Explain how hemoglobin’s oxygen binding affinity changes with pH and CO2 (Bohr effect) and the physiological significance.

• A. Hemoglobin affinity for oxygen increases with lower pH and higher CO2.
• B. Decreased pH or increased CO2 lowers Hb affinity for O2, shifting curve to the right, promoting unloading; higher pH and lower CO2 shift left, promoting loading.
• C. Hemoglobin affinity is not affected by pH or CO2.
• D. Oxygen binding is enhanced when CO2 is high.

Answer: (B)

The Bohr effect shows how the chemical environment around hemoglobin changes its oxygen binding affinity. In tissues that are metabolically active, CO2 levels rise and pH drops. This causes protonation of certain amino acids and stabilizes the T (tense) state of hemoglobin, lowering its affinity for O2 and shifting the oxygen dissociation curve to the right so more O2 is released where it’s needed. In the lungs, CO2 is expelled and pH rises, which favors the R (relaxed) state with higher O2 affinity, promoting loading of O2 onto hemoglobin for transport to tissues. This mechanism efficiently matches oxygen delivery to demand and facilitates CO2 removal during metabolism and exercise.