Despite a number of difficulties in performing experiments during weightlessness, a great deal of information has been obtained concerning the effects of spaceflight on the regulation of body fluid and
electrolytes. Many paradoxes and questions remain, however. Although body mass, extracellular fluid volume, and plasma volume are reduced during spaceflight and remain so at landing, the changes in total body water are comparatively small. Serum or plasma
sodium and osmolality have generally been unchanged or reduced during the spaceflight, and fluid intake is substantially reduced, especially during the first of flight. The diuresis that was predicted to be caused by weightlessness, has only rarely been observed as an increased urine volume. What has been well established by now, is the occurrence of a relative diuresis, where fluid intake decreases more than urine volume does. Urinary excretion of
electrolytes has been variable during spaceflight, but retention of fluid and
electrolytes at landing has been consistently observed. The glomerular filtration rate was significantly elevated during the SLS missions, and water and
electrolyte loading tests have indicated that renal function is altered during readaptation to Earth's gravity. Endocrine control of fluid volumes and
electrolyte concentrations may be altered during weightlessness, but levels of
hormones in body fluids do not conform to predictions based on early hypotheses.
Antidiuretic hormone is not suppressed, though its level is highly variable and its secretion may be affected by
space motion sickness and environmental factors. Plasma
renin activity and
aldosterone are generally elevated at landing, consistent with
sodium retention, but inflight levels have been variable.
Salt intake may be an important factor influencing the levels of these
hormones. The circadian rhythm of
cortisol has undoubtedly contributed to its variability, and little is known yet about the influence of spaceflight on circadian rhythms.
Atrial natriuretic peptide does not seem to play an important role in the control of natriuresis during spaceflight. Inflight activity of the sympathetic nervous system, assessed by measuring
catecholamines and their metabolites and precursors in body fluids, generally seems to be no greater than on Earth, but this system is usually activated at landing. Collaborative experiments on the Mir and the International Space Station should provide more of the data needed from long-term flights, and perhaps help to resolve some of the discrepancies between U.S. and Russian data. The use of alternative methods that are easier to execute during spaceflight, such as collection of saliva instead of blood and urine, should permit more thorough study of circadian rhythms and rapid
hormone changes in weightlessness. More investigations of dietary intake of fluid and
electrolytes must be performed to understand regulatory processes. Additional
hormones that may participate in these processes, such as other
natriuretic hormones, should be determined during and after spaceflight. Alterations in body fluid volume and blood
electrolyte concentrations during spaceflight have important consequences for readaptation to the 1-G environment. The current assessment of fluid and
electrolyte status during weightlessness and at landing and our still incomplete understanding of the processes of adaptation to weightlessness and readaptation to Earth's gravity have resulted in the development of countermeasures that are only partly successful in reducing the postflight
orthostatic intolerance experienced by astronauts and cosmonauts. More complete knowledge of these processes can be expected to produce countermeasures that are even more successful, as well as expand our comprehension of the range of adaptability of human physiologic processes.