Discuss the process of the chloride shift in both the respiratory and the digestive systems.

Carbon dioxide is generated in tissues as a byproduct of normal metabolism. It dissolves in blood plasma to form carbonic acid (H2CO3); red blood cell (RBC) carbonic anhydrase catalyzes this reaction. Carbonic acid then spontaneously dissociates to form bicarbonate (HCO3-) and a hydrogen ion (H+). In response to the decrease in intracellular pCO2, more CO2 passively diffuses into the cell.

Cell membranes are generally impermeable to charged ions (i. e. H+, HCO3-) but RBCs are able to exchange bicarbonate for chloride using the anion exchanger protein Band 3. Thus, the rise in intracellular bicarbonate leads to bicarbonate export and chloride intake. The term "chloride shift" refers to this exchange. Consequently, chloride concentration is lower in systemic venous blood than in systemic arterial blood: high venous pCO2 leads to bicarbonate production in RBCs, which then leaves the RBC in exchange for chloride coming in. [2]