Physiological reactions of primates to 9-D immersion and head-down tilt

Purpose of the investigation was to compare physiological reactions of primates (Macaca mulatta) to microgravity simulated by immersion and head-down tilt (HDT). In immersion experiments, primates in waterproof suits were put into motion-restraining chairs and immersed into water (t=35.4 degrees C) breast-deep for 9 days. In 9-d HDT experiments, prone primates were motor restrained in dedicated tilt beds at -5 degrees. It was found that the CNS functioning was significantly affected, the plasma volume reduced and the marrow erythropoietic function declined. Atrophy developed in leg muscles on a backdrop of iliopectineal spongy osteopenia. Loss in hydration, inhibition of erythropoietic hemopoiesis and iliopectineal spongy osteopenia were more pronounced following immersion than HDT.