How is the bulk of carbon dioxide carried in blood? A. as carbonic acid in the plasma B. as the bicarbonate ion in the plasma after first entering the red blood cells C. chemically combined with the amino acids of hemoglobin as carbaminohemoglobin in the red blood cells D. chemically combined with the heme portion of hemoglobin

B. as the bicarbonate ion in the plasma after first entering the red blood cells

Explanation:

Carbon dioxide is exchanged from the deoxygenated tissues into the red blood cells. Inside the red blood cells, an enzyme known as carbonic anhydrase which catalyses reaction of CO₂ with H₂O and form H₂CO₃ (carbonic acid) which further dissociates into bicarbonate ion, HCO₃⁻ and H⁺.

CO₂ + H₂O ⇄ HCO₃⁻ + H⁺

This bicarbonate ion thus formed is exchanged to the plasma in the exchange of the chloride ions, Cl⁻. Thus, Carbon dioxide is transported in the blood in this form.

Hence, Option B is correct.