One definition of an acid which is useful for biology is that of a proton, or positive hydrogen ion, donor. When excess acid builds up in the blood, this is called acidemia. When it builds up in the tissues, this is called acidosis. Causes of acidosis may be broadly categorized as respiratory acidosis and metabolic acidosis. The category of metabolic acidosis can be subdivided into an anion gap and normal anion gap metabolic acidosis.
Respiratory Acidosis
Carbon dioxide, or CO2, is a waste product of metabolism. It exits the cells and travels in the blood to be breathed out through the lungs. While in the blood it can combine with water to form carbonic acid. Therefore, if the lungs are not expelling enough CO2, respiratory acidosis results. Causes include nerve, muscular and skeletal problems interfering with ventilation. Drugs such as narcotics can slow down the respiratory rate. Lung diseases interfering with gas exchange or obstructions to the airways also cause respiratory acidosis according to the Merck Manual.
Normal Anion Gap Acidosis
A metabolic acidosis is a buildup of acid in the body from a source other than the lungs. One useful way of determining the cause of a metabolic acidosis is to look at a calculation called the anion gap. Basically, this calculation gives the relative number of positively charged molecules to negatively charged molecules. If it is normal, the causes are narrowed down generally to the loss of bicarbonate in the GI tract such as with diarrhea, or loss of bicarbonate from the kidneys.
Anion Gap Acidosis
If the measurement of the anion gap is elevated, this means there are extra anions, or negatively charged molecules in the blood. A mnemonic for the causes or substances found in the blood during anion gap metabolic acidosis is "MUD PILERS," according to Family Practice Notebook. "M" stands for methanol, commonly known as moonshine. "U" stands for urea, a waste product of protein breakdown that builds up when the kidneys fail. "D" is diabetic acidosis, as occurs in uncontrolled diabetes, and "P" stands for two medications: paraldehyde, which is a seizure medication, and phenformin, a diabetes drug. "I" is for isopropyl alcohol, also known as rubbing alcohol, and isoniazid, a tuberculosis drug. "L" stands for lactic acid, a product of metabolism that signals a lack of oxygen. "E" stands for ethylene glycol, or anti-freeze, and ethanol, or drinking alchohol. "R" is for rhabdomyolysis, a condition of muscle breakdown, and "S" stands for salicylates, which are drugs in the same class as aspirin.
