Plasma concentration and urinary excretion of certain electrolytes during supine work.

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Abstract
The arterial plasma concentrations of Na, K, and phosphate, and the urinary excretion of these ions were studied during supine exercise of varying intensity on a bicycle ergometer in 17 healthy male volunteers. The plasma concentration of Na remained constant during exercise while K and phosphate concentrations were found to increase significantly. The Na excretion was found to decrease during severe exercise while the excretion varied in milder exercise. The K excretion increased in most subjects during milder exercise but showed a tendency to decrease during severe exercise. The phosphate excretion was found to increase markedly during exercise. It is suggested that the decreased Na excretion during exercise is influenced not only by changes in glomerular filtration rate but also by change in the tubular reabsorption of Na, that K excretion is increased as long as the renal blood flow is not decreased below a critical level, and that the increased phosphate excretion is due to decreased relative reabsorption of phosphate.