Which condition promotes oxygen unloading from hemoglobin according to the Bohr effect?

• A. Low pH and high CO2
• B. High pH and low CO2
• C. High oxygen concentration in lungs
• D. Low body temperature with normoxia

Answer: (A)

Bohr effect: hemoglobin’s affinity for oxygen decreases when acidity rises or carbon dioxide levels are high, which promotes oxygen release in tissues that are metabolically active. In tissues, CO2 produced by respiration combines with water to form carbonic acid, dissociates to bicarbonate and hydrogen ions, lowering pH. The extra protons stabilize the deoxygenated form of hemoglobin, making it easier for O2 to be released where it’s needed. CO2 can also bind to hemoglobin directly (carbaminohemoglobin), further reducing O2 affinity. In the lungs, CO2 is expelled and pH rises, increasing hemoglobin’s affinity for O2 so it can load O2 efficiently. So the condition that promotes unloading is a low pH together with high CO2. The other options would either raise hemoglobin’s oxygen affinity or promote loading rather than unloading.