Final answer:
ADH, also known as vasopressin, increases water permeability at the inner medullary collecting duct of the kidney by inserting aquaporin channels into the apical membrane of the principal cells in the collecting ducts.
Step-by-step explanation:
ADH, also known as vasopressin, causes an increase in water permeability at the inner medullary collecting duct of the kidney by inducing the insertion of aquaporin channels into the apical membrane of the principal cells in the collecting ducts. This allows water to be osmotically pulled from the collecting duct into the surrounding interstitial space and into the peritubular capillaries. As a result, more water is reabsorbed into the bloodstream and the final urine becomes more concentrated.