Physiological mechanisms of adaptation to weightlessness. Data from experiments with animals in earth-orbit...

Four experiments were carried out with mammals (white laboratory rats) in Kosmos biosatellites. The experimental results indicate that rather prolonged weightlessness does not cause pathological changes in internal organs. Changes were discovered in the metabolic and hormonal status of the organisms, allowing us to consider an 18-22-day space flight as a moderately stressful activity. Changes in the musculoskeletal system involved atrophy of particular muscle groups, adaptive transformation of the contractile properties of some of them, osteoporosis, and decreased durability of bony tissue. There was a decrease in the ATPase activity of myocardial myosin, and there were changes in the erythrocytic system: decreased erythrocytic hemopoietic activity, increased levels of spontaneous hemolysis of erythrocytes, etc. All the described changes were reversible, and examination of the animals 25 days after their return to Earth showed practically complete normalization of the parameters studied.