Acid-Base Disorders

Key points

• • Blood pH is kept near 7.4; a disorder is called an acidosis if it tends to lower the pH and an alkalosis if it tends to raise it.
• • There are four primary disorders, arising from a metabolic cause (a change in bicarbonate) or a respiratory cause (a change in carbon dioxide).
• • Metabolic acidosis (low bicarbonate) comes from making too much acid (as in uncontrolled diabetes or a build-up of lactic acid) or from losing bicarbonate (as in diarrhoea); metabolic alkalosis (high bicarbonate) comes from losing acid, as in prolonged vomiting.
• • Respiratory acidosis (high carbon dioxide) comes from poor breathing that fails to blow off carbon dioxide, while respiratory alkalosis (low carbon dioxide) comes from over-breathing, as in anxiety or at high altitude.
• • The body compensates: the lungs adjust breathing within minutes for a metabolic problem, and the kidneys adjust bicarbonate over days for a respiratory problem.
• • The anion gap helps sort out a metabolic acidosis: a high gap reflects added unmeasured anions (from accumulated acids), while a normal gap points to a loss of bicarbonate.