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An increase in extracellular fluid H+ concentration stimulates renal glutamine metabolism and, therefore, increases the formation of NH4+ and new HCO3- to be used in H+ buffering; a decrease in H+ concentration has the opposite effect.

Under normal conditions, the amount of H+ eliminated by the ammonia buffer system accounts for about 50 per cent of the acid excreted and 50 per cent of the new HCO3- generated by the kidneys. However, with chronic acidosis, the rate of NH4+ excretion can increase to as much as 500 mEq/day. Therefore, with chronic acidosis, the dominant mechanism by which acid is eliminated is excretion of NH4+. This also provides the most important mechanism for generating new bicarbonate during chronic acidosis.

Figure 30-8

Production and secretion of ammonium ion (NH4+) by proximal tubular cells. Glutamine is metabolized in the cell, yielding NH4+ and bicarbonate. The NH4+ is secreted into the lumen by a sodium-NH4+ pump. For each glutamine molecule metabolized, two NH4+ are produced and secreted and two HCO3- are returned to the blood.

Renal interstitial fluid

Collecting tubular cells

Tubular lumen

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