Question

after intense exercise, a patient is sweating heavily and becomes dehydrated. blood tests show elevated plasma sodium concentration.
which sequence of physiological events most likely explains how the body will respond to restore fluid balance?

• increased plasma sodium concentration causes osmoreceptors to shrink → activates supraoptic nuclei → adh release → insertion of aquaporins in kidney tubules → water reabsorption → plasma sodium concentration decreases.
• sweating directly activates kidney sodium pumps → adh released → water retained → plasma sodium concentration increases.
• increased plasma sodium concentration stimulates osmoreceptors to swell → adh is blocked → na/ k pumps in kidney get activated → water excreted in urine → urine sodium concentration decreases.
• sweating stimulates baroreceptors → inhibits adh release from posterior lobe of pituitary gland → aquaporins removed from kidney → water excreted → urine sodium concentration decreases.

Explanation:

To solve this, we analyze each option:

• Option 1: Elevated plasma Na⁺ (sodium) increases osmolarity, causing osmoreceptors (in hypothalamus) to shrink (due to water loss via osmosis). This activates supraoptic nuclei, triggering ADH (antidiuretic hormone) release from posterior pituitary. ADH increases aquaporins in kidney tubules, enhancing water reabsorption. More water in blood dilutes Na⁺, reducing its concentration. This matches the body’s homeostatic response to dehydration.
• Option 2: Sweating doesn’t directly activate kidney sodium pumps. Also, retaining water should dilute Na⁺, not increase its concentration. So this is incorrect.
• Option 3: Increased Na⁺ would make osmoreceptors shrink (not swell) due to water loss. ADH is released (not blocked) to retain water, so this is wrong.
• Option 4: Sweating (dehydration) should activate osmoreceptors, not baroreceptors (which detect blood pressure). ADH release is stimulated (not inhibited) to retain water, so this is incorrect.

Answer:

A. Increased plasma sodium concentration causes osmoreceptors to shrink → activates supraoptic nuclei → ADH release → insertion of aquaporins in kidney tubules → water reabsorption → plasma sodium concentration decreases.