Chapter 26: Fluid, Electrolyte and Acid-Base Balance

Water movement between these compartments occurs primarily through osmotic forces, governed by solute concentrations and membrane permeability. Sodium and chloride ions predominate in extracellular environments, while potassium and phosphate ions concentrate intracellularly, establishing critical electrochemical gradients. The chapter explains how water balance is regulated through the thirst center in the hypothalamus and urinary output control mediated by antidiuretic hormone, aldosterone, and atrial natriuretic peptide, with each hormone playing distinct roles in modulating water reabsorption and sodium handling. Electrolyte homeostasis receives detailed treatment, particularly for sodium regulation through hormonal mechanisms affecting blood volume and osmolarity, potassium control through renal mechanisms critical for cardiac and neural function, and calcium balance through the coordinated actions of parathyroid hormone, calcitonin, and vitamin D on intestinal absorption, bone remodeling, and renal reabsorption. The chapter systematically presents acid-base physiology through chemical buffering systems including bicarbonate, phosphate, and protein buffers that resist pH changes, supplemented by respiratory compensation through ventilatory rate adjustments and renal compensation through selective bicarbonate reabsorption and hydrogen ion secretion. Clinical presentations of respiratory and metabolic acidosis and alkalosis are examined, demonstrating compensatory mechanisms and the consequences of pathological conditions such as emphysema, vomiting, diarrhea, and renal dysfunction. The integration of respiratory, renal, and endocrine system coordination emphasizes how multiple physiological systems collaborate to maintain the narrow pH range necessary for enzymatic function and cellular survival.