CO2 Transport — FRCA Primary MCQ
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Correct answer: B — Dissolved CO2, bicarbonate ions and carbamino compounds
Explanation lettering: E = shown as A · A = shown as B · B = shown as D · D = shown as E
Option A is correct. Carbon dioxide is transported in three principal forms: most as bicarbonate, a smaller proportion as carbamino compounds, and a small amount physically dissolved. In erythrocytes, carbonic anhydrase rapidly catalyses hydration of CO2 to carbonic acid, which dissociates into H+ and HCO3−. Much of the bicarbonate leaves the erythrocyte in exchange for chloride—the chloride or Hamburger shift. Carbamino compounds form when CO2 binds to terminal amino groups on proteins, predominantly haemoglobin; deoxygenated haemoglobin carries more CO2 because of the Haldane effect. Options B–D each omit one genuine mode of carriage, while option E incorrectly excludes both dissolved and protein-bound CO2.
Reference: Occhipinti R, et al. Role of Carbonic Anhydrases and Inhibitors in Acid-Base Physiology: Insights from Mathematical Modeling, section on respiratory CO2 carriage and Figure 9, 2019. https://pubmed.ncbi.nlm.nih.gov/31390837/