NASA Logo

NTRS

NTRS - NASA Technical Reports Server

Back to Results
Cardiovascular and Body Fluid Adjustments During Bed Rest and Space FlightAlthough a few scientific bed rest (BR) studies were conducted soon after World War II, advent of the space program provided impetus for utilizing prolonged (days-months) BR, which employed the horizontal or 6 degree head-down tilt (HDT) body positions, to simulate responses of healthy people to microgravity. Shorter (hours) HDT protocols were used to study initial mechanisms of the acclimation-deconditioning (reduction of physical fitness) syndromes. Of the major physiological factors modified during BR, reduced force on bones, ligaments, and muscles, and greatly reduced hydrostatic pressure within the cardiovascular system, the latter: which involves shifts of blood from the lower extremities into the upper body, increase in central venous pressure, and diuresis, appears to be the initial stimulus for acclimation. Increase in central venous pressure occurs in subjects during weightless parabolic flight, but not in astronauts early during orbital flight. But significant reduction in total body water (hypohydration) and plasma volume (hypovolemia) occurs in subjects during both BR and microgravity. Response of interstitial fluid volume is not as clear, It has been reported to increase during BR, and it may have increased in Skylab II and IV astronauts. Reduction of total body water, and greater proportional reduction of extracellular volume, indicates increased cellular volume which may contribute to inflight cephalic edema. Cerebral pressure abates after a few days of HDT, but not during flight. accompanied by normal (eugravity) blood constituent concentrations suggesting some degree of acclimation had occurred. But during reentry, with moderately increased +Gz (head-to-foot) acceleration and gravitational force, the microgravity "euhydration" becomes functional progressive dehydration contributing to the general reentry syndrome (GRS) which, upon landing the Shuttle, can and often results in gastrointestinal distress, disorientation, vertigo, fatigue, and fainting. Various pre-reentry hyperhydration procedures have been utilized to counteract the GRS. Thus, the somewhat decreased central venous pressure and lack of diuresis early in spaceflight suggests mechanisms other than the Gauer-Henry reflex are more important for maintaining fluid volume homeostasis in astronauts. Inflight hypohydration and hypovolemia are more important for maintaining fluid volume homeostasis in astronauts.
Document ID
20020038908
Acquisition Source
Ames Research Center
Document Type
Conference Paper
Authors
Greenleaf, John E.
(NASA Ames Research Center Moffett Field, CA United States)
Tomko, David L.
Date Acquired
August 20, 2013
Publication Date
January 1, 1995
Subject Category
Aerospace Medicine
Meeting Information
Meeting: International Symposium on Occupational Health
Location: Kitakyushu
Country: Japan
Start Date: October 18, 1995
End Date: October 20, 1995
Funding Number(s)
PROJECT: RTOP 199-18-12-07
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

Available Downloads

There are no available downloads for this record.
No Preview Available