The P50 of the oxyhemoglobin dissociation curve represents which of the following?
- A The percentage saturation achieved at a PO2 of 50 mmHg
- B The PO2 at which hemoglobin is 50 percent saturated with oxygen ✓
- C The PO2 at which half the bound oxygen is unloaded per unit time
- D The time taken for hemoglobin to reach 50 percent saturation
Correct answer: B. The PO2 at which hemoglobin is 50 percent saturated with oxygen
Explanation
P50 is defined as the partial pressure of oxygen at which hemoglobin is 50 percent saturated; its normal value is approximately 26 to 27 mmHg. It is a convenient index of oxygen affinity: a rightward shift raises P50 (lower affinity) and a leftward shift lowers it. Option A confuses the axis variables, since saturation at a fixed PO2 of 50 mmHg is roughly 85 percent, not the definition.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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