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.

Respiratory acidosis

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, and PaCO₂ will normally be high (>45mmHg / 6kPa).

Metabolic acidosis

Metabolic acidosis results from disturbances in the ability to excrete acid via the kidneys or from excess metabolic production acidic substances. Examples are 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.

In blood gas tests, it is characterised by a low pH and normal or low PaCO₂.