Short-Term Physiological EffectsIn order to deal with the effects of increased gravity loading (g loading) during the acceleration
of launch and deceleration of reentry, crews in both U.S. and Soviet spacecraft generally have been placed so that the g loads are experienced in the chest-to-back position rather than the head-to-foot position. The human body can withstand a much higher g loading in the former mode. The g loads themselves, which have varied from 3 to 8.2 times the gravity experienced at the Earth's surface, have created no problems. In the U.S. Space Shuttle, however, crews take a reentry decelerative force of about 1.5 g in a feet-first position, which causes blood to push toward the feet. To counteract this effect, Shuttle crews are provided with suits that produce pressure on the lower part of the body.Physiological changes start to occur from the first moment of achieving orbit. Body fluids redistribute themselves toward the head. As they do, the face puffs out, and some astronauts experience sensations similar to those of a head cold. The intestines also tend to float upward, and as a result an astronaut may lose as much as 10 cm (4 in) in girth. The curve of the spine also straightens out somewhat, so that persons may gain an inch or so in height while in the weightless environment. Many astronauts have experienced a mild lower backache as a result of the different stresses being placed on the back muscles in space.The vestibular system consists of the otolith and the semicircular canals of the inner earÑwhich constitute the body's organs of balance. When they are disturbed, motion sickness can result. In space, this is known as "space adaptation syndrome." Its symptoms include loss of appetite, cold sweating, mild dizziness, stomach awareness, nausea, and vomiting.The symptoms occur as early as an hour after launch or as late as the second day of flight, and they can persist for as long as four days. Approximately half of all astronauts and cosmonauts have experienced these symptoms. The causes of the disorder are not yet known. However, most astronauts report symptom relief after an intramuscular injection of Promethazine (also known as Phenargan), although this drug can cause drowsiness. Because the symptoms disappear shortly, even if untreated, space adaptation syndrome is considered adaptive and self-limiting.Other physiological changes associated with short-term spaceflight include some cardiac deconditioning and a decrease in the volume of body fluids. These changes return to preflight norms within a short time.