Pathophysiology
Clinical meaning
The respiratory system compensates for metabolic acid-base disorders through chemoreceptor-mediated changes in ventilation. Central chemoreceptors in the medulla respond to CSF pH changes driven by arterial CO2, while peripheral chemoreceptors in the carotid and aortic bodies detect arterial pH and PaO2. In metabolic acidosis (pH < 7.35, HCO3 < 22 mmol/L), the respiratory centre increases ventilation rate and depth (Kussmaul breathing) to lower PaCO2. Winter's formula (expected PaCO2 = 1.5 x HCO3 + 8 +/- 2) predicts respiratory compensation. In metabolic alkalosis, ventilation decreases to retain CO2 though compensation is limited by hypoxic drive. The clinician must interpret ABGs systematically and calculate the anion gap (Na - Cl - HCO3, normal 8-12 mmol/L).