Figure 1: Proton production and its transporting kinetics in intracellular and extracellular fluid in metabolic tissues. The production of organic acids such as lactic acid and ketone bodies is accelerated by elevating glycolytic anaerobic metabolism and β-oxidation in metabolic cells. Body fluid pH is strictly maintained by buffering systems, efflux across plasma membrane, and acid excretion. Monocarboxylate transporter (MCT) and Na+/H+ exchanger (NHE) contribute to proton extrusion from the cytosol to the extracellular space. In contrast to intracellular fluid and blood containing pH buffers such as Hb (hemoglobin) and albumin, the interstitial fluid pH could be easily reduced by acid stress owing to the limited availability of the buffering factors such as proteins.