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The contractile vacuole allows free-living protists to expel extra water. <b>The contractile vacuole allows free-living protists to expel extra water <br /></b>.
1 ADH Absent. 2  When ADH is absent the volume of water in the tubule is relatively high in the distal collecting duct, thus diluting the concentration of tubular urea and making the driving force for passive resorption of urea low.