Overview
Introduction
Acid base homeostasis maintains arterial pH within a narrow range (7.35 7.45) through three integrated systems: chemical buffers (immediate, seconds), respiratory compensation (...
Acid-base homeostasis maintains arterial pH within a narrow range (7.35-7.45) through three integrated systems: chemical buffers (immediate, seconds), respiratory compensation (minutes to hours), and renal compensation (hours to days). The bicarbonate buffer system (H2CO3 ↔ H+ + HCO3-) is the most clinically important, described by the Henderson-Hasselbalch equation: pH = 6.1 + log([HCO3-] / [0.03 x PaCO2]). Normal values: pH 7.35-7.45, PaCO2 4.7–6.0 kPa, HCO3- 22–26 mmol/L, PaO2 10.7–13.3 kPa, SaO2 >95%, base excess -2 to +2. Primary metabolic acidosis (HCO3- <22) triggers respiratory compensation through peripheral chemoreceptor stimulation of the medullary respiratory center, increasing minute ventilation (Kussmaul breathing) to lower PaCO2. Winter's formula predicts expected PaCO2: PaCO2 = (1.5 x HCO3-) + 8 ± 2. If measured PaCO2 differs from predicted, a concurrent respiratory disorder exists. Primary respiratory acidosis (PaCO2 >45) triggers renal compensation: proximal tubular cells increase H+ secretion and HCO3- reabsorption; type A intercalated cells in collecting duct increase H+ secretion via H+-ATPase. Acute compensation: HCO3- rises 1 mEq/L per 1.3 kPa PaCO2 increase. Chronic compensation (3-5 days): HCO3- rises 4 mmol/L per 1.3 kPa PaCO2 increase....
