It is a process which occurs in a cardiovascular system and
refers to the exchange of bicarbonate (HCO3−) and chloride (Cl−)
across the membrane of red blood cells (RBCs).
A rise in intracellular bicarbonate causes chloride intake and
bicarbonate export. The term "chloride shift" refers to this
exchange.
As a result, blood chloride concentration is lower in systemic
venous blood than in systemic arterial blood or in pulmonary
circulation because the levels of CO2 and therefore bicarbonate are
higher in systemic venous blood, providing less of a driving force
for exchange.