CO2 is carried in blood in three different ways. Most of it (about 80%90%) is converted to bicarbonate ions HCO3− by the enzyme carbonic anhydrase in the red blood cells. 5%10% is dissolved in the plasma and 5%10% is bound to hemoglobin as carbamino compounds. The exact percentages vary depending whether it is arterial or venous blood.
The CO2 bound to hemoglobin does not bind to the same site as oxygen; rather it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO2 does decrease the amount of oxygen that is bound for a given partial pressure of oxygen.
Carbon dioxide may be one of the mediators of local autoregulation of blood supply. If it is high, the capillaries expand to allow a greater blood flow to that tissue.[citation needed]
Bicarbonate ions are crucial for regulating blood pH. As breathing rate influences the level of CO2 in blood, too slow or shallow breathing causes respiratory acidosis, while too rapid breathing, hyperventilation, leads to respiratory alkalosis.
