Describe: Transport of $CO_{2}$.

(N/A) $CO_{2}$ is carried by haemoglobin as carbamino-haemoglobin (about $20-25$ percent).
This binding 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 $pO_{2}$ 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.
$RBCs$ contain a very high concentration of the enzyme, carbonic anhydrase, and minute quantities of the same are present in the plasma too.
This enzyme facilitates the following reaction in both directions:
$CO_{2} + H_{2}O \rightleftharpoons[\text{Carbonic anhydrase}]{\text{Carbonic anhydrase}} H_{2}CO_{3} \rightleftharpoons[\text{Carbonic anhydrase}]{\text{Carbonic anhydrase}} HCO_{3}^{-} + H^{+}$
At the tissue site where partial pressure of $CO_{2}$ is high due to catabolism, $CO_{2}$ diffuses into blood and forms $HCO_{3}^{-}$ 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 $H_{2}O$.
Thus, $CO_{2}$ trapped as bicarbonate at the tissue level and transported to the alveoli is released out.