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chloride shift The movement of chloride ions (Cl–) into red blood cells. Carbon dioxide reacts with water to form carbonic acid in the red blood cells (see carbonic anhydrase). The carbonic acid then dissociates into hydrogencarbonate ions (HCO3–) and hydrogen ions (H+). The plasma membrane is relatively permeable to negative ions. Therefore the hydrogencarbonate ions diffuse out of the cell into the plasma, leaving the hydrogen ions, which create a net positive charge; this is neutralized by the diffusion of chloride ions from the plasma into the cell.