Na+ and Cl- ions are actively pumped out of the ascending limb, accumulating in the interstitial fluid, lowering the water potential.  , Water can't osmotically follow Na+ and Cl- ions as the ascending limb is impermeable to water. , Water is drawn out of the descending limb and into the interstitial fluid by osmosis, making it more concentrated., When fluid in the descending limb has reached the bottom of the limb, it has lost a lot of water and is very concentrated. , The fluid surrounding the bottom of the loop (in the medulla) is also very concentrated because of the accumulation of Na+ and Cl- ions, Fluid then enters the ascending limb, as it moves up, sodium ions are actively pumped out., The fluid at the top of the ascending limb is very dilute., Fluid empties into the collecting duct, passing through the very concentrated medullary region., ADH influences the wall of the collecting duct to become permeable to water., Water is osmotically drawn out of the collecting duct and into the blood capillaries in the region, concentrating urine., By regulating the permeability of the collecting duct, the amount of water in the blood and the concentration of the urine can be controlled. .

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