Question

Carbon dioxide is carried in the blood, mainly as

• A. Sodium bicarbonate
• B. Potassium bicarbonate
• C. Carbamino-haemoglobin
• D. Dissolved gas in plasma

Answer 1

The correct option is A Sodium bicarbonate

Nearly 20-25 per cent of CO${}_2$ is transported by RBCs whereas 70 per cent of it is carried as bicarbonate.About 7 per cent of CO${}_2$ is carried in a dissolved state through plasma.

RBCs contain a very high concentration of the enzyme, carbonic anhydrase and minute quantities of the same is present in the plasma too. This enzyme facilitates the following reaction in both directions.

At the tissue site where partial pressure of CO${}_2$ is high due to catabolism, CO${}_2$ diffuses into blood (RBCs and plasma) and forms HCO${}_3$ and H+. This HCO${}_3$ combines with Na${}^{+}$ to form sodium bicarbonate. 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 as CO${}_2$.

So, the correct option is 'Sodium bicarbonate'.