Post-flight water-salt metabolism in cosmonauts: The long-duration MIR missions

Investigations of 28 MIR crews (60 cosmonauts) after 115- to 438-day missions showed a typically minor increase in ionized calcium and reduction in blood serum potassium independent of mission length. Reasoning from the diminished extracellular space and decreased renal excretion of liquid and main electrolytes, the positive water balance, and activation of antidiuretic and antinatriuretic hormones, most of the cosmonauts returned hypohydrated to the Earth's gravity. These physiological shifts are considered as an adaptive reaction directed towards re-establishment of water-mineral homeostasis and maintenance of orthostatic tolerance at 1 g. In addition, data of pre-flight investigations of healthy subjects in the normal environment can be used as the normal range of water-salt metabolic parameters.