Clinical meaning
Dehydration refers to a deficit of total body water that disrupts normal physiological processes. It occurs when fluid output exceeds fluid intake over a sustained period. Water comprises approximately 60% of adult body weight (higher in infants at 70-80%, lower in elderly and obese individuals), making fluid balance essential for cellular function, circulatory volume, thermoregulation, and organ perfusion. Dehydration is classified by the relationship between water and sodium loss into three types. Isotonic (isonatraemic) dehydration involves proportional loss of water and sodium, keeping serum sodium within the normal range (135-145 mEq/L). This is the most common type, occurring with GI losses (vomiting, diarrhoea), haemorrhage, and third-spacing. The primary deficit is intravascular volume loss, leading to reduced cardiac output and tissue perfusion. Hypotonic (hyponatraemic) dehydration involves greater sodium loss relative to water (serum sodium less than 135 mEq/L). This creates an osmotic gradient that pulls water from the extracellular space into cells, causing cellular swelling. This is particularly dangerous for brain cells, which can swell within the rigid skull, causing cerebral oedema with neurological symptoms (headache, confusion, seizures). Hypertonic (hypernatraemic) dehydration...