Studies on the Renal Concentrating and Diluting Mechanisms in the Premature Infant12

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Abstract
It is known that young premature infants (p. ins.) are unable to concentrate urine to the same degree as adults. The question of whether decreased rate of solute excretion is a factor in this lowered concning. capacity of the premature infant could be answered by loading (p. ins.) with a 25% soln. of mannitol given intraven. Tests were made in p. ins., wt. approx. 1500 g., aged 5-23 days. During osmotic diuresis in hydropenia, the p. ins. reabsorbed a smaller quantity of fixed H2O than did the adult under similar circumstances. Under conditions descr. above, a constant vol. of reabsorbed H2O measured as TcH2O appeared to be a factor limiting concn. during high urine flows. The p. in. attained a urine osmotic U/P ratio of about 2.5 during water deprivation. Adults under similar circumstances attained a value of about 4. This lowered TCH2O and osmotic U/P ratio of the p. in. are probably dependent upon the immaturity of renal tubular function. The same p. ins. during water diuresis dilute their urine to values comparable to those in the adult, suggesting that the concning. and diluting mechanisms of the kidney in the p. in. mature at different rates, and may involve different functional segments of the tubule. Na is excreted at a higher percentage of the total urinary osmoles in p. ins. than in adults during osmotic loading.