Chapter 4: Water, Acids, Bases, and Buffers

Water comprises approximately 60 percent of total body weight and exists in distinct intracellular and extracellular compartments, with its dipolar structure enabling hydrogen bonding, solvent capacity, and thermal stability critical for cellular function. The concept of osmolality and osmotic pressure governs water movement between fluid compartments, and clinical perturbations such as dehydration, osmotic diuresis from hyperglycemia, and intravenous fluid administration directly alter fluid distribution and electrolyte concentrations. The chapter introduces quantitative acid-base chemistry through dissociation constants, the distinction between strong and weak acids, and the Henderson-Hasselbalch equation, which mathematically relates pH, pKa, and the ratio of conjugate base to acid concentration. Buffer systems, composed of weak acid-conjugate base pairs, resist dangerous pH fluctuations and operate throughout the body through multiple mechanisms: the bicarbonate buffer system in blood, hemoglobin buffering within red blood cells, and phosphate and protein buffers in intracellular environments. Normal cellular metabolism continuously generates acids including lactic acid, ketone bodies, sulfuric acid, and carbon dioxide, requiring coordinated excretion through respiratory pathways, which eliminate CO2 as volatile acid, and renal mechanisms, which excrete titratable acid and ammonium ions. The chapter integrates three clinical cases demonstrating pH disturbance: diabetic ketoacidosis in a type 1 diabetes patient showing ketone accumulation and Kussmaul breathing patterns, aspirin toxicity illustrating respiratory alkalosis followed by metabolic acidosis, and anxiety-induced hyperventilation producing acute respiratory alkalosis. Electrolyte balance involving sodium, potassium, chloride, and bicarbonate ions interconnects with buffer function, while gastric acid secretion and renal ammonium metabolism illustrate organ-specific contributions to acid-base homeostasis. Blood gas analysis and pH measurement serve as critical diagnostic tools, with HbA1C reflecting chronic glucose control in diabetes and demonstrating the intimate relationship between metabolic disorders and acid-base disturbance.