Introduction
This topic covers the alkaline side of acid-base disorders: how alkalaemia is defined and measured, how the lungs and kidneys regulate bicarbonate and carbon dioxide, and the causes, clinical picture, compensation and treatment of metabolic alkalosis and respiratory alkalosis. Examiners expect you to link each laboratory parameter to its mechanism.
Basic concepts and parameters
Alkalosis is a primary process that raises arterial pH above 7.45 (normal 7.35 to 7.45). Acid-base status is described by the Henderson-Hasselbalch relation:
- pH: the net result of the primary disorder and compensation.
- pCO₂ (about 4.7 to 6.0 kPa, or 35 to 45 mmHg): the respiratory component; it falls in respiratory alkalosis.
- Standard bicarbonate (about 22 to 26 mmol/L): the metabolic component; it rises in metabolic alkalosis.
- Base excess (about -2 to +2 mmol/L): positive values indicate a metabolic alkalosis component.
- Electrolytes: chloride, potassium and ionised calcium, which are needed to find the cause and to plan treatment.
Regulation of acid-base balance
- Buffers (bicarbonate, haemoglobin, phosphate, proteins) act within seconds and do not remove acid from the body.
- Lungs eliminate carbon dioxide through ventilation within minutes; hyperventilation lowers pCO₂, hypoventilation raises it.
- Kidneys reabsorb filtered bicarbonate, excrete hydrogen ions as titratable acid and ammonium, and regenerate bicarbonate; they act over hours to days and are the only route for lasting correction of metabolic disorders.