Delivering O₂ to tissues is only half the respiratory function of blood. Blood also carries CO₂, a metabolic waste product, away from tissues (Figure 48.14). CO₂ is highly soluble and readily diffuses through cell membranes, moving from its site of production in the tissues into the blood, where P_CO2 is lower. However, only about 5 percent of the CO₂ carried by the blood is dissolved CO₂. About 20 percent of the CO₂ carried to the lungs in the blood is in chemical combination with hemoglobin. And most CO₂ produced by the tissues is transported to the lungs in the form of bicarbonate ions (HCO₃^–). CO₂ diffusing from cells is converted to HCO₃^–, transported to the lungs, and then converted back to CO₂ to be exhaled.

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When CO₂ dissolves in water, some of it slowly reacts with the water molecules to form carbonic acid (H₂CO₃), some of which then dissociates into a proton (H⁺) and a bicarbonate ion (HCO₃^–). This reversible reaction is expressed as follows:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃^–

In the extracellular fluid, the reaction between CO₂ and H₂O proceeds slowly. But it is a different story in the endothelial cells of the capillaries and in the RBCs, where the enzyme carbonic anhydrase speeds up the conversion of CO₂ to H₂CO₃. The newly formed H₂CO₃ dissociates, and the resulting bicarbonate ions enter the plasma in exchange for Cl^– (see Figure 48.14). By converting CO₂ to H₂CO₃, carbonic anhydrase reduces the P_CO2 in these cells and in the plasma, facilitating the diffusion of CO₂ from tissue cells to endothelial cells, plasma, and RBCs.

In the lungs, the reactions involving CO₂ and bicarbonate ions are reversed. Remember that an enzyme such as carbonic anhydrase only speeds up a reversible reaction; it does not determine its direction. The direction is determined by concentrations of reactants and products. Ventilation keeps the P_CO2 in the alveoli low, so CO₂ diffuses from the blood plasma into the alveoli, lowering the P_CO2 in the blood, which favors the conversion of HCO₃^– into CO₂. Also, the oxygenation of hemoglobin in the alveoli decreases its affinity for CO₂, a phenomenon known as the Haldane effect.