Question
Explain the transport of carbon dioxide.
✓
Answer
1. Dissolved in plasma: About $7-10 \%$ of $CO _2$ is transported in a dissolved form in the plasma.
2. Bound to haemoglobin: About $20-25 \%$ of dissolved $CO _2$ is bound and carried in the RBCs as carbaminohaemoglobin $\left( Hb CO _2\right)$
$
CO _2+ Hb \rightarrow Hb CO { }_2
$
3. As bicarbonate ions in plasma about $70 \%$ of $CO _2$ is transported as bicarbonate ions. This is influenced by $pCO _2$ and the degree of haemoglobin oxygenation. $RBCs$ contain a high concentration of the enzyme, carbonic anhydrase, whereas small amounts of carbonic anhydrase is present in the plasma. At the tissues, the $pCO _2$ is high due to catabolism and diffuses into the blood to form $HCO _2$ and $H ^{+}$ions. When $CO _2$ diffuses into the RBCs, it combines with water forming carbonic acid $\left( H _2 CO _2\right)$ catalyzed by carbonic anhydrase. Carbonic acid is unstable and dissociates into hydrogen and bicarbonate ions. Carbonic anhydrase facilitates the reaction in both directions.
The $HCO _3{ }^{-}$moves quickly from the $RBCs$ into the plasma, where it is carried to the lungs. At the alveolar site where $pCO _2$ is low, the reaction is reversed leading to the formation of $CO _2$ and water. Thus $CO _2$ trapped as $HCO _3{ }^{-}$at the tissue level is transported to the alveoli and released out as $CO _2$. Every $100 mL$ of deoxygenated blood delivers $4 mL$ of $CO _2$ to the alveoli for elimination.
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