18 pH of blood
Blood pH expresses the acidity of the blood. It is kept within a very narrow range, 7.35–7.45, because enzymes, ion channels, heart rhythm and the binding of oxygen to haemoglobin depend on it. The main buffer is the carbonic acid–bicarbonate system: the lungs regulate carbon dioxide (an acid), and the kidneys regulate bicarbonate (a base) and excrete acid. A primary change in CO₂ is followed by a compensating change in bicarbonate, and vice versa. A pH below 7.35 is acidaemia, above 7.45 alkalaemia.
The four primary disorders
| Disorder | Primary change | Typical causes |
|---|---|---|
| Metabolic acidosis | bicarbonate ↓ | diabetic ketoacidosis, lactic acidosis, kidney failure, diarrhoea, poisoning |
| Respiratory acidosis | CO₂ ↑ | COPD exacerbation, respiratory depression (opioids, sedatives), neuromuscular weakness |
| Metabolic alkalosis | bicarbonate ↑ | vomiting, diuretics, low potassium, excess mineralocorticoids |
| Respiratory alkalosis | CO₂ ↓ | hyperventilation (anxiety, pain), pneumonia, pulmonary embolism, pregnancy, early sepsis, high altitude |
Assessment follows a stepwise approach: pH, then PaCO₂ and bicarbonate to identify the primary disorder, the expected compensation and, in metabolic acidosis, the anion gap.
for rapid or very slow breathing, confusion or drowsiness, persistent vomiting, severe weakness or palpitations, especially in people with diabetes, kidney disease or lung disease.
References
This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.
Published on 9 May 2024