Concept of the Week: Everybody Do the Chloride Shift!

The majority of carbon dioxide is transported in the blood as bicarbonate ion (60-70%). The chemical reaction that forms the bicarbonate takes place within the erythrocytes (red blood cells) and is catalyzed by the enzyme carbonic anhydrase. Carbon dioxide reacts with water in the cytosol to form carbonic acid, which then dissociates into a negatively charged bicarbonate ion and a positively charged hydrogen ion. The bicarbonate ions quickly diffuse out into the blood plasma. This, however, would result in a net loss of negative charge within the RBC if the process were not counterbalanced. The chloride shift balances the loss of bicarbonate as negatively charged chloride ions diffuse into the RBC from the plasma. Thus, the loss of negatively charged bicarbonate is offset by an equal acquisition of negatively charged chloride, allowing the RBC to maintain its normal membrane potential. What happens to the hydrogen ion? It binds to hemoglobin, which acts as a buffer to prevent intracellular pH from dropping too low. It drops just low enough to trigger the Bohr effect, which reduces the affinity of hemoglobin for oxygen and facilitates oxygen release to the tissues.