CO2 Release in Lung Capillaries

Steps:

• Identify gas exchange in lungs: O2 enters blood, CO2 exits to alveoli.
• Recall CO2 transport: In tissues, CO2 forms HCO3- via carbonic anhydrase for blood carry.
• In lungs, low CO2 partial pressure reverses process: HCO3- converts back to CO2.
• Confirm enzyme role: Carbonic anhydrase catalyzes HCO3- + H+ → H2CO3 → CO2 + H2O.

Why D is correct:

• In lungs, carbonic anhydrase reverses bicarbonate formation (HCO3- + H+ ⇌ H2CO3 ⇌ CO2 + H2O), releasing CO2 for diffusion into alveoli.

Why the others are wrong:

• A: Dissociation of H2CO3 to CO2 + H+ occurs in tissues, not lungs.
• B: CO2 to HCO3- happens in tissues for transport, opposite in lungs.
• C: Carbaminohemoglobin forms in tissues; in lungs, it releases CO2.

Final answer: D