**2. Normal acid-base balance**

The body maintains its normal physiology by the strict balance of acid and base. The body maintains its normal arterial pH close to 7.4 at a range between 7.36–7.44, and the intracellular pH of the human body is 7.2 [11]. Normal acid-base balance is the balance between each hydrogen increase by the intake or production, and that is decreased by elimination. Acid-base balance is measured by measuring pH, CO2 and HCO3. In general, consuming animal protein add acid in the body, and consuming cereals and vegetables add alkali in the body. In oxidative metabolism, CO2 is produced in the tissue, and at a similar rate, that is eliminated by the lungs. So, pCO2 persists at about 5.33 kPa (40 mm of Hg). Different buffer systems of the body play a crucial role in removing excess H<sup>+</sup> . Metabolism of carbohydrate and fat uses O2 and produce CO2 and H2O. Normal lungs efficiently remove most of the CO2. In oxidation of amino acids, carbon dioxide and water are produced along with the liberation of nitrogen as ammonia, a toxic material in the body. In the liver, the urea cycle utilizes the ammonia, where this toxic NH3 combines with CO2, and produce urea. In the proximal tubule and other renal epithelial cells, ammonia and bicarbonate are also produced from glutamine metabolism. Some of it returns to the body fluid through the renal veins and is metabolized in the liver. And the rest of the NH3 excreted in the lumen. So, NH3 does not exist in the body fluid. Most of the NH3 is excreted in the urine, and it plays an important role in removing H<sup>+</sup> to maintain normal acid-base balance. In the urine, NH3 binds hydrogen ion to produce NH4, and it prevents excessive acidification of urine.
