Gas exchange in pulmonary capillaries favors O2 uptake

Steps:

• Locate the site: Capillary next to alveolus is in lungs, where blood arrives deoxygenated with high PCO2 and low PO2.
• Identify reaction 1: Hemoglobin oxygenation (Hb + O2 ⇌ HbO2), which proceeds forward due to high alveolar PO2.
• Identify reaction 2: CO2 hydration (CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-), which occurs in tissues for CO2 loading.
• Conclude: Only reaction 1 occurs, as CO2 reactions reverse for unloading in lungs.

Why B is correct:

• Reaction 1 aligns with the definition of pulmonary gas exchange, where high PO2 drives O2 binding to hemoglobin per the equilibrium law.

Why the others are wrong:

• A includes 2, but CO2 hydration is tissue-specific, not in lungs.
• C includes 3, likely a systemic or irrelevant process like acid buffering.
• D selects only 2, which reverses in lungs for CO2 release, not forward hydration.

Final answer: B