The key factor in the transpo of carbon dioxide as bicarbonate is
Since the Pco2 in the tissue is high,CO2 diffuses from tissues into the plasma. From there it enters the RBC which contains plenty of carbonic anhydrase. In blood,70% of CO2 is transpoed as HCO3 by the action of carbonic anhydrase. Reference:Textbook of medical physiology-Geetha N,2nd edition,page no:245.