CO2 unloading reactions in pulmonary capillaries

Steps:

• In alveoli capillaries, CO2 diffuses from blood to air, reversing tissue-loading reactions.
• Bicarbonate buffer reverses: HCO3- + H+ → H2CO3 → CO2 + H2O (reaction 4).
• Oxygenated hemoglobin releases H+ : HbH+ → Hb + H+ (reaction 3), driving reaction 4.
• Reactions 1 and 2 represent CO2 loading in tissues, not unloading here.

Why B is correct:

• CO2 transport via bicarbonate (70%) unloads through reaction 4; hemoglobin buffering releases H+ via reaction 3 to maintain pH and facilitate diffusion.

Why the others are wrong:

• A: Reactions 1 and 2 occur in systemic capillaries during CO2 uptake from tissues.
• C: Omits bicarbonate reversal (4), the primary CO2 unloading pathway.
• D: Omits H+ release (3), essential for shifting bicarbonate equilibrium.

Final answer: B