Carbon Dioxide Transport Mechanisms

Steps:

• CO2 diffuses from tissues into blood plasma and red blood cells (RBCs).
• Inside RBCs, CO2 combines with water via carbonic anhydrase to form carbonic acid (H2CO3), which dissociates into bicarbonate (HCO3-) and H+ ions.
• HCO3- exits the RBC in exchange for chloride ions (Cl-) via the chloride shift, allowing transport in plasma.
• At the lungs, the process reverses: HCO3- re-enters RBCs, reforms CO2, which diffuses out for exhalation.

Why C is correct:

• Diagram C illustrates the bicarbonate pathway and chloride shift, accounting for ~70% of CO2 transport per physiological definitions.

Why the others are wrong:

• A: Depicts only dissolved CO2 (~7%), omitting dominant bicarbonate and carbaminohemoglobin forms.
• B: Shows CO2 binding solely to plasma proteins, ignoring RBC-specific reactions.
• D: Reverses the sequence, placing lung events before tissue uptake.

Final answer: C