8.2.2—Chloride shift and its importance

The chloride shift is the exchange of hydrogencarbonate ions and chloride ions across the red-cell membrane during carbon-dioxide transport. It allows hydrogencarbonate to leave the red cell without leaving an electrical imbalance.

1. CO2 enters the red cell: in respiring tissues, carbon dioxide diffuses from cells into the blood and then into red blood cells.
2. Hydrogencarbonate forms: carbonic anhydrase catalyses CO2 + water → carbonic acid; carbonic acid dissociates into hydrogencarbonate ions (HCO3−) and hydrogen ions (H+).
3. HCO3− leaves: negatively charged hydrogencarbonate ions move out of the red cell through a membrane transport protein into the plasma, where they can be carried in solution.
4. Cl− enters: chloride ions move into the red cell through the same exchange system → the negative charge leaving as HCO3− is balanced by negative charge entering as Cl− → electrical neutrality is maintained and conversion can continue.
5. Lung-side reversal: at the lungs, the exchange runs in the opposite direction: HCO3− returns to the red cell and Cl− leaves. The carbon-dioxide chemistry can then be reversed so CO2 is regenerated for removal in exhaled air.

Boundary: this is an ion-exchange mechanism that supports CO2 transport; it is not the Bohr shift, which describes the effect of CO2/H+ conditions on haemoglobin oxygen affinity.

Chloride does not replace carbon dioxide as the transported gas. HCO3− is the carbon-dioxide-derived form carried mainly in plasma; Cl− enters to balance charge. No image generated or bound.