During a marathon run in a hot environment, a runner loses 3 L of sweat (hypotonic fluid). Which combination of physiological responses is expected?
- A Increased plasma osmolality → stimulated ADH → concentrated urine → stimulated thirst; also reduced ECF volume activates RAAS ✓
- B Decreased plasma osmolality → suppressed ADH → decreased thirst → dilute urine
- C Isoosmotic ECF contraction → no ADH change → normal urine concentration → RAAS activation only
- D Hypervolemia from sympathetic-mediated adrenal cortisol release mobilizing fluid from ICF
Correct answer: A. Increased plasma osmolality → stimulated ADH → concentrated urine → stimulated thirst; also reduced ECF volume activates RAAS
Explanation
Sweat is hypotonic (approximately 50 mEq/L Na⁺), so losing 3 L of sweat removes proportionally more water than solute, raising plasma osmolality. This hyperosmolality is the dominant stimulus for ADH release (osmoreceptors in OVLT and SFO threshold ~280 mOsm/kg), producing concentrated urine (up to 1200 mOsm/kg). Simultaneously, ECF volume contraction activates baroreceptors → RAAS activation. Thirst is stimulated both by hyperosmolality (osmoreceptors) and angiotensin II. Options A and C are incorrect because osmolality rises (sweat is hypotonic, not isotonic).
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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