 Dear students, in this topic we shall discuss the regulation of pH by the kidneys. The pH of blood is maintained within a narrow alkyne range of 7.35 to 7.45. Outside this range, pH becomes incompatible with life because it denatures enzymes and proteins. Maintain pH by two ways. Number one by reabsorbing bicarbonate ions and by removing the fixed acids and hydrogen ions. First we shall discuss in detail the mechanism of reabsorption of bicarbonate ions in the kidneys. Bicarbonate ions actually make a buffer system with carbon dioxide that minimizes changes in pH. Kidneys help to adjust pH by reabsorbing all of the bicarbonate ions in the glomerular filtrate. Out of this reabsorption, 85 to 90 percent of bicarbonate reabsorption occurs in the proximal convoluted tibule, while the rest are reabsorbed by ascending limb of loop of Henle, distal tibule and the collecting ducts. Reabsorption is facilitated by secreting hydrogen ions in the lumen of proximal convoluted tibule. These hydrogen ions are secreted by capturing bicarbonate ions. This way, these hydrogen ions combine with the filtered bicarbonate ion to form carbon dioxide and water. Since carbon dioxide lipids soluble, it diffuses into the tibular cells. Inside the cells, it combines with hydroxyl ions to regenerate bicarbonate. The bicarbonate ions in this way reabsorbed in this way cross the basolateral membrane of the tubular cell through a sodium bicarbonate simporter. This involves transfer of three bicarbonate ions into the blood for every one sodium ion. Dear students, now we shall discuss the mechanism or process of removal of fixed acids and hydrogen ions. Kidneys excrete about 7200 millimole of fixed acids and associated acidic hydrogen ions per day. Fixed acids are acidic anions that is citrate, acetate, gluconate and lactate. This excretion helps to maintain the plasma hydrogen ion concentration of only 40 nanomoles per liter. Hydrogen ions are removed as ammonium ions and as bound to some filtered buffers. The ammonium ions are produced from the filtered glutamine. Glutamine is reabsorbed by the cells of the proximal convoluted tubule. In these cells, glutamine is metabolized to ammonium ions and bicarbonate ions. Bicarbonate ions are diffused into the blood, whereas ammonium ions are secreted into the tubule and excreted in the urine. Because one ammonium ion is equivalent to one ammonium molecule and one hydrogen ion, so it contributes to the removal of acidic hydrogen ions. Dear students, these secreted hydrogen ions may also be eliminated by combining with some filtered buffer. For example, phosphate buffer. Every phosphate buffer binds two hydrogen ions or is bound and this method excretes about 36 millimole of hydrogen ions each day.