Effects of gravity on the circulatory system

Gravitational forces may have a profound influence on blood flow through the circulatory system. As a result, VR and CO may be affected. Imagine that the circulatory system is a column of blood that extends from the heart to the feet. As in any column of fluid, the pressure at the surface is equal to zero. Due to the weight of the fluid, the pressure increases incrementally below the surface. This pressure is referred to as the hydrostatic pressure. 

Blood tends to pool in the highly distensible veins. Furthermore, the excessive filtration of fluid out of the capillaries and into the tissues that occurs causes edema or swelling of the ankles and feet. As a result, VR and therefore CO are decreased, leading to a decrease in MAP. This fall in MAP can cause a decrease in cerebral blood flow and, possibly, syncope (fainting). 

Compensatory mechanisms in the circulatory system are needed to counteract the effects of gravity. Two important mechanisms include  

Baroreceptors are sensitive to changes in MAP. As VR, CO, and MAP decrease, baroreceptor excitation is diminished. Consequently, the frequency of nerve impulses transmitted from these receptors to the vasomotor center in the brainstem is reduced. This elicits a reflex that will increase HR, increase contractility of the heart, and cause vasoconstriction of arterioles and veins. The increase in CO and TPR effectively increases MAP and therefore cerebral blood flow. Constriction of the veins assists in forcing blood toward the heart and enhances venous return. Skeletal muscle activity associated with simply walking decreases venous pressure in the lower extremities significantly. Contraction of the skeletal muscles in the legs compresses the veins and blood is forced toward the heart. 

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