Chapter 8: Disorders of Fluid, Electrolyte, and Acid-Base Balance

Body fluids are partitioned into intracellular and extracellular spaces, with the distribution of water and dissolved substances governed by concentration gradients and transport mechanisms including diffusion and osmosis. Cell volume regulation depends on the tonicity of the surrounding fluid environment, classified as isotonic, hypotonic, or hypertonic conditions that determine whether cells maintain, gain, or lose water. Imbalances in the exchange of fluid between capillaries and interstitial tissue can result in pathological fluid accumulation such as edema or sequestration in third spaces. Sodium and water homeostasis are maintained through an integrated system of regulatory mechanisms including the thirst mechanism, sympathetic nervous system activation, the renin-angiotensin-aldosterone system, and antidiuretic hormone secretion; dysfunction in these pathways produces disorders such as diabetes insipidus and syndrome of inappropriate antidiuretic hormone secretion. The regulation of other critical electrolytes including potassium, calcium, phosphorus, and magnesium involves coordinated control by the gastrointestinal tract, kidneys, bone tissue, vitamin D metabolism, and parathyroid hormone signaling. Acid-base homeostasis is maintained through chemical buffering systems, particularly the bicarbonate buffer system, and physiological compensatory mechanisms mediated by respiratory and renal function. Derangements in acid-base status are classified as metabolic or respiratory in origin and present as acidosis or alkalosis, each triggering characteristic compensatory responses. Clinical assessment relies on laboratory measurements including arterial carbon dioxide levels, serum bicarbonate concentrations, and anion gap calculations to identify the underlying disorder and guide therapeutic intervention.