How is the majority of carbon dioxide transported from the tissues to the lungs?

Correct answer: D. As bicarbonate ion dissolved in plasma

• A. ❌ Carbaminohemoglobin accounts for roughly 20–23% — significant, but not the majority.
• B. ❌ Only about 7–10% travels as physically dissolved CO₂, though this fraction determines the measured PaCO₂.
• C. ❌ Albumin is a carrier for drugs, bilirubin and calcium — not a meaningful CO₂ transporter.
• D. ✅ CORRECT — About 70% of CO₂ travels as BICARBONATE. Carbonic anhydrase within the red cell converts CO₂ and water to carbonic acid, which dissociates into H⁺ and HCO₃⁻; the bicarbonate leaves the cell in exchange for chloride (the chloride shift) and is carried in plasma.

Note: Carbonic anhydrase is the ENZYME that enables the conversion; bicarbonate is the FORM in which CO₂ is carried. The question asks for the form.