A patient drinks 2 L of pure water rapidly. Which physiological changes will occur in steady state after maximal renal compensation?
- A Plasma osmolality decreases, ECF expands but ICF contracts due to ADH release
- B Plasma osmolality decreases, ECF and ICF both expand, ADH is suppressed and water is excreted ✓
- C Both compartments shrink due to dilutional reduction of COP
- D Plasma osmolality is unchanged as pure water distributes only to ICF
Correct answer: B. Plasma osmolality decreases, ECF and ICF both expand, ADH is suppressed and water is excreted
Explanation
Free water (hypotonic) distributes across all body fluid compartments in proportion to their volumes (ICF ~67%, ECF ~33%), lowering osmolality in all compartments. This suppresses ADH (antidiuretic hormone), leading to dilute urine production (aquaporin-2 channels are not inserted in collecting duct) and excretion of the water load within 45–90 minutes. Both ECF and ICF expand transiently. With intact renal function, this is rapidly corrected — demonstrating the efficiency of the osmoregulatory system.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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