<P> The collecting duct system is the final component of the kidney to influence the body's electrolyte and fluid balance . In humans, the system accounts for 4--5% of the kidney's reabsorption of sodium and 5% of the kidney's reabsorption of water . At times of extreme dehydration, over 24% of the filtered water may be reabsorbed in the collecting duct system . </P> <P> The wide variation in water reabsorption levels for the collecting duct system reflects its dependence on hormonal activation . The collecting ducts, in particular, the outer medullary and cortical collecting ducts, are largely impermeable to water without the presence of antidiuretic hormone (ADH, or vasopressin). </P> <Ul> <Li> In the absence of ADH, water in the renal filtrate is left alone to enter the urine, promoting diuresis . </Li> <Li> When ADH is present, aquaporins allow for the reabsorption of this water, thereby inhibiting diuresis . </Li> </Ul> <Li> In the absence of ADH, water in the renal filtrate is left alone to enter the urine, promoting diuresis . </Li>

How does adh act on the collecting duct
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