Which statement best describes the primary regulation of water balance in newborns?

• A. Primarily regulated by aldosterone
• B. Primarily regulated by natriuretic peptides
• C. Not influenced by hormones
• D. Primarily regulated by ADH

Answer: (D)

Antidiuretic hormone (ADH) is the primary regulator of water balance. ADH is released in response to increased plasma osmolality or decreased blood volume and acts on the kidney’s collecting ducts to insert aquaporin-2 channels, increasing water reabsorption and concentrating the urine. This direct effect on water transport makes ADH the main driver of maintaining proper hydration in newborns.

Aldosterone mainly controls sodium reabsorption (and potassium excretion) in the distal nephron, indirectly influencing fluid volume but not directing water balance itself. Natriuretic peptides promote natriuresis and diuresis to reduce volume, but they function more as modulators of volume status rather than the primary regulator of water balance. Hormonal influence is real, and in newborns, the concentrating ability can be immature, but ADH remains the key hormone governing water balance.