Urea excretion in human sweat as a tracer for movement of water within the secreting gland.

Previous workers have found that the concentration of urea in human sweat exceeds the concentration of urea in plasma. This fact has been confirmed in the present work and extended by measurements of the concentrations of urea in the sweat and the corresponding concentrations of urea in the plasma over a range of 25- fold variation in the latter. An analysis of the data showed that the ratio of the concentration of urea in sweat to the concentration of urea in plasma, S/P, was independent of the plasma concentration, independent of the absolute flux of urea or water, and independent of the calculated energy expenditure. Two experiments showed that the concentration of urea in sweat followed a rising concentration in plasma with a time lag of approximately 5 minutes; this fact indicates that the accumulation of urea in sweat is a continuous process which is in operation during the observed discharge. These observations suggest that urea is carried into the gland in some precursor solution and subsequently raised to a higher concentration by the reabsorption from this solution of a constant fraction of the water.