Carbon dioxide is the end product of cellular oxidative metabolism. Being a small neutral molecule it diffuses readily across all cell membranes and forms carbonic acid by the hydration reaction as shown in the figure. This reaction is accelerated by the enzyme carbonic anhydrase (CA) which is present in high concentration in many cells including RBC and renal epithelial cells. It is named volatile because it is a gas and as such is readily excreted by the lung.
In the peripheral tissue capillaries about 10% of the CO2 remains in the plasma and 90 % of the CO2 enters the RBC where, facilitated by CA, most of it is converted to H2CO3 which dissociates spontaneously into H+ and HCO3-. The HCO3- enters the plasma in exchange for a Cl- and the H+ is buffered by hemoglobin. This is illustrated in figure 1.
Physiological Definition of Acids:
In the lung the process is reversed. HCO3- enters the RBC in exchange for Cl- and is converted to CO2 which leaves the RBC and is excreted in the expired gas. This is illustrated in figure 2.
The transmembrane movement of CO2 and the exchange of Cl- and HCO3- are very rapid and are completed in the time required for the RBC to traverse the peripheral tissue or the lung capillary. This process allows the lungs to excrete up to 20 mols of CO2 produced per day and thereby efficiently prevents the acidosis that would develop from the retention of the CO2. It is also the source of HCO3- in the CO2 - HCO3- buffer system. Because H2CO3 can be converted to the gaseous state, i.e. CO2, and excreted as such by the lungs it is referred to as a volatile acid.