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.

In an upright adult, the hydrostatic pressure of the blood in the feet may be as high as 90 mmHg. When this pressure is added to pressure in the veins generated by the pumping activity of the heart, the total pressure in veins in the feet may be as high as 100 mmHg. The valves in the veins effectively prevent the backward flow of blood toward the feet. However, the valves have no effect on the build-up of pressure in the veins in the lower extremities. The capillaries in the feet are also subjected to the effects of gravity. Pressure in these vessels may be in the range of 135 mmHg. Increased hydrostatic pressures in the veins and capillaries have two very detrimental effects on the circulatory system:

• Pooling of blood

• Edema formation

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:

• Baroreceptor reflex

• Skeletal muscle activity

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.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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