Chapter 2: Water and Sodium

Water and Sodium systematically examines the compartmentalization of body water into intracellular and extracellular spaces, detailing how osmotic gradients, hydrostatic forces, and colloid osmotic (oncotic) pressure regulate essential fluid shifts across capillary and cellular membranes. The text heavily emphasizes the neuroendocrine regulation of these pathways, contrasting the water-retaining and osmoregulatory functions of antidiuretic hormone (arginine vasopressin) on renal collecting ducts with the blood pressure-stabilizing and sodium-conserving actions of the renin-angiotensin-aldosterone system. Students and healthcare professionals will find detailed discussions on calculating and measuring plasma osmolality versus osmolarity, including critical clinical caveats like identifying the osmolar gap and recognizing pseudohyponatraemia in the presence of severe hyperlipidaemia or hyperproteinaemia. Furthermore, the chapter categorizes fluid balance disorders into isosmolar volume depletion, predominant sodium depletion, and predominant water depletion, thoroughly explaining their distinct pathophysiological consequences, such as dangerous cellular dehydration or overhydration. It offers an exhaustive review of the etiologies, diagnostic algorithms, and precise treatment protocols for varying states of hyponatraemia (categorized as hypovolaemic, euvolaemic, or hypervolaemic, and notably detailing the syndrome of inappropriate antidiuretic hormone secretion) alongside hypernatraemia, while heavily underscoring the severe neurological risks of rapid osmotic correction, such as central pontine myelinolysis. Finally, the material outlines the rigorous clinical investigation of polyuria, differentiating between psychogenic polydipsia, osmotic diuresis, and both cranial and nephrogenic diabetes insipidus through specialized diagnostics like the water deprivation and synthetic vasopressin (DDAVP) tests, ultimately equipping readers with the evidence-based rationale required for safe intravenous fluid resuscitation, daily fluid balance monitoring, and targeted electrolyte replacement.