Plasma volume and electrolyte shifts with heavy exercise in sitting and supine positions

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Abstract
Plasma volume (PV) and electolyte shifts were measured before and for 60 min after a continuous peak O2 uptake (.ovrhdot.VO2 peak) test in 4 men (26-45 yr) on a bicycle ergometer. Mean (.+-. SE) sitting .ovrhdot.VO2 peak (3.16 .+-. 0.32 l/min) was the same as supine .ovrhdot.VO2 peak (3.13 .+-. 0.33 1/min). In recovery (R + 1.5 min), mean PV had decreased by 477 ml (-16.1%, P < 0.05) in the sitting and by 548 ml (-17.6%, P < 0.05) in the supine positions, whereas total osmolality increased progressively with its peak at R + 3.5 min. The percentage losses of protein, total Ca2+, and ionized Cai2+ were about half as great as the percentage loss in PV, indicating a selective retention of these constituents. Calculated osmalality (.SIGMA. Na+, K+, Cl-, Cai2+) returned to control levels within 1.5 min after sitting exercise but required about 15 min after supine exercise. These small increases in protein concentration were not likely to significantly aid restitution of plasma volume and the ions were probably in equilirium across the capillary membrane. So a change in hydrostatic and/or systemic blood pressures most likely provided the force for resitution of plasma volume.