Question

What are the major transport mechanisms for CO₂? Explain.

Answer 1

Transport of Carbon dioxide CO₂ is carried by haemoglobin as carbaminohaemoglobin (about 20-25 per cent). This binding is related to the partial pressure of CO₂ , pO_{​​​​​​​2}is a major factor which could affect this binding.

When pCO_{​​​​​​​2} is high and pO_{​​​​​​​2} is low as in the tissues, more binding of carbon dioxide occurs whereas, when the pCO_{​​​​​​​2} is low and pO_{​​​​​​​2} is high as in the alveoli, dissociation of CO_{​​​​​​​2} from carbamino- haemoglobin takes place, i.e., CO_{​​​​​​​2} which is bound to Haemoglobin from the tissues is delivered at the alveoli.

RBCs contain a very high concentration of enzyme, carbonic anhydrase and minute quantities of the same is present in the plasma too. This enzyme facilitates the following reaction in both directions.

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At the tissue site where partial pressure of CO_{​​​​​​​2} is high due to catabolism, CO_{​​​​​​​2} diffuses into blood (RBCs and plasma) and forms HCO₃ and H⁺.  At the alveolar site where pCO_{​​​​​​​2} is low, the reaction proceeds in the opposite direction leading to the formation of CO_{​​​​​​​2} and H₂O.

Thus CO₂ trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO₂ . Every 100 ml of deoxygenated blood delivers approximately 4ml of CO₂ to the alveoli.