Definition: The Haldane effect refers to the phenomenon in which the binding of oxygen with hemoglobin tends to displace carbon dioxide from the blood. It is the reverse of the Bohr effect (in which a rise in CO2 displaces O2 from hemoglobin) and is quantitatively far more important in promoting CO2 transport than the Bohr effect is in promoting O2 transport.

Mechanism: When O2 combines with hemoglobin in the pulmonary capillaries, hemoglobin becomes a stronger acid. This increased acidity displaces CO2 from the blood into the alveoli through two pathways:

In the tissues, the opposite occurs: deoxygenation of hemoglobin (as O2 is unloaded) increases its affinity for CO2 and H+, facilitating CO2 uptake from the tissues into the blood.

Significance: The Haldane effect causes the CO2 dissociation curve to shift depending on the PO2, so that at a given PCO2, deoxygenated blood carries more CO2 than oxygenated blood. As demonstrated using the CO2 dissociation curves at PO2 of 40 mm Hg (tissue capillaries) versus 100 mm Hg (pulmonary capillaries), if the curve did not shift due to the Haldane effect, CO2 content would fall by only about 2 volume percent as blood passes through the lungs; because of the Haldane effect, it actually falls by about 4 volume percent. Thus the Haldane effect approximately doubles the amount of CO2 released from blood in the lungs and doubles the amount of CO2 picked up by blood from the tissues, making it a major contributor to efficient CO2 transport and exchange.