The Bohr effect describes how pH and CO2 influence Hb-O2 binding. Which statement best describes it?

• A. Higher pH or lower CO2 decreases Hb affinity for O2
• B. Lower pH or higher CO2 decreases Hb affinity for O2
• C. Changes in pH do not affect Hb affinity for O2
• D. Hb affinity for O2 increases with higher CO2

Answer: (B)

The Bohr effect describes how carbon dioxide and hydrogen ions (lower pH) modulate hemoglobin’s oxygen-binding affinity. When tissues metabolize, CO2 levels rise and pH falls, causing hemoglobin to release O2 more readily. CO2 entering red blood cells forms carbonic acid, which lowers pH; the increased H+ binds to hemoglobin and stabilizes the deoxygenated form, shifting the oxygen dissociation curve to the right. This means a decreased affinity for O2 under these conditions, promoting oxygen delivery to tissues. In the lungs, CO2 is expelled and pH rises, shifting the curve back to the left to increase Hb’s affinity for O2 for loading. The statement that lower pH or higher CO2 decreases Hb affinity for O2 matches this mechanism.