Tubular filtrate is diluted as it travels through the ascending limb of the loop of Henle, so all the kidney needs to do secrete dilute (hypo-osmotic) urine is allow the filtrate to continue on its way into the renal pelvis. When ADH (Antidiuretic hormone) is not being released by the posterior pituitary, that is exactly what happens, The collecting ducts remain essentially impermeable to water due to absence of aquaporins in their luminal cell membranes, and no further water reabsorption occurs. Moreover, as noted, Na+ and selected other ions can be removed from the filtrate by distal convoluted tubule and collecting duct cells so that urine becomes even more dilute before entering renal pelvis. The osmolality of urine can plunge as low as 50 mOsm, about one-sixth the concentration of lomerular filtrate or blood plasma.
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