Most carbon dioxide molecules entering the plasma quickly enter RBCs. The reactions that convert carbon dioxide to bicarbonate ions for transport mostly occur inside RBCs. CO2+H2O⇌H2CO3 (carbonicacid)⇌H+ +HCO3−(bicarbonate ion). Although this reaction also occurs in plasma, it is thousands of times faster in RBCs because they contain carbonic anhydrase, an enzyme that reversibly catalyzes the conversion of carbon dioxide and water to carbonic acid. Hydrogen ions released during the reaction (as well as CO2 itself) bind to Hb, triggering the Bohr effect. In this way CO2 loading enhances O2 release. Once generated, HCO3− moves quickly from the RBCs into the plasma, where it is carried to the lungs. To counterbalance the rapid outrush of these anions from the RBCs, chloride ions move from the plasma into the RBCs. This ion exchange process, called the chloride shift, occurs via facilitated diffusion through an RBC membrane protein.