Pathophysiology
Clinical meaning
The body maintains arterial pH within a narrow range (7.35-7.45) through three compensatory mechanisms. Chemical buffers respond within seconds: the bicarbonate buffer system (H2CO3 ↔ H+ + HCO3-) is the most important extracellular buffer; phosphate and protein buffers operate intracellularly. Respiratory compensation acts within minutes to hours: central chemoreceptors in the medulla and peripheral chemoreceptors in the carotid/aortic bodies detect pH/PCO2 changes and adjust ventilation (hypoventilation retains CO2 to compensate metabolic alkalosis; hyperventilation eliminates CO2 to compensate metabolic acidosis). Renal compensation takes 3-5 days for full effect: proximal tubule reabsorbs filtered HCO3-, distal tubule secretes H+ and generates new HCO3-. Compensation adjusts the ratio of HCO3- to PCO2 toward normal but NEVER fully normalizes pH (except in chronic respiratory alkalosis).