s weight amounts to the force required to support the person's mass against the force produced by the acceleration of gravity. Weightlessness would occur under such circumstances if the elevator's cable were to be suddenly cut. The scale would almost immediately register zero. Although no change would have occurred in either the person's mass or the Earth's gravitational field, the external reactionary force would have been removed. Therefore, during the state of weightlessness, a "gravitational force is constantly accelerating the body in relation to the center of gravity of the Earth without the intervention of counteracting reactional forces" (10:505-521).
Various studies have indicated that weightlessness may alter the bodily fluids. Space travel typically causes a substantial loss of body mass. About half of the weight lost during spaceflight is water. This loss may or may not be augmented by increased renal fluid excretion. Typically though, a negative water balance results from astronauts' decreased fluid intake. In addition to water, numerous electrolytes are also excreted. These may include sodium, chloride, and other minerals. Such changes may cause decreased serum osmolality and decreased serum sodium. Further, spaceflight has been associated with an increase in the plasma and urine levels of certain hormones. For example, those hormones usually associated with stress (e.g., angiotensin I, aldosterone, and cortisol) are sometimes found to be elevated. Such changes may involve an adaptive form of volume regulation (7:A74-A78).
Research into gravitational stresses and fluid volume shifts has determined that adaptation to microgravity occurs in two different stages. During the first stage, or the dynamic phase, the human cardiovascular and renal/endocrine systems rapidly respond to their changed circumstances. This alteration may include, for example, a decrease in leg volume, a headward shift of tissu...