In this reaction scheme, the double-headed arrows indicate the equilibrium relation between reactants and products. The equilibrium between H2CO3 and CO2 + H2O is catalyzed by carbonic anhydrase. CO2 levels in the plasma can be regulated rapidly by respiration, but the finite body stores of HCO3— limit the buffering capacity. When the protons from the fixed acids ingested and formed by metabolism are buffered by HCO—, the HCO— concentration falls as it reacts with the protons to form H2CO3, as shown by the change in the equilibrium represented by a directional arrow (! or g) and the concentration changes by up or down arrows (" or g):
As the hydrogen ions are buffered, carbonic acid and CO2 concentrations rise, but this is countered by an increase in respiration to maintain a normal CO2 tension (PCO2) in the plasma. As the reaction proceeds constantly to the right, the amount of HCO— in the extracellular fluid is reduced and would eventually be exhausted. To maintain homeostasis, the HCO— consumed in buffering reactions must be restored. This is accomplished by the kidneys, which excrete H+ in the urine while returning HCO— to the plasma. Therefore, the kidneys essentially reverse the reaction in Eq.  and return the plasma HCO— concentration to normal.
Not all individuals are faced with a net dietary acid load. On a vegetarian diet, there is an excess intake of the salts of weak acids. The anions such as oxalate, citrate, butyrate, lactate, and others represent a base load (proton acceptors) that is buffered by all body buffers, including the HCO—/CO2 buffer system. The HCO—/CO2 buffer system is also effective in preventing alkaline shifts in plasma pH because of ingestion of strong bases or the loss of hydrogen ions. For example, an excess intake of hydroxyl ions, as occurs in poisoning with lye, is buffered by CO2:
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