Solute and Water Secretion in Sweat.

In man sweat is a hypotonic fluid. Except in rare instances (1) the ratio of sweat osmolality to serum osmolality is always less than one. In those studies where osmolality and the component solutes have been measured, the principal ions of extracellular fluid, sodium, and chloride are evidently the principal ions of sweat, and potassium, lactate, urea, and bicarbonate play a smaller osmotic role (1, 2). There are few studies of osmolality in conditions of altered sweat physiology such as Addison's disease, cystic fibrosis of the pancreas, and acclimatization; but the electrolyte analyses in these conditions and in normal individuals of varying ages suggest the following approximations. The acclimatized individual may have a sweat to serum osmolality ratio of about 0.1 (3). The ratio in normal unacclimatized preadolescent children may range from 0.1 to 0.3 (4), whereas the range in normal unacclimatized men and women may be from 0.1 to 0.5 (4). In Addison's disease the ratio would be about 0.7 (5), and in cystic fibrosis the expected range would be from 0.4 to 1.0 (6). In contrast to the above is the sweat to serum ratio of urea concentrations, which is always greater than one. Schwartz, Thaysen, and Dole (7) reported an average ratio to be 1.8 with a range of 1.0 to 3.2, whereas Bulmer (8) noted a ratio that was somewhat lower, but still significantly greater than 1. Of further interest was Schwartz's observation that this ratio is inde-