The kidney in health and disease: VIII. Renal concentrating and diluting mechanisms.

The capacity to elaborate urine either more dilute or more concentrated than the plasma from which it is derived enables the kidney to regulate the solute concentrations and hence the osmolality of body fluids within narrow physiologic limits despite wide fluctuations in salt and water intake. The underlying mechanisms are examined in detail, with emphasis on the role of urea in the countercurrent system of Henle's loop.