Which of the following determines the volume of the extracellular fluid?

• A. Renal sodium concentration
• B. Serum sodium concentration
• C. Total sodium content
• D. Plasma potassium

Answer: (C)

Extracellular fluid volume is governed by the total body sodium content because Na+ is the principal osmolyte in the extracellular space and water follows sodium osmotically. When the total sodium load increases, the body retains more water to maintain osmotic balance, expanding the extracellular compartment; when sodium is lost, water shifts out, reducing ECF volume. Serum sodium concentration reflects current osmolality and water status but does not by itself set the volume of the extracellular fluid. Urinary (renal) sodium concentration shows kidney handling at a moment in time, not the total body Na+ pool. Plasma potassium is more about cellular function and overall potassium balance than determining extracellular fluid volume.