TIME-COURSE AND MECHANISMS OF DECREASED PLASMA-VOLUME DURING ACUTE PSYCHOLOGICAL STRESS AND POSTURAL CHANGE IN HUMANS

In this study, we examined the effects of psychological stress on calc ulated plasma volume, the relationship of these changes to changes in hematologic and hemodynamic factors, and the time course of plasma vol ume changes. Plasma volume changes were assessed using mass densitomet ry techniques, which measured changes in blood and plasma density. Hem atologic and hemodynamic variables were assessed in 40 healthy men and women during a mental arithmetic (math) or benign reading task and du ring postural change (standing). Results indicated that math and postu re change produced a significant decrease in plasma volume (ps < .001) and increases in blood pressure (ps < .001), blood and plasma density (ps < .001), and total plasma protein (p < .001). Correlations were o bserved between plasma volume changes and changes in systolic (r = .55 , p < .0001), diastolic (r = .61, p < .0001), and mean (r = .65, p < . 0001) arterial pressure during math; plasma volume and blood pressure returned to baseline within 12 min following the math task. These resu lts suggest that an important mechanism for stress-induced decreases i n plasma volume is increased blood pressure leading to increased fluid shifts from the vasculature into the interstitial spaces.