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Approximately 70% of CO₂ is transported as bicarbonate (HCO₃⁻) in plasma. Inside red blood cells, carbonic anhydrase catalyzes ( \text{CO₂} + \text{H₂O} \leftrightarrow \text{H₂CO₃} \leftrightarrow \text{H⁺} + \text{HCO₃⁻} ). Bicarbonate then exits into plasma in exchange for chloride (the chloride shift). About 20–23% binds to hemoglobin’s globin chains as carbaminohemoglobin (B), and roughly 7–10% remains dissolved (A). Little circulates as intact carbonic acid (D) because it rapidly dissociates at physiological pH. This chemistry underlies CO₂ transport and acid–base balance: tissues producing CO₂ drive the reaction rightward, increasing H⁺ and prompting oxygen unloading (Bohr effect); in the lungs, CO₂ is exhaled, shifting leftward and facilitating oxygen loading. TEAS often asks you to choose the predominant form—remember: bicarbonate in plasma is the main carrier, aided by rapid erythrocyte enzymatic conversion and ionic exchange mechanisms.