What is critical temperature? Give its value for `CO_2`.

Explanation

Carbondioxide is transported in three ways.
1. In dissolved state : 7% of `CO_2` is transported in dissolved state through plasma.
`CO_(2) + H_(2) to H_(2) CO_(3)`.
2. As Carbamino compounds: About 20-25% of `CO_2` combine directly with free amino group of haemoglobin and forms Carbamino haemoglobin in a reversible manner.
`Hb- NH_(2) + CO_(2) to Hb - NHCOO^(-) + H^(+)`
`pCO_2` and `pO_2` could affect the binding of `Co_2` to haemoglobin.
— when `pCO_2` is high and `pO_2` is low as in the tissues, binding of more `CO_2` occurs.
— when `pCO_2` is low and `pO_2` is high as in the alveoli, dissociation of `CO_2` carbamino - haemoglobin takes place, (i.e., `CO_2` which is bound to haemoglobin from the tissues is delivered at the alveoli)
3. As Bicarbonates : About 70% of `CO_2` is transported as bicarbonate. RBCs contain a very high concentration of the enzyme, carbonic anhydrase and a minute quantity of the same is present in plasma too. This enzyme facilitates the following reaction in both the directions.
`CO_(2) + H_(2)O overset("Carbonic anhydrase") to H_(2)CO_(2) overset("Carbonic anhydrase") to HCO_(3)^(-) + H^(+)`.
At the tissues where partial pressure of C02 is high due to catabolism, `CO_2` diffuses into the blood and forms carbonic acid which dissociates into `HCO_3^(-) + 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 alveoli where it is dissociated out as `CO_2`.
Every 100 ml of deoxygenated blood delivers approximately 4 ml of `CO_2` to the alveolar air.