an acidosis is a process going on in the body which tends to lower its pH. This is different than acidemia, which is the state of having an abnormally low blood pH (less than 7.35; normal blood pH is 7.35-7.45). An acidosis might not actually lower the body's pH enough to cause acidemia, because of competing processes, the body's buffering capabilities, or compensatory mechanisms.

The two most important classifications of acidosis are metabolic vs respiratory acidosis, and anion-gap vs non-anion gap acidosis.

Both metabolic and respiratory processes frequently coincide. If a person has a metabolic acidosis, a normal response is to quickly compensate by creating a respiratory alkalosis. Similarly, a metabolic alkalosis will quickly be compensated for with a respiratory acidosis. A primary respiratory process takes longer to compensate for metabolically, but a long-standing condition affecting respiration will certainly trigger metabolic compensation. Occasionally a toxin can both affect respiration and act metabolically as well, creating a combined respiratory and metabolic acidosis, for example. This would be much more likely to cause acidemia than would an appropriately-compensated process. Of course, in ill patients, normal compensatory mechanisms may not function properly.

A respiratory acidosis is one that has to do with carbon dioxide elimination, which is done primarily through the lungs, though bicarbonate can be excreted in much smaller quantities by the kidneys. If a person increases respiration, more carbon dioxide will be excreted, and this will tend to raise the body pH, causing an alkalosis. In contrast, decreasing respiration will reduce the amount of carbon dioxide that is excreted, creating a respiratory acidosis.

Ventilation consists of both the tidal volume of each breath, as well as the ventilation rate. While rapid, shallow breathing is thought to be more efficient for blowing off carbon dioxide than is slow, deep breathing, either increasing rate or volume will work. A person with a metabolic acidosis may thus find themselves breathing more deeply and rapidly in order to produce a compensatory respiratory acidosis.

A metabolic acidosis is any other form of acid-generating process. This can be anything from ketoacidosis to ingestion of acid. Typically, metabolic acidosis is worked up by calculating the anion gap (blood sodium - chloride - bicarbonate). This total should be about 10-14. It can also be calculated adding (sodium+potassium-chloride-bicabonate), which would add about 4 to this total. The significance of the anion gap is that since blood contains as many anions as cations, a widened gap between these common blood solutes indicate a large number of unmeasured organic anions.

A normal anion gap metabolic acidosis is typically due to dehydration, diarrhea, or the like. These typically respond to hydration and electrolyte repletement.

An elevated anion gap metabolic acidosis requires further workup. The mnemonic CATMUDPILES is often used to represent sources of elevated anion gap metabolic acidosis.

Carbon Monoxide or Cyanide
Diabetic Ketoacidosis
Iron or Isoniazid
Lactic acid
Ethylene Glycol