Ever stand up too quickly from a sitting or lying position and feel dizzy or disoriented for a brief moment? The downward push of Earth's gravity naturally causes blood to settle in the lower areas of the human body, and occasionally, with a quick movement—such as rising swiftly from a chair—the body is not able to adjust fast enough to deliver an adequate supply of blood to the upper parts of the body and the brain. This sudden, temporary drop in blood pressure is what causes brief feelings of lightheadedness upon standing. In essence, when the heart pumps blood to different parts of the body, it is working against the physical phenomenon of gravity in its efforts to send blood up to the brain.
In more cases than not, the body is able to make the necessary adjustments to ensure proper blood flow and pressure to the brain; but when the disorientation lasts a long time and/or become chronic, individuals may have a condition called orthostatic intolerance. According to the American Journal of Physiology–Heart and Circulatory Physiology, an estimated 500,000 Americans are affected by orthostatic intolerance. Symptoms range from occasional fainting, blurry vision, and pain or discomfort in the head and the neck, to tiredness, weakness, and a lack of concentration. Though research indicates that the condition is not life-threatening, it could impact the quality of life and contribute to falls that result in serious injuries.
The condition is a prominent concern for NASA, since astronauts have to readjust to the gravitational environment of Earth after spending days in the weightlessness of space. NASA's Exploration Systems Mission Directorate has found that roughly 20 percent of astronauts coming off of short-duration space flights experience difficulty maintaining proper blood pressure when moving from lying down to either sitting or standing during the first few days back on Earth. The difficulties are even more severe for astronauts coming off of longduration missions, according to the mission directorate, as 83 percent of these crewmembers experience some degree of the condition.
Cardiovascular experts at NASA have found that the blood that normally settles in the lower regions of the body is instead pulled to the upper body in the microgravity environment of space. Blood volume is subsequently reduced as some cardiovascular reflexes are no longer being used, and less blood flows to the legs. Additionally, the muscles weaken, especially in the lower portion of the body, because they are not working (contracting) as hard as they usually do. This is not so much a concern for the astronauts while they are in space, since the action of floating around takes the place of putting center-of-gravity pressure on their legs. (They do exercise strenuously while in microgravity, though, to keep their muscles and circulatory systems conditioned, thus preparing their bodies for the return to gravity as best they can.) When they return to Earth's gravity, however, more blood returns to the legs. Since there is a lower volume of blood, the flow that is supposed to be traveling to the brain can be insufficient. That is when orthostatic intolerance can set in.