FIGURE 11-28 Carbon dioxide transport in blood requires a Cl⁻/HCO₃⁻ antiporter. (a) In systemic capillaries, carbon dioxide gas diffuses across the erythrocyte plasma membrane and is converted into soluble HCO₃⁻ by the enzyme carbonic anhydrase; at the same time, oxygen leaves the cell and hemoglobin binds a proton. The anion antiporter AE1 (purple) catalyzes the reversible exchange of Cl⁻ and HCO₃⁻ ions across the membrane. The overall reaction causes HCO₃⁻ to be released from the cell, which is essential for maximal CO₂ transport from the tissues to the lungs and for maintaining pH neutrality in the erythrocyte. (b) In the lungs, where carbon dioxide is excreted, the overall reaction is reversed. See text for additional discussion.