Renal physiology series: Part 3 of 8. Urinary concentration and dilution.

The maintenance of extracellular volume depends on the appropriate excretion of both salt and water. When extracellular volume is expanded, the excretion of a dilute urine facilitates the return of extracellular volume to normal. When extracellular volume is contracted, the retention of both salt and water and the excretion of a small amount of concentrated urine contributes to the reexpansion of extracellular volume. The formation of either a dilute or concentrated urine is dependent on the physical arrangement of the loops of Henle, collecting tubules, and vasa recta within the kidney medulla, the transport properties of each segment; and the appropriate presence or absence of antidiuretic hormone (ADH), a hormone synthesized in the hypothalamus and secreted from the posterior pituitary gland.