What are the major transport mechanisms for `CO_(2)`?- Explain.

Explanation

Transport of Carbon Dioxide : `CO_2` is transported in three ways.
i) In dissolved state : 7 per cent of `CO_2` is carried in a dissolved state (physical solution) through plasma.
`CO_2 + H_2O to H_2CO_3`
ii) As carbamino compounds : About 20-25 per cent of `CO_2` combines directly with free amino group of the haemoglobin and forms carbamino-haemoglobin in a reversible manner.
`Hb - NH_2 + CO_2 to Hb - NHCOO^(-) + H^(+)`
This binding of `CO_2` is related to the partial pressure of `CO_2`. `pO_2` is a major factor which could affect this binding. When `pCO_2` is high and `PCO_2` is low as in the tissues, binding of more carbon dioxide occurs. When `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. Carbamino compounds are also formed by the union of `CO_2` with plasma proteins.
iii) As Bicarbonates : About 70 per cent of `CO_2` is transported asbicarbonate. RBCs contain a very high concentration of the enzyme, carbonic anhydrase and a minute quantity of the same is present in the plasma too. This enzyme facilitates the following reaction in both the directions.
`CO_2 + H_2O overset("Carbonic anhydrase")(rarr) H_2CO_2 overset("Carbonic anhydrase")(rarr) HCO_3 + H^(+)`
At the tissues where partial pressure of `CO_2` is high due to catabolism, `CO_2` diffuses into the blood (RBC and Plasma) and forms carbonic acid, which dissociates into `HCO_3^(-)` 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 water. Thus `CO_2` is mostly trapped as bicarbonate at the tissues and transported to the alveoti where it is released out as `CO_2`.