Muscle fluid and electrolyte balance during and following exercise.

During shortlasting exhausting exercise plasma volume is decreased by up to 20%. Water is probably taken up by muscle cells since the concentration of small plasma solutes like Na+ and Cl- increases in parallel with the hemoglobin concentration. During the first 4 min of the recovery phase the extracellular distribution volume for NaCl seems to be restored as evidenced by normalization of the plasma sodium and chloride concentrations. However, the plasma volume is restored at a much slower rate by recapture of isotonic fluid. Interestingly, arterial lactate concentrations are normalized at the same rate. We propose that immediately after end of shortlasting exercise the swollen muscle cells rapidly reduce their volume. Fluid will accumulate in the interstitium since reuptake of fluid into the capillaries is a much slower process.