The transport mechanism of CO₂ takes place mainly by the haemoglobin. The carbon dioxide dissolved in the blood combines with the haemoglobin to form carbamino-haemoglobin (about 20-25 per cent), which is carried to the alveoli from the tissue. This binding is related to the partial pressure of CO₂. Partial pressure of O₂ is a major factor, which could affect this binding. When pCO₂ is high and pO₂ is low as in the tissues, more binding of carbon dioxide occurs whereas, when the pCO₂ is low and pO₂ is high as in the alveoli, dissociation of CO₂ from carbamino-haemoglobin takes place. During the dissociation of carbamino-haemoglobin the CO₂ bound to haemoglobin from the tissues is delivered at the alveoli. RBCs contain a very high concentration of the enzyme, carbonic anhydrase and minute quantities of the same is present in the plasma too. This enzyme facilitates the following reaction in both directions. CO₂ + H₂O → H₂CO₃ HCO^-₃ + H⁺ In the above reaction CO₂ combines with H₂O in the presence of carbonic anhydrase to form H₂CO₃ this is futher split into to HCO₃ ^- +H⁺ . At the alveolar site where pCO₂ is low, the reaction proceeds in the opposite direction leading to the formation of CO₂ and H₂O. Thus, CO₂ trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO₂. By these methods every 100 ml of blood transports about 4ml of carbon dioxide to the alveoli to be exhaled.