Clinical meaning
Acid-base physiology encompasses the mechanisms by which the body maintains hydrogen ion concentration within the physiologically compatible range. The bicarbonate buffer system is the most important extracellular buffer, described by the Henderson-Hasselbalch equation: pH = 6.1 + log([HCO3-] / [0.03 x PaCO2]). Normal values: pH 7.35-7.45, PaCO2 35-45 mmHg, HCO3 22-26 mEq/L. The respiratory system provides rapid compensation (minutes) by adjusting alveolar ventilation: hypoventilation retains CO2 (acidifying), hyperventilation eliminates CO2 (alkalinizing). The renal system provides slower but more powerful compensation (hours to days) through three mechanisms: proximal tubular bicarbonate reclamation (reclaiming 80-90% of filtered bicarbonate via Na+/H+ exchange and carbonic anhydrase), distal tubular hydrogen ion secretion (type A intercalated cells secrete H+ via H+/K+ ATPase and H+ ATPase, generating new bicarbonate), and ammoniagenesis (proximal tubule cells produce NH3 from glutamine, which combines with secreted H+ to form NH4+ -- a titratable acid excreted in urine, representing new bicarbonate generation). The clinician applies Stewart's strong ion approach for complex cases: pH is determined by three independent variables -- strong ion difference (SID), total concentration of weak acids (primarily albumin and phosphate), and PaCO2. This approach explains why hypoalbuminemia alkalinizes (reduces weak acid) and hyperchloremia acidifies (reduces SID).