Role of osmolality and plasma volume during rehydration in humans

J Appl Physiol (1985). 1988 Jul;65(1):325-31. doi: 10.1152/jappl.1988.65.1.325.


To determine how the sodium content of ingested fluids affects drinking and the restoration of the body fluid compartments after dehydration, we studied six subjects during 4 h of recovery from 90-110 min of a heat [36 degrees C, less than 30% relative humidity (rh)] and exercise (40% maximal aerobic power) exposure, which caused body weight to decrease by 2.3%. During the 1st h, subjects rested seated without any fluids in a thermoneutral environment (28 degrees C, less than 30% rh) to allow the body fluid compartments to stabilize. Over the next 3 h, subjects rehydrated ad libitum using tap water and capsules containing either placebo (H2O-R) or 0.45 g NaCl (Na-R) per 100 ml water. During the 3-h rehydration period, subjects restored 68% of the lost water during H2O-R, whereas they restored 82% during Na-R (P less than 0.05). Urine volume was greater in H2O-R than in Na-R; thus only 51% of the lost water was retained during H2O-R, whereas 71% was retained during Na-R (P less than 0.05). Plasma osmolality was elevated throughout the rehydration period in Na-R, whereas it returned to the control level by 30 min in H2O-R (P less than 0.05). Changes in free water clearance followed changes in plasma osmolality. The restoration of plasma volume during Na-R was 174% of that lost. During H2O-R it was 78%, which seemed to be sufficient to diminish volume-dependent dipsogenic stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Publication types

  • Clinical Trial
  • Controlled Clinical Trial
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adult
  • Algorithms
  • Blood Volume*
  • Body Weight
  • Dehydration / physiopathology*
  • Electrolytes / analysis
  • Hot Temperature
  • Humans
  • Male
  • Osmolar Concentration*
  • Physical Exertion
  • Sodium / administration & dosage*
  • Sodium / analysis
  • Thirst
  • Urine
  • Water*


  • Electrolytes
  • Water
  • Sodium