Renal Acid Excretion and Ammonia
The kidney completes acid-base homeostasis by reclaiming filtered bicarbonate and by excreting the daily net acid load generated by metabolism. It does so by secreting hydrogen ions that are buffered in the urine as titratable acid and, most importantly, as ammonium, while regenerating the bicarbonate consumed in systemic buffering.
Definition
Renal acid excretion is the process by which the kidney maintains acid-base balance, reabsorbing essentially all filtered bicarbonate, secreting hydrogen ions into the tubular fluid, and excreting net acid as titratable acid and ammonium while regenerating bicarbonate; ammonia (as ammonium) is the principal and most adjustable component.
Scope
The topic covers proximal bicarbonate reabsorption, distal hydrogen-ion secretion, the two forms of net acid excretion (titratable acid and ammonium), renal ammonia synthesis and transport, and how net acid excretion adapts to acid-base demands. It is framed as physiology rather than clinical guidance.
Core questions
- How is filtered bicarbonate reabsorbed, and why does this not by itself excrete acid?
- What are titratable acid and ammonium, and why is ammonium the dominant adjustable component?
- How is ammonia synthesised, secreted, and transported along the nephron?
- How does net acid excretion increase in response to an acid load?
Key concepts
- Bicarbonate reabsorption
- Hydrogen-ion secretion
- Titratable acid
- Ammonium excretion
- Renal ammoniagenesis
- Net acid excretion
- Bicarbonate regeneration
Key theories
- Net acid excretion model
- Defines the kidney's contribution to acid-base balance as urinary titratable acid plus ammonium minus any excreted bicarbonate, with ammonium production being the principal mechanism by which net acid excretion is increased during an acid challenge.
Mechanisms
The proximal tubule reabsorbs most filtered bicarbonate by secreting hydrogen ions that combine with luminal bicarbonate, a step that conserves base but does not by itself remove acid from the body. New bicarbonate is generated only when secreted hydrogen ions are buffered by urinary phosphate (titratable acid) or by ammonia. Ammonia is synthesised in proximal tubular cells, chiefly from glutamine, secreted into the lumen, and recycled and concentrated in the medulla before being trapped as ammonium in the collecting duct and excreted; for each ammonium excreted, a bicarbonate is returned to the blood. Because ammoniagenesis can increase several-fold during acidosis, ammonium excretion is the principal way the kidney raises net acid excretion to meet a sustained acid load, whereas titratable acid is limited by the available filtered buffer.
Clinical relevance
Impaired renal acid excretion underlies the metabolic acidosis of chronic kidney disease and the renal tubular acidoses, and urinary ammonium estimation is used in evaluating these disorders. This entry explains the underlying physiology and does not provide dosing or individualised treatment recommendations.
Evidence & guidelines
The mechanisms of bicarbonate reabsorption, net acid excretion, and renal ammonia metabolism are well established in physiology reviews and texts (Weiner and Verlander, 2013; Hamm and colleagues, 2015; Berend and colleagues, 2014). The account here is descriptive physiology and not a clinical protocol.
History
The recognition that ammonium excretion, rather than free hydrogen-ion excretion, accounts for most renal acid removal emerged through twentieth-century studies of renal ammoniagenesis and urinary buffering. Later work identified the specific transporters, including the Rhesus glycoproteins, that move ammonia across renal epithelia, refining the molecular picture of how net acid excretion is regulated.
Key figures
- I. David Weiner
- Jill W. Verlander
- L. Lee Hamm
Related topics
Seminal works
- weiner-verlander-2013
- hamm-2015
Frequently asked questions
- Why is ammonium so important for renal acid excretion?
- Because ammonia production can be increased several-fold during acidosis, ammonium excretion is the main adjustable route by which the kidney raises net acid excretion and regenerates bicarbonate, whereas titratable acid is limited by available buffer.
- Does reabsorbing filtered bicarbonate excrete acid?
- No. Reabsorbing filtered bicarbonate only conserves existing base; new bicarbonate is generated, and acid is removed, only when secreted hydrogen ions are buffered as titratable acid or ammonium and excreted in the urine.