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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>.
That water concentrations are regulated by a hormone called ADH. That ADH effects the permeability of the distal convoluting tubule and the collecting duct. That aldosterone controls sodium concentrations in the blood. The Collecting Duct Hormones control the permeability of the walls of the collecting duct.If the walls are water-permeable, water leaves the ducts to pass into the hyperosmotic surroundings and concentrated urine is produced.If the ducts are impermeable to water the final urine will be less concentrated.he Collecting Duct Hormones control the permeability of the walls of the collecting duct. If the walls are water-permeable, water leaves the ducts to pass into the hyperosmotic surroundings and concentrated urine is produced. If the ducts are impermeable to water the final urine will be less concentrated.