Chapter 4: Acid–Base Disturbances

Acid–Base Disturbances establishes the foundational principles of chemical buffering, emphasizing the central role of the bicarbonate buffer system, governed by the Henderson-Hasselbalch equation, in mitigating the continuous production of metabolic acids. The text systematically details the primary physiological control mechanisms, highlighting the pulmonary regulation of carbon dioxide partial pressure through alveolar ventilation and the vital renal and erythrocyte mechanisms driven by the enzyme carbonate dehydratase. Renal tubular functions, specifically bicarbonate reclamation and generation coupled with the utilization of urinary buffers such as phosphate and ammonia, are thoroughly explained. The chapter meticulously categorizes clinical acid-base disorders into metabolic and respiratory disturbances, offering detailed diagnostic frameworks and compensatory behaviors for each. Metabolic acidosis is differentiated using the plasma anion gap concept, distinguishing between high anion gap conditions like lactic acidosis, diabetic ketoacidosis, and toxic ingestions, versus normal anion gap or hyperchloremic states such as renal tubular acidosis and gastrointestinal fluid loss. Conversely, metabolic alkalosis is explored through the lens of volume depletion, chloride responsiveness, and mineralocorticoid excess. Respiratory acid-base imbalances, including hypercapnic respiratory acidosis resulting from impaired alveolar ventilation and hypocapnic respiratory alkalosis triggered by hyperventilation, are critically analyzed alongside their respective acute and chronic physiological renal compensations. Furthermore, the material covers the essential clinical application of arterial blood gas analysis, detailing the intricacies of the oxyhemoglobin dissociation curve, the Bohr effect, and the clinical differentiation of hypoxemic (type I) and hypercapnic (type II) respiratory failure. Essential diagnostic parameters, including base excess, standard bicarbonate, urinary anion gaps, Stewart's strong ion difference hypothesis, and the clinical limitations of pulse oximetry, are integrated to provide medical and biochemistry students with a robust clinical diagnostic toolkit.