title: Plasma colloid osmotic pressure increases in humans during simulated microgravity.
abstract: BACKGROUND
  On exposure to microgravity, astronauts lose up to 12% of their plasma volume which may contribute to post-flight orthostatic intolerance.
  
  
  OBJECTIVE
  Whole-body dehydration during prolonged microgravity, simulated by 6(0) head-down tilt (HDT), may increase plasma colloid osmotic pressure (COP).
  
  
  METHODS
  There were seven healthy male subjects (30-55 yr of age) were placed in 6(0) HDT for 16 d. Plasma COP was measured from blood samples drawn immediately before HDT, on day 14 of HDT, and 1 h following bed rest termination using a 20 muL colloid osmometer. Plasma volume was determined before HDT, on day 16 of HDT, and 1 h following bed rest termination using a modified Evans blue dye technique.
  
  
  RESULTS
  Plasma COP on day 14 of bed rest (29.9 +/- 0.7 mm Hg) was higher (p = 0.01) than pre-HDT value (23.1 +/- 0.8 mm Hg), coinciding with a decrease of plasma volume. At 1 h of upright recovery following HDT, plasma volume stayed below baseline and plasma COP remained elevated (26.6 +/- 0.6 mm hg; p = 0.003) as compared with the pre-HDT value.
  
  
  CONCLUSIONS
  Our results indicate that reduced plasma volume and significantly elevated plasma COP probably reflect an overall loss of extracellular fluids during simulated microgravity.
authors.affiliation: Gravitational Research Branch, NASA-Ames Research Center, Moffett Field 94035-1000, USA.
authors.forename: S T
authors.initials: ST
authors.lastname: Hsieh
authors.forename: R E
authors.initials: RE
authors.lastname: Ballard
authors.forename: G
authors.initials: G
authors.lastname: Murthy
authors.forename: A R
authors.initials: AR
authors.lastname: Hargens
authors.forename: V A
authors.initials: VA
authors.lastname: Convertino
mesh_terms: D000328:Adult; D001510:Bed Rest / Q000009:adverse effects; D001798:Blood Proteins / Q000032:analysis*; D003102:Colloids / Q000032:analysis*; D003681:Dehydration / Q000097:blood / Q000209:etiology* / Q000503:physiopathology*; D018495:Fluid Shifts; D018475:Head-Down Tilt / Q000009:adverse effects; D006801:Humans; D008297:Male; D008875:Middle Aged; D009997:Osmotic Pressure; D010953:Plasma Volume; D013997:Time Factors; D018474:Weightlessness Simulation / Q000009:adverse effects*
chemical_list: D001798:Blood Proteins; D003102:Colloids