Abstract
From the first space flight in 1961, numerous studies have shown that weightlessness (microgravity) causes alterations of metabolism which result in the serious impairment of health in astronauts. During space flight, the metabolism of body fluid undergoes marked changes. The decrease of gravitation during the flight results in a body fluid shift from the lower part of the body to the upper part (cephalad fluid shift). This upward fluid shift has been considered as being one of the causes of space motion sickness due to brain edema and increased endolymph in the vestibular system. The initial fluid increase in the upper body could be ameliorated by an increased urine excretion. During a long-term stay in space, however, astronauts show an edmatous face, while total plasma volume may be reduced. Upon return to the earth (normal gravity), downward fluid shift occasionally causes cardiac deconditioning because of decreased plasma volume. Thus, body fluid metabolism has become one of the important subjects in space medicine. The present article describes the observed alteration in fluid and electrolyte metabolism in actual space flight and simulated weightless conditions. In simulation studies such as head-out immersion, increased central venous pressure is followed by suppression of ADH secretion (Gauer's reflex). Also renin-angiotensin-aldosterone system is suppressed. On the other hand, atrial natriuretic polypeptide secretion is increased. In space flight, however, upward fluid shift is often not associated with suppressed ADH secretion, and activation of renin-angiotensin-aldosterone system may be observed. Thus, the changes in fluid-electrolyte metabolism in space can not always be reproduced in simulation studies on earth. Some of the hormones released during stressful conditions are involved in control of fluid and electrolyte metabolism. The contribution of these stress-related hormones in fluid electrolyte metabosim in space has not been studied in detail. The understanding of fluid-electrolyte metabolism in space awaits further investigation.
