Question

Bulk of carbon dioxide ($CO_2$) released from body tissues into blood is present as ,
A. $70\%$carbamino haemoglobin and $30\%$ as bicarbonate
B. Carbamino haemoglobin in RBCs
C. Bicarbonate in blood plasma and RBCs
D. Free $CO_2$ in blood plasma

Answer 1

$CO_2$ is transported in the blood in three ways; as a hydrogen carbonate ($HCO3^{-}$), as carbamino compounds and as dissolved $CO_2$. The maximum amount of carbon dioxide is transported as bicarbonate in blood plasma and red blood cells.

Complete answer:
As we know from the hint given above that carbon dioxide can be transported by three ways in the body; as a hydrogen carbonate, as carbamino compounds and as dissolved $CO_2$.
About $60\% - 70\%$ of all $CO_2$ is transported through production of $HCO3^{-}$ ions in the red blood cell. When the red blood cells reach the lungs, oxygen binds to the haemoglobin and allows the release of $H^{+}$ ions. These hydrogen ions then become free to react with bicarbonate ions to produce $CO_2$ and $H_{2}O$, where the $CO_2$ is exhaled. Thus the high $O_2$ concentrations reduces the $CO_2$ carrying capacity of blood by decreasing it’s binding affinity with the haemoglobin.
About $30\%$ of all $CO_2$ is transported in the form of carbamino compounds. At high concentrations carbon dioxide directly binds to amino acids and also with the amine groups of haemoglobin to create carbamino haemoglobin.
The remaining amount of all $CO_2$ is transported by dissolving in plasma. The amount of carbon dioxide dissolved in the blood depends on its solubility and the partial pressure of the carbon dioxide.

Hence, the correct answer is option (C).

Note: Approximately $75\%$ of carbon dioxide is transported through blood and $25\%$ through the plasma. Carbon dioxide is very soluble in water but despite that only a small amount of carbon dioxide is transported in dissolved form in the blood.