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

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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: $\cong$ 70% (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.



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