Metabolic acidosis is characterized as a pH imbalance in which there is a significant increase in plasma acidity due to excess hydrogen ions. This occurs when the body's production of organic acids such as lactic acid exceeds its rate of elimination due to an excessive loss of bicarbonate ions or renal tubular acidosis. The kidney functions as a buffering agent in two ways: it reabsorbs bicarbonate ions which bind to hydrogen ions and excretes it from the body as water and carbon dioxide, in addition to excreting excess hydrogen ions in the form of ammonium. Renal symptoms associated with metabolic acidosis include kidney stones, increased excretion of ammonium ions, as well as increased levels of plasma bicarbonate ions.
Kidney Stones
In metabolic acidosis, the kidney becomes unable to excrete excess amounts of hydrogen ions thus calcium and sodium ions are mobilized into the kidney to help buffer the acidic conditions. Under extreme acidic conditions, waste products cling to the cellular walls of the nephrons and the mobilized calcium is crystallized in the collecting ducts of the kidney resulting in kidney stones. Thus, the formation of kidney stones due to excess, crystallized calcium is known as hypercalciuria . In the October 2009 issue of the "Cleveland Clinic of Journal Medicine," Dr. Phillip Hall states that chronic metabolic acidosis causes hypercalciuria by lowering the levels of citrate, an inhibitor of calcium salt crystallization.
Increased Ammonium Excretion
The kidney works to increase the plasma pH during periods of metabolic acidosis by excreting excess hydrogen ions in the form of ammonium ions. As blood filters in the glomureli, excess hydrogen ions enter the distal tubular lumen and decrease the urine pH. As urine pH falls, ammonium secretion increases. According to Anaesthesia Education website, increased ammonium excretion proves extremely important during chronic metabolic acidosis. It is noted that the level of ammonium secreted is directly related to the severity of acidosis and can increase to a level of about 300mmol/day in an individual suffering with chronic metabolic acidosis.
Increased Plasma Bicarbonate Concentration
The kidney regulates plasma pH by reabsorbing bicarbonate ions that will bind to hydrogen ions and excretes them from the body in the form of water and carbon dioxide. Excessive generation of bicarbonate ions in the proximal tubule is a symptom of metabolic acidosis since the kidney generates bicarbonate ions and adds it to blood plasma neutralize its acidity, says the Anaesthesia Education website.
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