The formation of concentrated urine depends on the medullary osmotic gradient and the presence of antidiuretic hormone. In the distal tubules, the filtrate osmolality is approximately 100 mOsm, but as the filtrate flows through the collecting ducts and is subjected to the hyperosmolar conditions in the medulla, water rapidly leaves the filtrate, followed by urea. Depending on the amount of antidiuretic hormone released (which is keyed to the level of body hydration), urine concentration may rise as high as 1200 mOsm, the concentration of the interstitial fluid in the deepest part of the medulla. With maximal antidiuretic hormone secretion, up to 99% of the water in the filtrate is reabsorbed and returned to the blood, and a half liter per day of highly concentrated urine is excreted. The ability of kidneys to produce such concentrated urine is critically tied to ability to survive without water. Water reabsorption that depends on the presence of antidiuretic hormone is called facultative water reabsorption.
Antidiuretic hormone is released more or less continuosly unless the blood solute concentration drops too low. Released of antidiuretic hormone is enhanced by any event that raises plasma osmolality above 300mOsm, such as sweating or diarrhea, or by greatly reduced blood volume or blood pressure. Although release of antidiuretic hormone is the “signal” to produce concentrated urine that opens the door of water reabsorption (through aquaporins), the kidney’s ability to respond this signal depends on the high medullary osmotic gradient,