Blood volume reduction counteracts fluid shifts in water immersion

Six healthy men were bled by 15% of their total blood volume (TBV) before 7 h of seated water immersion to test the hypothesis that some of the major physiological responses to an expansion of central blood volume can be counteracted by prior reduction of TBV. Subjects were their own controls under two conditions: seated dry in air and seated immersed to the suprasternal notch in water. Immersion without prior reduction of TBV (WC = Wet Control) caused a statistically significant 22% increase in cardiac output (CO), 368% increase in urine production, and 200% increase in sodium excretion relative to dry control (DC) sessions. When TBV was reduced before immersion, CO was the same as during DC sessions; however there were significant increases above DC in urine flow (+73%) and sodium excretion (+120%), although they were significantly reduced from WC values. Potassium excretion was similar during DC and WC sessions, but was significantly increased (+75%) when subjects were immersed after 15% reduction of TBV.