Endocrine responses in long-duration manned space flight

The bioassay of body fluids experiment is designed to evaluate the biochemical adaptation resulting from extended exposure to space flight environment by identifying changes in hormonal and associated fluid and electrolyte parameters reflected in the blood and urine of the participating crewmen. The combined stresses of space flight include weightlessness, acceleration, confinement, restraint, long-term maintenance of high levels of performance, and possible desynchronosis. Endocrine measurements to assess the physiological cost of these stresses have been considered from two aspects. Fluid and electrolyte balance have been correlated with weight loss, changes in the excretion of aldosterone and vasopressin and fluid compartments. The second area involves the estimation of the physiological cost of maintaining a given level of performance during space flight by analysis of urinary catecholamines and cortisol. Inter-individual variability was demonstrated in most experimental indices measured; however, constant patterns have emerged which include: body weight change; increases in plasma renin activity; elevations in urinary catecholamines, ADH, aldosterone and cortisol concentrations. Plasma cortisol decreases in immediate postflight samples with subsequent increase in 24-hour urines. The measured changes are consistent with the prediction that a relative increase in thoracic blood volume upon transition to the zero-gravity environment is interpreted as a true volume expansion resulting in an osmotic diuresis. This diuresis in association with other factors ultimately results in a reduction in intravascular volume, leading to an increase in renin and a secondary aldosteronism. Once these compensatory mechanisms are effective in reestablishing positive water balance, the crewmen are considered to be essentially adapted to the null-gravity environment. Although the physiological cost of this adaptation must reflect the electrolyte deficit and perhaps other factors, it is assumed that the compensated state is adequate for the demands of the environment; however, this new homeostatic set is not believed to be without physiological cost and could, except with proper precautions, reduce the functional reserve of exposed individuals.