Carbon dioxide or CO2 is released as a by-product of aerobic respiration and is carried in blood in three different ways. Most of it is converted to bicarbonate ions HCO3- by the enzyme carbonic annhydrase, present in red blood cells (RBCs), by the reaction: CO2 + H2O → H2CO3 → H+ + HCO3-. Another 5% – 10% is dissolved in the plasma, the fluid portion of the blood, and 5% – 10% is bound to hemoglobin as carbamino compounds. Carbamino refers to a compound generated by the addition of carbon dioxide to a free amino group in an amino acid or a protein, such as hemoglobin, forming carbaminohemoglobin. The veins, which carry deoxygenated blood back to the right atrium of the heart appear bluish due to the distinctive blue color of carbaminohemoglobin.
Haemoglobin , the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the CO2 bound to hemoglobin does not bind at the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. Because of allosteric effects on the haemoglobin molecule, the binding of CO2 decreases the amount of oxygen that is bound at a given partial pressure of oxygen. The decreased binding of 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. Carbon dioxide 'unloading' from the red blood cells will be increased at the lungs where levels of oxygen are relatively high. The likelihood of CO2 associating with the globin protein in Hemoglobin will accordingly be increased when oxygen levels are low. Other physiological imbalances such as a rise in the partial pressure of CO2 or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr Effect .