A 70 kg man has a measured plasma volume of 3.0 L using Evans blue dye. If his hematocrit is 45 percent, what is his approximate total blood volume?
- A 3.0 L
- B 4.5 L
- C 5.5 L ✓
- D 7.0 L
Correct answer: C. 5.5 L
Explanation
Total blood volume is calculated as plasma volume divided by (1 minus hematocrit). With a plasma volume of 3.0 L and hematocrit of 0.45, total blood volume equals 3.0 divided by 0.55, which is approximately 5.5 L. This assumes the measured plasma volume accurately reflects total circulating plasma. The hematocrit indicates the fraction of blood occupied by red cells, so the remaining fraction is plasma.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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