Which of the following is NOT listed as a regulator of body water balance?

• A. Thirst center (hypothalamus)
• B. Antidiuretic hormone (ADH)
• C. Renin-angiotensin-aldosterone system (RAAS)
• D. Hematocrit

Answer: (D)

Body water balance is controlled by mechanisms that regulate fluid intake and how the kidneys handle water. The thirst center in the hypothalamus senses changes in plasma osmolality or blood volume and motivates drinking. Antidiuretic hormone acts on the kidney’s collecting ducts to increase water reabsorption, helping conserve water when needed. The renin-angiotensin-aldosterone system responds to reduced renal perfusion or low sodium by promoting sodium and water retention and by influencing thirst, thus helping to maintain circulating volume and blood pressure. Hematocrit, the proportion of red blood cells in blood, reflects hydration status as a consequence of volume changes but does not actively regulate water balance. Therefore, hematocrit is not a regulator of body water balance.