Gas exchange reactions in pulmonary capillaries

Steps:

• Recall lung anatomy: pulmonary capillaries surround alveoli for O2 uptake and CO2 release.
• Identify key reactions: hemoglobin oxygenation (O2 + Hb → HbO2) and CO2 elimination (H+ + HCO3- → CO2 + H2O via carbonic anhydrase).
• Eliminate irrelevant processes: glycolysis and Krebs cycle occur in mitochondria, not capillaries.
• Match to options: only reaction 4 (CO2 dehydration) is specific to lung capillaries.

Why D is correct:

• Reaction 4 is the reverse carbonic anhydrase reaction (HCO3- + H+ → CO2 + H2O), essential for CO2 unloading in pulmonary capillaries per respiratory physiology.

Why the others are wrong:

• A includes 1 and 2, which are tissue-level reactions like CO2 hydration, not lung-specific.
• B pairs 3 (O2 diffusion, alveolar) with 4, but 3 occurs at alveolar membrane, not strictly capillary.
• C selects only 3, ignoring the primary capillary CO2 reaction (4).

Final answer: D