward the heart it has to battle with the effects of gravity.

As you move around this contracts and tenses your leg muscles which squeeze the veins forcing the blood upward. However, when the veins are squeezed by the contractive force the blood could move in either direction, up or down. But veins have valves in them which prevent the blood from flowing back down the leg. This forces the blood to only move in an upward direction.

To aid this process, veins has a muscular layer which have the ability to contract. This increases the amount of blood pushed upward. This can be used in combination with the squeezing of veins due to contraction of muscles around it.