Deoxygenation of the blood increases its ability to carry carbon dioxide; this property is the Haldane effect. Conversely, oxygenated blood has a reduced capacity for carbon dioxide. This is a consequence of the fact that reduced (deoxygenated) hemoglobin is a better proton acceptor than the oxygenated form.
In red blood cells, the enzyme carbonic anhydrase catalyzes the conversion of dissolved carbon dioxide to carbonic acid, which rapidly dissociates to bicarbonate and a free proton:
CO2 + H2O → H2CO3 → H+ + HCO3-
By Le Chatelier's principle, anything that stabilizes the proton produced will cause the reaction to shift to the right, thus the enhanced affinity of deoxyhemoglobin for protons enhances synthesis of bicarbonate and accordingly increases capacity of deoxygenated blood for carbon dioxide. The majority of carbon dioxide in the blood is in the form of bicarbonate. Only a very small amount is actually dissolved as carbon dioxide, and the remaining amount of carbon dioxide is bound to hemoglobin.
In addition to enhancing removal of carbon dioxide from oxygen-consuming tissues, the Haldane effect promotes dissociation of carbon dioxide from hemoglobin in the presence of oxygen. In the oxygen-rich capillaries of the lung, this property causes the displacement of carbon dioxide to plasma as venous blood enters the alveolus and is vital for alveolar gas exchange.
The general equation for the Haldane Effect is: H+ + HbO2 ←→ H+.Hb + O2
This partially explains the observation that some patients with emphysema might have an increase in PaCO2 (partial pressure of arterial dissolved carbon dioxide) following administration of supplemental oxygen even if content of CO2 stays equal.
Alkalosis causes a left shift in the oxygen dissociation curve, which enhances the ability of hemoglobin to pick up oxygen in the lungs but makes it less available at the tissue level. ["Mechanical Ventilation, physiological and clinical applications", Author Susan P Pilbeam, 1998; Mosby Inc.]