Plasma volume during stress in man: osmolality and red cell volume

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Abstract
The study tested the hypothesis that in man there is a range of plasma osmolality within which the red cell volume (RCV) and mean corpuscular volumne (MCV) remain essentially constant and determined the upper limit of this range. During a variety of stresses.sbd.submaximal and maximal exercise, heat and altitude exposure, +GZ acceleration, and tilting.sbd.changes in plasma osmolality between -1 and +13 mosmol/kg resulted in essentially no change in the regression of percent change in plasma volume (PV) calculated from a change in Hct + Hb, i.e., the RCV and MCV were constant. Factors that did not influcnce RCV were the level of metabolism, heat exposure at rest, and short-term orthostasis (head-to-foot acceleration). Factors that may influence RCV were exposure to high altitude and long-term orthostasis (head-up tilting). Factors that definity influence RCV were prior dehydration and extended (> 2 h) period of stress. Thus, the Hct or the Hct + Hb equations could be used to calculate percent changes in PV under short-term (< 2 h) periods of stress when the change in plasma osmolality was < 13 mosmol/kg.