A 70 kg man is given 2 litres of isotonic saline (0.9% NaCl) intravenously. Assuming normal distribution, how much of the infused volume remains in the intravascular compartment after equilibration?
- A 2000 mL
- B 500 mL ✓
- C 1000 mL
- D 250 mL
Correct answer: B. 500 mL
Explanation
Isotonic saline distributes throughout the entire extracellular fluid (ECF) compartment, which constitutes approximately 1/3 of total body water (about 20 L in a 70 kg man). The ECF is divided into interstitial (~15 L, 75%) and intravascular (~5 L, 25%). Therefore 25% of the 2 L infusion = 500 mL remains in the plasma. It does not enter the intracellular compartment because it is isoosmolar. This is why large volumes of saline are needed to expand plasma volume, and why colloids are more efficient plasma expanders.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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