Figure 4.

Schematic diagram of processes involved in N balance in the kidney.

Notes: Urinary excretion of NH₄⁺ is the most important factor in the control of acid–base balance. NH₄⁺ is derived primarily from the metabolism of blood glutamine, which is metabolized in the proximal tubule. A complex set of NH₄⁺, NH₃, and HCO₃^– tubule- and collecting duct-transport processes regulate NH₄⁺ excretion. For each NH₄⁺ molecule excreted, an HCO₃^– molecule is added to the systemic circulation. In addition, some of the glutamine is either used as an energy source or converted to alanine.