structural:expansion
Carbon dioxide transport

CO is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood). Most of it (about 70%) is converted to bicarbonate ions HCO − 3 by the enzyme carbonic anhydrase in the red blood cells by the reaction CO + H O → H CO → H + HCO − 3; about 7% is dissolved in the plasma; and about 23% is bound to hemoglobin as carbamino compounds.

Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the CO bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO decreases the amount of oxygen that is bound for a given partial pressure of oxygen. The decreased binding to carbon dioxide in the blood due to increased oxygen levels is known as the Haldane effect, and is important in the transport of carbon dioxide from the tissues to the lungs. A rise in the partial pressure of CO or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr effect.