What is the effect of a decrease in pH on the oxygen dissociation curve?

• A. Shifts to the left
• B. No change
• C. Shifts to the right
• D. Increases maximum saturation

Answer: (C)

Lower pH increases hydrogen ion binding to hemoglobin, which decreases its affinity for oxygen. This makes the oxygen-hemoglobin dissociation curve shift to the right. In practical terms, at any given PO2, hemoglobin holds onto less O2 and releases more to tissues—the PO2 at which Hb is 50% saturated (P50) goes up. This is the Bohr effect: in metabolically active tissues where CO2 and H+ are high, oxygen is released more readily to meet demand. The maximum level of saturation at very high PO2 doesn’t rise because the change is in affinity and unloading, not in the total capacity of Hb to carry O2. A left shift would imply higher affinity (more O2 binding), which isn’t what a pH decrease does.