Physiology & Biochemistry · single question
In what form is the majority of carbon dioxide transported in blood back to the lungs?
Show answer and explanation
Correct answer: B. As bicarbonate ion (HCO3-)
- A. Only about 5–10% of CO2 travels dissolved directly in plasma.
- B. Correct. Roughly 70% of CO2 is carried as BICARBONATE. Inside the red cell, carbonic anhydrase converts CO2 + H2O to carbonic acid, which dissociates into H+ and HCO3-. Bicarbonate then exits into the plasma via the chloride shift. About 20–25% travels as carbaminohaemoglobin bound to the globin chains, and the remainder is dissolved.
- C. Carbonic anhydrase is the ENZYME that catalyses the reaction — it is not a carrier molecule.
- D. Carboxyhaemoglobin is haemoglobin bound to carbon MONOXIDE (CO), a completely different and toxic species.
Practice Physiology & Biochemistry questions Physiology & Biochemistry study notes