Accession Number : ADA318434

Title :   Clinical Aspects of the Control of Plasma Volume at Microgravity and During Return to One Gravity,

Corporate Author : ARMSTRONG LAB BROOKS AFB TX AEROSPACE MEDICINE DIRECTORATE

Personal Author(s) : Convertino, Victor A.

PDF Url : ADA318434

Report Date : DEC 1995

Pagination or Media Count : 9

Abstract : Plasma volume is reduced by 10%-20% within 24 to 48 h of exposure to simulated or actual microgravity. The clinical importance of microgravity-induced hypovolemia is manifested by its relationship with orthostatic intolerance and reduced VO2max after return to one gravity (1G). Since there is no evidence to suggest plasma volume reduction during microgravity is associated with thirst or renal dysfunctions, a diuresis induced by an immediate blood volume shift to the central circulation appears responsible for microgravity-induced hypovolemia. Since most astronauts choose to restrict their fluid intake before a space mission, absence of increased urine output during actual spaceflight may be explained by low central venous pressure (CVP) which accompanies dehydration. Compelling evidence suggests that prolonged reduction in CVP during exposure to microgravity reflects a 'resetting' to a lower operating point which acts to limit plasma volume expansion during attempts to increase fluid intake. In groudbase and spaceflight experiments, successful restoration and maintenance of plasma volume prior to returning to an upright posture may depend upon development of treatments that can return CVP to its baseline 10 operating point. Fluid-loading and LBNP have not proved completely effective in restoring plasma volume, suggesting that they may not provide the stimulus to elevate the CVP operating point. On the other, exercise, which can chronically increase CVP, has been effective in expanding plasma volume when combined with adequate dietary intake of fluid and electrolytes. The success of designing experiments to understand the physiological mechanisms of and development of effective countermeasures for the control of plasma volume in microgravity and during return to one gravity will depend upon testing that can be conducted under standardized controlled baseline condi

Descriptors :   *GRAVITY, *EXPOSURE(PHYSIOLOGY), *BLOOD PLASMA, *BLOOD PRESSURE, MAINTENANCE, OUTPUT, CONTROL, LOW PRESSURE, REDUCTION, BASE LINES, SHIFTING, FLUIDS, EXPANSION, WEIGHTLESSNESS, ELECTROLYTES, SPACE FLIGHT, DIET, CIRCULATION, SPACE MISSIONS, VEINS, URINE, DEHYDRATION, DIURESIS, BLOOD VOLUME, ASTRONAUTS, THIRST.

Subject Categories : Medicine and Medical Research
      Stress Physiology
      Astronautics

Distribution Statement : APPROVED FOR PUBLIC RELEASE