Question

What's the relationship between CO2, bicarbonate (HC03- ), and carbonic acid (H2CO3)? In what form is most CO2 carried in our blood? What is the role of carbonic anhydrase in gas exchange?

Answer 1

The relationship between

Carbon dioxide (CO₂) dissolves in with water to form carbonic acid (H₂CO₃) this reaction is speedup by the enzyme carbonic anhydrase, then carbonic acid dissociates in to bicarbonate (HC03^-).

Overall reaction is represented as CO₂ + H₂O ^{carbonic anhydrase}  -à H₂CO₃ + HCO₃^- + H⁺ (this is a reversible reaction and carbonic anhydrase can reverse this reaction – this happens in once RBC reaches lungs)

carbonic acid (H₂CO₃) and bicarbonate (HC03^-) exist in 1:20 in the blood

CO₂ / H₂CO₃ = 1000 / 1

H₂CO₃ / HCO₃^- = 1/20

Major form of dissolved CO2 carried in our blood is in the form of bicarbonate (HC03^-), 70-75% (rest 25% dissolved in serum or combined with serum protiens and heamoglobin) of the CO₂ in our blood is converted in to carbonic acid (H₂CO₃), which dissociate in to bicarbonate (HC03^-) as we know carbonic acid (H₂CO₃) to bicarbonate (HC03^-) ratio is 1:20, that means nearly 71.25% of carbonic acid (H₂CO₃) gets dissociated in to bicarbonate (HC03^-)   

As described above carbonic anhydrase can help H₂CO₃ + HCO₃^- formation and reverse reaction of forming CO₂ + H₂O , In tissues due to high CO₂ concentration the reaction is forward (lowers the activation energy of combining C O₂ + OH^- to give HC03^-), whereas in lungs because of low CO₂ concentration and high O₂ concentration the backward reaction is favored to release CO₂ (lowers the activation energy of hydrolyzing HCO₃^- to CO₂ + OH^- and release CO₂ + H₂O)