Carbon dioxide transport in blood. Carbon dioxide transport in blood. CO2 produced during cell metabolism reaches the blood by simple diffusion driven by a partial pressure gradient (higher in tissue, lower in blood). To allow CO2 to be cleared from tissues, this gradient must remain high. A series of reactions keeps CO2 in solution low. Once in plasma, CO2 diffuses into red cells, where carbon anhydrase catalyzes the reaction with water to produce carbonic acid (H2CO3), which subsequently dissociates into hydrogen (H+) and bicarbonate (HCO3−). Once again, the accumulation of either H+ or HCO3− would stop those reactions. However, protons are buffered by hemoglobin, and bicarbonate is exchanged for extracellular chloride (Cl−) by AE1 (Band 3). For more details, see text. Gaston Murias et al. Respir Care 2014;59:1795-1807 (c) 2012 by Daedalus Enterprises, Inc.