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Gravity, simply put, is the force of attraction between the earth and the objects and beings on it. Gravity is ubiquitous and constant. Its effects are so predictable in everyday life that we rarely ever think about it. Nevertheless, a brief orientation to the effects of gravity on fluids may be helpful for understanding the medical problems that result when the “anti-gravitational” system of the circulation becomes defective.

Of course, gravity affects fluids as well as solids. The effect of gravity on liquids determines weight and is expressed as hydrostatic pressure. The concept of the weight of water can be envisioned easily using the dam as an example. A dam is constructed with a narrow top (where the pressure of the water is small) that becomes much thicker toward the bottom (where the pressure is great). Taken to the extremes for illustration is the Hoover Dam; at the top there is a roadway. Here the surface pressure on water is 14.7 pounds per square inch, representing atmospheric pressure from the weight of air above it. The bottom of the dam is 660 feet thick at its depth of 726 feet. The weight of water exerts a pressure of 323 pounds per square inch!

For an illustration on a lesser scale, visualize a barrel full of water shot up by a rapid-fire sharp-shooter. Imagine that each bullet hits the barrel one above the other at almost the same instant. Water spurting from each of the holes in the barrel will project different distances. Water from the lowermost hole (under the greatest pressure) will spout farthest.

Any tube when filled with fluid and laid horizontally exerts a very low hydrostatic pressure throughout its length. When turned into the vertical position, the fluid now at the top will not have any appreciable change in pressure; the pressure at the low end of the tube will be increased in proportion to the height of the tube. The width of the tube is irrelevant in determining hydrostatic pressure. The analogy is exactly fitting for the dynamics of a human going from lying down to standing up.

Blood vessels are tubes upon which hydrostatic principles operate. Veins of the neck drain downward and, without sustaining the weight of blood, are virtually collapsed. Strong chest pressure, as in forced exhalation or singing, will cause neck veins to distend. Veins in the upward stretched arm are without hydrostatic pressure and are flattened into a virtually collapsed state. When the arm bends downward, the veins in the hand become distend.

It is easy to observe the effect of gravity on filling and emptying of veins in the hand. Hold an arm straight ...

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