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Okay, so tell me if I understand this correctly. The descending loop is permeable to water and therefore by the bottom of the loop, the filtrate is pretty concentrated. The ascending loop is permeable to salt and therefore by the time the filtrate reaches the distal convoluted tubule, it is fairly dilute. In the presence of ADH, the collecting duct is permeable to water and therefore loses it, resulting in the production of a small amount of concentrated urine. In the absence of ADH, the collecting duct is not permeable to water, resulting in the production of a large amount of dilute urine. In the presence of a diuretic, such as caffeine, sodium reabsorption is inhibited, which results in water also being retained in the filtrate. This ends in the production of a large amount of concentrated urine?