Dehydration

Dehydration is a physiological condition in which the body lacks sufficient total body water to carry out normal metabolic processes. This deficit arises when free water loss exceeds intake, commonly due to excessive sweating, illness, or insufficient drinking. Even mild dehydration, defined as a 1–2% loss of body water, can impair cognitive performance. Most people tolerate a 3–4% loss without difficulty, but a 5–8% loss typically causes fatigue and dizziness. Losses exceeding 10% lead to physical and mental deterioration with severe thirst, while a 15–25% loss is fatal. Mild cases usually resolve with oral rehydration, but severe cases may require intravenous fluids. Dehydration can cause hypernatremia (elevated blood sodium), which is distinct from hypovolemia (loss of blood plasma volume). Chronic dehydration contributes to kidney stones and chronic kidney disease. Hallmarks include thirst, headaches, loss of appetite, nausea, decreased urine volume, confusion, tiredness, purple fingernails, and seizures. In people over 50, thirst sensation diminishes, yet fluid intake does not differ from younger individuals; fatigue is the most common symptom in the elderly, and dehydration increases morbidity during hot weather. Risk factors include exertion in heat, high altitude, endurance sports, infancy, childhood, chronic illness, and certain medications. In the elderly, blunted thirst or inadequate access to water, especially with hyperglycemia, are main causes. Water loss occurs via sensible routes (sweat, vomiting, diarrhea, osmotic diuresis) and insensible routes (skin, respiratory tract). The body’s water is distributed as intracellular fluid (57% of total, rich in potassium and magnesium) and extracellular fluid (43%, including blood and interstitial fluid, rich in sodium and chloride). When free water is lost, serum osmolarity rises, detected by hypothalamic osmoreceptors, which trigger antidiuretic hormone release. ADH increases water reabsorption in the kidneys and, in severe low blood pressure, constricts blood vessels to raise pressure. Diagnosis involves examining dry mucous membranes, sunken eyes, poor skin turgor, and orthostatic hypotension. Severe dehydration can cause seizures or respiratory arrest, and overly rapid rehydration risks osmotic cerebral edema.