What is the Bohr effect in relation to hemoglobin?

• A. Higher pH increases Hb affinity for oxygen
• B. Oxygen binds more strongly in tissues when CO2 is high
• C. CO2 binds Hb in a way that decreases oxygen unloading
• D. Lower pH and higher CO2 decrease Hb affinity, promoting oxygen unloading

Answer: (D)

The Bohr effect describes how acidity (pH) and CO2 levels influence hemoglobin’s affinity for oxygen. In actively metabolizing tissues, CO2 builds up and pH falls, which makes hemoglobin release oxygen more readily. This happens because the additional H+ ions bind to hemoglobin, stabilizing the form that has a lower affinity for O2, effectively shifting the oxygen-hemoglobin dissociation curve to the right. CO2 can also directly bind to hemoglobin (carbaminohemoglobin), further reducing oxygen affinity. In the lungs, CO2 is expelled and pH rises, increasing hemoglobin’s affinity for O2 and promoting loading. So, lower pH and higher CO2 promote oxygen unloading to tissues.