Haemoglobin's gas binding affinities order

Steps:

• Identify affinity as the strength of haemoglobin (Hb) binding to each gas.
• Note CO binds to Hb ~250 times more strongly than O2, blocking oxygen transport.
• O2 binds reversibly to Hb for normal transport.
• CO2 binds weakly to Hb (mainly at different sites) and is mostly transported as bicarbonate.

Why B is correct:

• Hb affinity order is CO > O2 > CO2, per the binding constant where CO's affinity is 200–250 times that of O2, and CO2's is far weaker.

Why the others are wrong:

• A: Falsely ranks CO2 highest, ignoring its weak binding.
• C: Illogically repeats CO and starts with O2 over CO.
• D: Wrongly ranks O2 above CO, reversing their affinities.

Final answer: B