Erythrocytes circulating through the capillaries of the lung must exchange carbon dioxide (CO2) for oxygen (O2) during their short (0.5-1 sec.) transit time in pulmonary tissue (Reviewed in Jensen 2004, Esbaugh and Tufts 2006, Boron 2010). CO2 bound as carbamate to the N-terminus of hemoglobin and protons (H+) bound to histidine residues in hemoglobin are released as hemoglobin (HbA) binds O2. Bicarbonate (HCO3-) present in plasma is taken up by erythrocytes via the band3 anion exchanger (AE1, SLC4A1) and combined with H+ by carbonic anhydrases I and II (CA1/CA2) to yield water and CO2 (Reviewed by Esbaugh and Tufts 2006). CO2 is passively transported out of the erythrocyte by AQP1 and RhAG. HCO3- in plasma is also directly dehydrated by extracellular carbonic anhydrase IV (CA4) present on endothelial cells lining the capillaries in the lung.