Mechanisms of the decrease in human postural stability during prolonged head-down tilt.

In 3 identical experiments with head-down bed rest lasting 60, 90, and 120 days and involving 18 volunteers, dynamics of the development of cardiovascular system (C.V.S) deconditioning was studied. A set of radioisotopic research techniques was used. Volumes of hemocirculation, body fluids, and metabolic activity of the bone marrow were investigated. Functions of the central and peripheral hemodynamics were studied. To determine the extent of C.V.S. deconditioning during the baseline period, on days 60, 90, and 120 of hypokinesia and during recovery, an orthostatic test was performed. The degree of gravitational blood shifting in regions (the head, thorax, the abdomen, the lower extremities) was recorded. Critical thresholds of blood shifting in the body were determined. It was established that the blood pooled in the splanchnic region participates in the decrease of central hypovolemia. Because of the insufficient number of observations, this research should be continued. During recovery, the sign of (CVS) deconditioning noted demonstrated a clear tendency to normalization.