The Kidney Can Produce Either Concentrated or Diluted Urine

An elegant system has evolved in the mammalian kidney that allows excretion of either concentrated or diluted urine as cir­cumstances warrant. This system has three main components: (1) generation of a hypertonic medullary interstitium, which allows excretion of concentrated urine; (2) dilution of the tubule fluid by the thick ascending limb and the distal convoluted tubule, which allows excretion of dilute urine; and (3) variability in the water permeability of the collecting duct in response to antidiuretic hormone (ADH, vasopressin), which determines the final urine concentration.