Acidosis is an increased acidity (i.e. hydrogen ion concentration) of blood plasma. Generally acidosis is said to occur when arterial pH falls below 7.35, while its counterpart (alkalosis) occurs at a pH over 7.45. Arterial blood gas analysis and other tests are required to separate the main causes.
Strictly speaking, the term acidemia would be more appropriate to describe the state of low blood pH, reserving acidosis to describe the processes leading to these states. Nevertheless, most physicians use the terms interchangeably. The distinction may be relevant where a patient has factors causing both acidosis and alkalosis, where the relative severity of both determines whether the result is a high or a low pH.
The rate of cellular metabolic activity affects and, aththe same time, is affected by the pH of the body fluids. In mammals, the normal pH of arterial blood lies between 7.35 and 7.50 depending on the species (e.g. healthy human-arterial blood pH varies between 7.39 and 7.44). Blood pH values compatible with life in mammals are limited to a pH range between 6.8 and 7.8. Changes in the pH of arterial blood, and therefore the extracellular fluid outside this range, result in irreversible cell damage (Needham, 2004).
Respiratory acidosis results from a build-up of carbon dioxide in the blood due to hypoventilation. It is most often caused by pulmonary problems, although head injuries, drugs (especially anaesthetics and sedatives), and brain tumors can also bring it on. Emphysema, chronic bronchitis, asthma, severe pneumonia, and aspiration are among the most frequent causes. It can also occur as a response to chronic metabolic alkalosis.
Blood gases show pH below 7.35 as above mentioned, and PaCO2 will be high (>45mmHg / 6kPa).
The key to distinguish between respiratory and metabolic acidosis is that in respiratory acidosis, the CO2 is increased while the bicarb is either normal (uncompensated) or increased (compensated). These latter two words indicating an acute onset or chronic condition.
Metabolic acidosis results from disturbances in the ability to excrete acid via the kidneys or from excess metabolic production acidic substances. Examples are lactic acid (i.e. resulting from hypoxemia), acidic poisons, iron etc. It can also result from decreased production of bicarbonate.
Metabolic acidosis can result in stimulation of chemoreceptors and so increase alveolar ventilation, leading to respiratory compensation, otherwise known as hyperventilation. Should this situation persist the patient is at risk for exhaustion leading to respiratory failure.
In blood gas tests, it is characterised by a low pH and normal or low PaCO2.
In addition to arterial blood gas one can use the anion gap to differentiate between possible causes.
The pH of the blood is expressed in terms of the Henderson-Hasselbach equation:
pH = pKA + log [HCO-3]/[H2CO3] ([H2CO3] α PCO2)
Needham, A. 2004. Comparative and Environmental Physiology Acidosis and ALkalosis.