Though the collecting duct is normally impermeable to water, it becomes permeable in the presence of antidiuretic hormone (ADH). ADH affects the function of aquaporins, resulting in the reabsorption of water molecules as it passes through the collecting duct. Aquaporins are membrane proteins that selectively conduct water molecules while preventing the passage of ions and other solutes. As much as three-quarters of the water from urine can be reabsorbed as it leaves the collecting duct by osmosis. Thus the levels of ADH determine whether urine will be concentrated or diluted. An increase in ADH is an indication of dehydration, while water sufficiency results in low ADH allowing for diluted urine.
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