Breathing and Exchange of Gases Question 21
21. Approximately seventy percent of carbondioxide absorbed by the blood will be transported to the lungs:
[2014]
(a) as bicarbonate ions
(b) in the form of dissolved gas molecules
(c) by binding to R.B.C.
(d) as carbamino - haemoglobin
Show Answer
Answer : a
Hints & Solution
(a) $CO_2$ from the respiratory tissues to the lungs is transported by the blood in 3 ways: (i) In dissolved state or as a physical solution: Very small amount is physically dissolved in plasma ( $7 %$ i.e. $\cong \mathbf{0} .3 ml$ of $CO_2$ by each 100 $ml$ of blood). (ii) Bicarbonate ions: $\cong \mathbf{7 0 %}$ (i.e. $\cong \mathbf{2 . 5} ml$ per $100 ml$ of blood) $CO_2$ diffuses in plasma & then into RBCs where it (in the presence of carbonic anhydrase) combines with $H_2 O$ to form carbonic acid which is almost spontaneously dissociated into hydrogen ion and bicarbonate ions. (iii) Carbaminohaemoglobin : $\cong 23 %$ (i.e. $\cong 1 ml$ of $CO_2$ per $100 ml$ of blood) combines with haemoglobin forming an unstable compound.
