Abstract
When a person stands up, he or she assumes that the body will automatically make any changes necessary to compensate for the increased gravitational stress this change of position brings. Indeed, standing causes gravity to try to pull nearly one quarter of the body’s blood downward toward the lower arms, legs, and abdomen, reducing the amount of blood available to keep the brain supplied with oxygen. To maintain a constant oxygen supply to the brain and upper body, standing is normally accompanied by an automatic increase in heart rate and the force with which the heart contracts and, most important, a tightening of the blood vessels in the lower part of the body. The combination of these 3 actions pushes blood upward against the force of gravity, thereby maintaining an uninterrupted flow of blood to the brain. The aspect of the nervous system that governs these …
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