Osmoreceptors in the hypothalamus regulate blood osmolality. A 1% to 2% shift in osmolality typically causes a ______ change in circulating concentration of ADH.

Osmoreceptors in the hypothalamus regulate blood osmolality. A 1% to 2% shift in osmolality typically causes a ______ change in circulating concentration of ADH.

• A. Twofold
• B. Fourfold
• C. Eightfold
• D. Tenfold

Answer: (A)

Osmoreceptors in the hypothalamus respond to small increases in plasma osmolality by signaling the posterior pituitary to release more ADH. This system is tuned so that a modest shift, about 1–2%, produces a noticeable, but not extreme, rise in circulating ADH—roughly a twofold increase. The reason this matters is that ADH acts on the kidneys to reabsorb more water in the collecting ducts (via V2 receptors and aquaporin-2 channels), which helps dilute the plasma back toward normal osmolality and concentrates the urine less, counteracting the osmotic rise. The twofold response reflects the need for a rapid yet proportionate adjustment to maintain fluid balance without overcorrection. Larger increases (fourfold, eightfold, tenfold) would imply much more drastic water retention than is typically required for ordinary osmotic shifts, and aren’t the standard physiologic response.

Osmoreceptors in the hypothalamus respond to small increases in plasma osmolality by signaling the posterior pituitary to release more ADH. This system is tuned so that a modest shift, about 1–2%, produces a noticeable, but not extreme, rise in circulating ADH—roughly a twofold increase. The reason this matters is that ADH acts on the kidneys to reabsorb more water in the collecting ducts (via V2 receptors and aquaporin-2 channels), which helps dilute the plasma back toward normal osmolality and concentrates the urine less, counteracting the osmotic rise. The twofold response reflects the need for a rapid yet proportionate adjustment to maintain fluid balance without overcorrection. Larger increases (fourfold, eightfold, tenfold) would imply much more drastic water retention than is typically required for ordinary osmotic shifts, and aren’t the standard physiologic response.