In what form is the majority of carbon dioxide transported from the tissues to the lungs?

Correct answer: B. As bicarbonate ion dissolved in plasma

• A. Carbonic anhydrase is the ENZYME that catalyses the conversion — it is not a carrier molecule and does not transport CO₂ anywhere.
• B. CORRECT. Roughly 70% of carbon dioxide travels as bicarbonate. CO₂ diffuses into the red cell, where carbonic anhydrase catalyses its hydration to carbonic acid, which dissociates into bicarbonate and a proton. The bicarbonate leaves the cell in exchange for chloride — the 'chloride shift' — while the proton is buffered by haemoglobin. At the lung the whole sequence reverses and CO₂ is exhaled.
• C. Only about 7–10% of CO₂ travels physically dissolved in plasma. This small fraction nevertheless determines the measured PaCO₂.
• D. Carbaminohaemoglobin, formed when CO₂ binds the globin amino groups (not the haem iron, which carries oxygen), accounts for only about 20–23% of transport. It is real but not the majority.

Note: The three fractions in order: bicarbonate ~70%, carbamino ~23%, dissolved ~7%. This system is also the body's principal buffer, which is why hyperventilation blows off CO₂ and produces respiratory alkalosis — the mechanism behind the perioral tingling and carpopedal spasm you may see in an anxious dental patient.