Prolonged exercise in prepubertal boys. II. Changes in plasma volume and in some blood constituents.

A group of 10 prepubertal boys was studied during prolonged exercise (60 min) on bicycle ergometer and on treadmill at two levels of work load (appr. 40% and 60% VO2 max). The hematocrit, serum proteins, Cl- and K+ were followed, and from the blood hematocrit changes the plasma volume changes were calculated. At the exercises of lower intensity of both types a slight hemodilution was found (appr. +5% increase in plasma volume), at higher intensity practically no changes could be demonstrated. These findings are supported by the values of serum protein concentration, where no increase was found, and by the fact that at the lower work loads a rather decreasing trend was found for this blood constituent. These findings are at variance with those in adults under similar conditions. The authors suggest that different changes of plasma volume during exercise in boys than in adults could be related to the disparate lactate production and fate in these age groups.