The metabolic degradation of protein and of phosphate containing compounds produces strong acids which can not be converted to the gas state and cannot be excreted by the respiratory system. They are therefore referred to as nonvolatile or Fixed Acids and amount to about 70 to 80 mEq/day. Sulfuric acid (H2SO4) and phosphoric acid (H3PO4) are primary examples of nonvolatile acids. These acids dissociate into anions and H+. In the ECF most of these acids are buffered by reaction of the H+ with HCO3- forming H2CO3 which dissociates into CO2 and H2O. The CO2 can be excreted by the lung and the H2O by the kidney leaving the sodium salt of the acid in the ECF. This reaction minimizes the change in pH and in the process decreases the amount of HCO3- available for further buffering of nonvolatile acids. The kidney is responsible for elimination of the acid anions produced in this way while conserving the Na+ associated with them and regenerating the HCO3- lost in the buffering reaction.