- The main role of antidiuretic hormone (ADH), also known as vasopressin, in renal physiology is to increase water reabsorption in the collecting ducts of the kidneys.
- ADH acts on the collecting duct cells by binding to V2 receptors, which triggers the insertion of aquaporin-2 water channels into the apical membrane.
- This process allows water to be reabsorbed from the tubular fluid back into the bloodstream, thereby concentrating the urine and reducing water loss.

- It is important to note that ADH primarily affects water permeability and does not directly promote sodium excretion or stimulate aldosterone secretion.
- Aldosterone, a different hormone secreted by the adrenal cortex, regulates sodium reabsorption and potassium secretion.
- Therefore, options related to sodium excretion, aldosterone stimulation, or potassium secretion are not roles of ADH.

In summary, the essential function of ADH is to conserve water by enhancing water reabsorption in the renal collecting ducts, which helps maintain body fluid balance and blood pressure.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th Edition, Chapter 27, Page 330