Characteristics of adaptation and maladaptation of human cardiovascular system under space flight conditions

The goal of this study was to analyze and generalize hemodynamic data collected over 20 years from 26 cosmonauts, who had flown from 8 to 438 days aboard orbital stations Salut-7 and Mir. This paper describes the results of ultrasonographic studies of the heart and arterial and venous peripheral vessels in different parts of human body as well as the study of venous capacity by occlusion aeroplethysmography. It was established that, at rest, the key hemodynamic parameters (the pumping function of the heart and blood supply of the brain) and integral parameters (blood pressure and heart rate) were best “protected” and remained stable throughout long exposure in microgravity. In the absence of gravitational stimulation, arterial resistance decreased in almost all vascular regions below the heart level; i.e., the antigravity distribution of the vascular tone was gradually lost as unneeded in microgravity. Venous hemodynamics was found to be most sensitive to microgravity: changes in it were expressed earlier and were more pronounced than in the arterial part of the vasculature. The changes included deceleration of venous return, a decrease in the vascular resistance in the lower body, and an increase in the leg’s venous network capacity. The functional test with the lower body’s negative pressure revealed a deterioration of gravity-dependent responses, which increased with an increase in the duration of the space flight. Cardiovascular deconditioning clearly manifested itself after the return to the Earth’s gravity as a decreased g-tolerance during reentry and orthostatic instability in the post-flight period. The results of this study confirmed the multifactorial genesis of orthostatic instability during space flights including blood redistribution, changes in the regulation of vascular tone of arterial and venous vessels in legs, and hypovolemia.