A 70-kg man is used as a model for fluid compartments: total body water 42 L, intracellular fluid 28 L, interstitial fluid 11 L and plasma 3 L. He is given 1 L of 0.9% sodium chloride intravenously. After equilibration, approximately how much of the infused volume remains in the plasma?
- A. 70 mL
- B. 210 mL
- C. 330 mL
- D. 670 mL
- E. 1000 mL
Show the answer and explanation
Answer: B. 210 mL
Sodium is the main extracellular cation, so isotonic saline stays in the extracellular fluid (plasma plus interstitium, 14 L). Plasma is 3 L of this, so roughly 3/14 of the infused volume (about 210 mL) remains intravascular. This is why large volumes of crystalloid are needed to restore circulating volume and why oedema follows.
Why the other options are wrong
- A. This is the approximate figure for 5% glucose, which distributes throughout total body water (3/42).
- C. This would require plasma to be one third of the distribution volume, which it is not.
- D. This overestimates retention; crystalloids rapidly leak into the interstitium.
- E. Only colloids or blood would come close to remaining wholly intravascular, and even they do not.