This review has attempted to clarify the sources of metabolic acid in relation to renal acid excretion and metabolic handling of acid with effective buffering systems. Three potential acidifying factors are; 1) the generation of sulfuric acid through the metabolism of sulfur-containing amino acides, 2)organic acids produced by leakage from the TCA cycle and 3)the incomplete matabolic degradation of organic substances. To make physiological serum a constant pH=7.4, prerequisite conditions in which generated hydrogen ions are excreted and consumed bicarbonate ions that are regenerated are met by buffering systems, urea synthesis in the liver and hepato-renal interorgan metabolosm for glutamin.