Osmoreceptors in the hypothalamus are key to regulating blood osmolality. They primarily respond to changes in plasma osmolality to adjust the secretion of which hormone?

• A. ADH
• B. Aldosterone
• C. Epinephrine
• D. Insulin

Answer: (A)

The key idea is how the body keeps water balance and plasma osmolality in check. Osmoreceptors in the hypothalamus detect changes in plasma osmolality, especially when it rises (more solute per water). In response to increased osmolality, these receptors trigger the release of antidiuretic hormone from the posterior pituitary. ADH then acts on the kidneys, making the collecting ducts more permeable to water by promoting insertion of aquaporin-2 channels. This increases water reabsorption, concentrates the urine, and reduces water loss, bringing plasma osmolality back toward normal. Aldosterone, epinephrine, and insulin don’t directly mediate this osmolality-sensing response.