Which systems regulate water reabsorption in the kidney?

• A. RAAS
• B. ADH
• C. Both RAAS and ADH
• D. Aldosterone only

Answer: (C)

Water reabsorption in the kidney is controlled by two hormonal systems. Antidiuretic hormone (ADH) directly increases water reabsorption in the collecting ducts by making the ducts permeable to water—more ADH means more water is reabsorbed and urine becomes more concentrated. The renin-angiotensin-aldosterone system (RAAS) promotes water reabsorption indirectly by increasing sodium reabsorption, especially in the collecting ducts via aldosterone. Since water follows sodium osmotically, this also raises water reabsorption. Angiotensin II within RAAS also supports volume conservation by stimulating thirst and promoting ADH release, reinforcing the effect. Because both systems contribute to retaining water, the best answer is that water reabsorption is regulated by both RAAS and ADH.