Which factor is NOT a determinant of extracellular fluid volume?

• A. Total sodium content
• B. Renal sodium handling
• C. Serum sodium concentration
• D. Interstitial protein concentration

Answer: (C)

Extracellular fluid volume is governed by the amount of osmotically active solute—in practice, the total body sodium content and how the kidneys handle sodium—as well as forces that control fluid movement between plasma and the interstitial space, such as interstitial protein concentration influencing capillary oncotic pressure. When total body sodium increases, water is drawn into the extracellular space, expanding its volume; when sodium is lost or excreted, the extracellular volume decreases. Interstitial protein concentration can shift fluid between the intravascular and interstitial compartments, affecting how much fluid sits in each part of the extracellular space. Serum sodium concentration, however, reflects the balance of sodium and water and thus the osmolality or tonicity of body fluids, not the total amount of fluid in the extracellular space. You can have normal or even abnormal serum sodium without a direct, proportional change in extracellular fluid volume, depending on overall water balance and other factors.