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 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, at the 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 7.35 and 7.45). 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
Anaesthetic s and
Sedative s), and
Brain Tumor s 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 PaCO
2 will be high (>45 mmHg / 6
KPa ).
The key to distinguish between respiratory and metabolic acidosis is that in respiratory acidosis, the CO
2 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
Kidney s or from excess metabolic production acidic substances. Examples are
Lactic Acid (i.e. resulting from
Hypoxemia ), acidic
Poison s,
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 .
Mutations to the
V-ATPase 'a4' or 'B1' isoforms result in distal renal tubular acidosis—a condition that leads to metabolic acidosis—in some cases with sensorineural deafness.
In blood gas tests, it is characterised by a low
PH , low blood
HCO3 , and normal or low PaCO
2.
In addition to
Arterial Blood Gas one can use the
Anion Gap to differentiate between possible causes.
The
Henderson-Hasselbalch Equation is useful for calculating blood pH, because blood is a
Buffer Solution .
