Ammonia and renal acid-base physiology

Ammonia is an exceptional chemical in that it is manufactured in the renal tubular cells and excreted into the urine. For most other chemicals, the kidney operates by transferring them in one direction or the other between the tubular and peritubular fluids but without itself manufacturing the chemicals. The ammonia produced in the tubular cells is derived from glutamine, as already described. The other chemicals yielded by the splitting of ammonia from glutamine are neither acids nor alkalis this mechanism provides the kidney with a source of base which can be produced without incurring other acid -base alterations. 

In renal physiology, it is conventional to divide the renal mechanisms for the production of urine into three groups, glomerular filtration, tubular secretion and tubular reabsorption. The unique status of ammonia requires a fourth mechanism in renal physiology, which is the addition of newly-produced chemical to the urine. 

The role of ammonia is in buffering acid and it makes a contribution which is particularly important when the acid load is high. Ammonia produced in the tubular cell cytoplasm diffuses readily across the tubular cell membrane into the tubular fluid. Cell membranes consist largely of lipid material and so are in general permeable to non-polar particles such as ammonia The solubility of ionized particles such as ammonium ions in lipid is very low so that ions permeate cell membranes very slowly. 

The ammonia, once it diffuses into the luminal fluid, reacts with hydrogen ions to produce ammonium ions, according to the reaction 

When the tubular cells are vigorously transporting hydrogen ions into the tubular fluid as a compensation for acidosis, the concentration of hydrogen 

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