Carbon dioxide is transported in blood through three major mechanisms. Nearly 70 per cent of CO₂ is transported as bicarbonate (HCO₃⁻), about 20-25 per cent is carried by haemoglobin as carbamino-haemoglobin, and approximately 7 per cent is carried in a dissolved state through plasma.

The major portion of CO₂ (70 per cent) is transported as bicarbonate. At the tissue site where pCO₂ is high due to catabolism, CO₂ diffuses into blood (RBCs and plasma) and forms HCO₃⁻ and H⁺ with the help of the enzyme carbonic anhydrase. At the alveolar site where pCO₂ is low, the reaction proceeds in the opposite direction, releasing CO₂.

About 20-25 per cent of CO₂ is carried by haemoglobin as carbamino-haemoglobin. This binding is related to the partial pressure of CO₂. When pCO₂ is high and pO₂ is low as in the tissues, more binding of carbon dioxide occurs. When pCO₂ is low and pO₂ is high as in the alveoli, dissociation takes place.

About 7 per cent of CO₂ is carried in a dissolved state through plasma.

Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO₂ to the alveoli. The factors are favourable for the diffusion of CO₂ from tissues to alveoli.