Antidiuretic hormone (ADH), also known as vasopressin, directly increases water reabsorption in the collecting ducts of the nephron by promoting the insertion of aquaporin-2 water channels into the apical membrane of the duct cells. This process enhances water permeability, allowing more water to be reabsorbed back into the bloodstream, thus concentrating the urine and maintaining body fluid balance. Unlike aldosterone, which primarily affects sodium reabsorption, ADH specifically regulates water retention. This mechanism is crucial in conditions requiring volume conservation or when plasma osmolality is high.

Reference: Guyton and Hall Textbook of Medical Physiology, Arthur C. Guyton, 13th Edition.