The Antidiuretic hormone (ADH), also known as vasopressin, primarily promotes water reabsorption in the kidneys by increasing the permeability of the collecting ducts to water. ADH binds to receptors on the cells of the collecting duct, triggering the insertion of aqua­porin-2 channels into the membrane, allowing water to be reabsorbed back into the bloodstream. This action concentrates the urine and prevents dehydration. Unlike aldosterone, which mainly regulates sodium, ADH directly controls water balance, making it the key hormone for maintaining body fluid volume.

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