The bicarbonate buffer system is one of the most effective buffer systems in the body because the amount of dissolved CO2 is controlled by respiration (ie, it is an "open" system). Additional control of the plasma concentration of HCO3- is provided by the kidneys. Additional factors that make the carbonic-acid-bicarbonate system such a good biological buffer: The reaction CO2 + H2O ? H2CO3 proceeds slowly in either direction unless the enzyme carbonic anhydrase is present. There is no carbonic anhydrase in plasma, but there is an abundant supply in red blood cells, spatially confining and controlling the reaction. The presence of hemoglobin in the blood increases the buffering of the system by binding free H+ produced by the hydration of CO2 and allowing for movement of the HCO3- into the plasma. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 35. Gas Transpo & pH. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.