Which of the following methods carries the largest amount of CO2 to the lungs?

Question

A. binding to hemoglobin

B. diffusing into plasma

C. formation of carbonic acid

D. formation of carbonic anhydrase

E. binding to white blood cells

Answer 1

Most CO2 carried in the blood combines with water in a reaction catalyzed by the enzyme carbonic anhydrase:

CO2 + H20 ---> H2CO3 (carbonic acid)

however, carbonic acid is not stable and usually breaks down into bicarbonate (HCO3-) and hydrogen ions (H+)

H2CO3 ----> HCO3- + H+

Hydrogen ions bind to the hemoglobin molecule, facilitating the release of O2 in the tissues.  

When blood returns to the lungs, the above reaction goes in reverse ... O2 binds to hemoglobin causing H+ to dissociate and recombine with bicarbonate ... etc

So the correct answer is bicarbonate (HCO3-), which is not a choice :(

The closest answer is C

Check out the diagram halfway down on the link below: