A patient with SIADH has serum Na⁺ of 118 mEq/L. Which of the following BEST describes the shift in body fluid osmolality and compartment volumes that occurs?

• A Low plasma osmolality drives water into cells; ECF volume is mildly expanded and ICF is expanded; total body Na⁺ is normal or slightly increased ✓
• B Low plasma Na⁺ causes ECF contraction; cells shrink due to osmotic water loss into extracellular space
• C Both ECF and ICF are contracted; total body water is reduced due to renal wasting
• D Plasma osmolality is normal; hyponatremia is purely dilutional without shifts in ICF volume

Explanation

In SIADH, excess ADH causes inappropriately high water reabsorption by the collecting duct, expanding total body water. Plasma osmolality falls (hypo-osmolality); this osmotic gradient drives water from ECF into cells, expanding the ICF and also mildly expanding the ECF (though urinary sodium loss partially corrects ECF volume through pressure natriuresis). Total body sodium is normal or slightly decreased while total body water is increased. The hallmark is euvolemic hyponatremia. Option B is incorrect — low osmolality causes water influx into cells, not shrinkage. Options C and D are incorrect characterizations of SIADH.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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