Plasma volume changes during submaximal swimming

Summary

The hemodynamic and diuretic responses of six swimmers were evaluated during 35 min of resting immersion, free swimming, and land bicycle ergometry at approximately 70% exercise specific maximal oxygen uptake. Venous blood, sampled at 0, 5, 10, 15, 25, and 35 min of each trial, was analyzed for hemoglobin, hematocrit, and total plasma protein concentration. Urine Output was also measured. Resting immersion resulted in a significant (p<0.05) hemodilution (plasma volume=+2.5±1.5%) during the first 25 min and a return towards resting levels at the termination. During swimming exercise, plasma volume decreased by −15.9±1.3% (p<0.01). A similar −17.0±1.3% decrease was also evident during cycle ergometry. Urine Output averaged 3.1±0.9 ml·min⁻¹ during resting immersion, significantly greater than either swimming (1.0±0.1 ml·min⁻¹) or cycling (0.4±0.2 ml·min⁻¹). Total plasma protein concentration increased less during cycling compared to swimming (p<0.05) and was unchanged (p>0.05) during resting immersion. It was concluded that the hemodynamic and renal responses to immersion are suppressed during swimming. It was also apparent that, despite possible differences in hemodynamics, plasma shift during exercise may be similar on land and in the water. It appears that the amount of plasma lost from the intravascular space during exercise may be independent of the muscle mass involved, but related to a combination of the ratio of active to total mass for each individual and body position.