|MadSci Network: Medicine|
Your question is based upon a misconception. While it is true that muscle contraction in the lower limbs does assist blood flow up the veins to the body proper, the muscle contraction is not the only driving force and in fact is not required for return blood flow. The circulatory system in higher animals is a closed system. When the left side of the heart contracts, blood is ejected into the aorta and then flows into the remainder of the system. In tissues, the microcirculation moves blood through to the vienous system and it flows back to the heart. There is pressure on the arterial (supply) side of the system due to 1) cardiac contraction, 2) smooth muscle based contraction of arteries. This pressure is enough to push most of the blood volume back to the heart. When standing, it may, in some situations, fall short of being enough to move blood out of the lower limbs, and the skeletal muscle contraction assists. There are also valves in the large veins in the lower leg that reduce back flow when the heart is between contractions, this helps to keep the blood moving in the right direction. When laying down, gravity reduces the load in the leg and these pressures are adequate. Sitting can result in some situations, in pooling of some small amount of fluid in the lower limbs, pretty much if you stand the muscles assist, and if you lay down, the reduction of work against gravity assists. So the short answer is that the skeletal muscle contraction is not the primary source of blood return.
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