Question

Select the INCORRECT statements with respect to transport of \( \text{CO}_2 \) by RBCs and plasma.

i. \( 23\% \) of \( \text{CO}_2 \) released from tissue cells diffuses into plasma first and then into RBCs. 

ii. Within RBCs, carbonic acid immediately dissociates into \( \text{H}^+ \) and \( \text{HCO}_3^- \). 

iii. In RBCs, \( \text{CO}_2 \) combines with \( \text{H}_2\text{O} \) to form carbonic acid. 

iv. \( \text{H}^+ \) ions move out of RBCs and combine with \( \text{Na}^+ \) to form \( \text{NaHCO}_3 \). 

v. \( 7\% \) of \( \text{CO}_2 \) is transported in dissolved form as carbonic acid in plasma. 

In the light of the above statements, choose the correct answer from the options given below.

• A. i, ii and iii only
• B. iii, iv and v only
• C. ii and v only
• D. i and iv only

Hint:

CO$_2$ transport: 70% $\rightarrow$ Bicarbonate 23% $\rightarrow$ Hb bound 7% $\rightarrow$ Dissolved

Answer 1

The Correct Option is D

Concept: Carbon dioxide is transported in blood in three forms:
• $7\%$ dissolved in plasma
• $23\%$ as carbaminohaemoglobin in RBCs
• $70\%$ as bicarbonate ions ($\text{HCO}_{3}^{-}$)

Step 1: Analyze statement i.
CO$_2$ diffuses directly into RBCs, not first into plasma and then RBCs. Hence, incorrect.

Step 2: Analyze statement ii.
Carbonic acid dissociates into $\text{H}^{+}$ and $\text{HCO}_{3}^{-}$. Correct.

Step 3: Analyze statement iii.
CO$_2$ combines with water in RBCs to form carbonic acid (via carbonic anhydrase). Correct.

Step 4: Analyze statement iv.
$\text{H}^{+}$ ions do not leave RBCs; bicarbonate ions move out (chloride shift). Hence, incorrect.

Step 5: Analyze statement v.
7% CO$_2$ is transported dissolved in plasma (correct).

Step 6: Conclusion.
Incorrect statements are i and iv. Final Answer: Option (D)