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Renal Control of Acid Base The hydrogen ion (protons) content of body fluids must be kept constant so that detrimental changes in proteins, enzyme and cellular structure do not occur. Buffers are compounds that can both accept and donate protons, and so minimise changes in pH. The body has various buffers, the main one being the bicarbonate buffer system. This is an open system, which means carbon dioxide may be removed. Bicarbonate is produced as follows: CO2 + H20 ? ? H+ + HCO3-
The kidneys excrete hydrogen ions via the Na+/H+ antiport exchanger in the proximal tubules. There is also an active hydrogen ion ATPase pump in the collecting tubules. However, hydrogen ions cannot be excreted in their free form. Secreted hydrogen is bound to filtered buffers such as phosphate or ammonia. Extra-cellular pH regulates acid secretion. All bicarbonate filtered by the glomerulus must also be reabsorbed, as loss of bicarbonate would lead to an acidosis. In the proximal tubule, the luminal fluid is acidified by the excretion of hydrogen ions via the Na+/H- and Na+/NH4+ antiport exchangers. This process also requires that bicarbonate formed in the cell is returned to circulation via the 3HCO3-/Na symport exchanger. The energy for these transport processes is originally provided by the basolateral Na+/K+ ATPase pump. 90% of bicarbonate reabsorption occurs in the proximal tubule. The enzyme carbonic anhydrase is important in this process. It converts water and carbon dioxide into bicarbonate and hydrogen ions. This action allows both bicarbonate reabsorption and continued hydrogen ion secretion.
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