The kidneys go to a great deal of trouble to create the medullary osmotic gradient. What is its purpose? Without this gradient, you would not be able to raise the concentration of urine above 300 mOsm, the osmolality of interstitial fluid. As a result, you would not be able to excrete excess solutes to lower your body osmolality.
Controlling the reabsorption of water from filtrate in the collecting ducts in order to adjust the body's osmolality is the job of antidiuretic hormone (ADH). ADH inhibits diuresis, or urine output. It accomplishes this via a second messenger system using cyclic AMP that causes insertion of aquaporins into the luminal membrane of the principal cells of the collecting ducts. The amount of the ADH determines the number of aquaporins in the collecting ducts and so the amount of water that is reabsorbed there.
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