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Renal and Urinary Physiology

A patient with Type IV renal tubular acidosis (RTA) has a defect in aldosterone secretion and ammonium excretion. Which of the following statements about ammonia (NH3) is correct?

ExplanationAmmonia (NH3) is produced from amino acids in the cells of the renal tubules (mainly the proximal tubules), and its rate of production increases during acidosis. This is important in acidosis because it increases the total amount of H+ ion that can be excreted in a given volume of urine. The NH3 freely diffuses into the tubular lumen, and because of the high pKa (9.2) of the reaction, essentially all of it combines with H+ to form NH4+. This maintains the driving force for more NH3 to passively diffuse into the lumen. The NH4+ that is formed gets “trapped” in the tubules and excreted because the tubules are impermeable to this cation.