Gas exchange and buffering in active tissues

Steps:

• Active tissues generate CO₂ and lactic acid, raising blood H⁺ and PCO₂.
• Elevated H⁺ triggers hemoglobin to release O₂ (reaction 1: HbO₂ + H⁺ → Hb + O₂, Bohr effect).
• H⁺ from metabolism is buffered by HCO₃⁻ forming H₂CO₃ (reaction 3: HCO₃⁻ + H⁺ → H₂CO₃).
• CO₂ reacts with water to form carbonic acid, aiding transport (reaction 4, implied carbonic anhydrase activity).
• Reaction 2 (likely reverse or unrelated, e.g., O₂ binding) does not occur under acidic conditions.

Why B is correct:

• B selects reactions 1, 3, and 4, matching Bohr effect (O₂ unloading via H⁺) and bicarbonate buffer system (HCO₃⁻ + H⁺ → H₂CO₃ → CO₂ + H₂O) for O₂ delivery and CO₂ handling in tissues.

Why the others are wrong:

• A includes incorrect reaction 2, which opposes tissue conditions.
• C omits buffering (3) and CO₂ transport (4), incomplete for acid load.
• D excludes O₂ release (1) and CO₂ step (4), missing key gas exchange.

Final answer: B