[Dynamics of human cardiovascular responses in different periods of long-term exposure in weightlessness].

Purpose of the work was to determine trends in the human cardiovascular function at rest and during LBNP in different periods of short- (8-25 d) and long-term (126-438 d) space flights (SF) using the data of USI and leg occlusive plethysmography. Due to blood redistribution and hypovolemia in the first week on SF, the cardiovascular system decreased the left heart filling and output without loss in myocardium contractility, weakened the renal artery resistance, and increased the maximum capacity of leg veins. In 30-40 days, these developments became more pronounced and were followed by a relative stabilization of hemodynamics at rest. Arterial circulation in the brain was stable; however, passive congestion in the region grew as SF duration extended. The most dramatic changes were observed in leg arteries (weakening of resistance) and veins (increase in maximum capacity). Venous changes were more distinct than arterial. Despite the relative stabilization at rest, LBNP showed a decline of the gravity-dependent reactions which was also a function of SF duration. In the first month of SF, the downward trend of femoral arteries vasoconstriction was not detrimental to cerebral circulation. SF extension impacted regulation of the vascular tone and caused cerebral circulation deficiency during LBNP. In several cases, the hemodynamic reaction to LBNP was affected to the extent which could be qualified as a failure to adapt to the orthostatic condition.