CO2 Transport in Tissue Capillaries

Steps:

• CO2 produced by resting muscle diffuses into the capillary blood and enters the RBC.
• A portion of CO2 reacts with hemoglobin's amino groups to form carbaminohemoglobin.
• This binding increases carbaminohemoglobin levels in the RBC as it picks up CO2.
• Deoxygenation of hemoglobin facilitates this CO2 binding process.

Why D is correct:

• Carbaminohemoglobin forms via Hb-NH₂ + CO₂ → Hb-NH-COOH, transporting 20-30% of CO₂ from tissues to lungs.

Why the others are wrong:

• A: Chloride shift during HCO₃⁻ efflux causes Cl⁻ influx into the RBC, leading to net increase.
• B: Haemoglobinic acid (HHb) increases to buffer H⁺ from CO₂ hydration.
• C: Carbonic anhydrase is a fixed enzyme in RBCs; its amount does not change.

Final answer: D