Search

Summary o 1. The relative contributions of the ions and molecules of normal serum to the total osmotic pressure have been calculated. 2. Serum osmolality has been determined in 91 cases of chloride depletionand these cases have been divided into three groups. 3. In group A, sufficient urea and bicarbonate were retained to keep the osmolality within normal limits. In group B this compensatory mechanism failed and the osmolality was abnormally low. In group C, the osmolality was abnormally high due to excessive urea retention. 4. Reasons are given for believing that these groups of patients are distinctand separate groups, although there is the possibility that group C may be an extension of group A due to the onset of renal dysfunction.

A test of general renal function based on the measurement of urine osmolality has been applied to a group of normals, a group of control patients, and a series of patients suffering from renal disease of varying severity. The results obtained have been compared with the Van Slyke urea clearance tests carried out at the same time. In normal subjects and in the early stages of renal disease the two tests give reasonably similar results. In the advanced cases of renal disease, there is a marked discrepancy between the two assessments of renal function.

The majority of urine specimens, including the “morning specime”, can be shown to be mixtures of dilute and of concentrated urines. The differences in osmolality and the differences in concentration of these urines may be very considerable. The osmolality of a urine specimen after dehydration for 12–14 hours is a poor measure of the ability of the urinary system to elaborate a concentrated urine. If the urine flow rate is kept within the limits of 14 to 75 ml per hour, either by mild dehydration or by the smoking of tobacco, then there is a linear relationship between the water and the solute output in unit time.