Clinical meaning
Sodium (Na+) is the most abundant cation in extracellular fluid, with a normal serum level of 135-145 mEq/L. Sodium is the primary determinant of extracellular fluid osmolality and volume. Because water follows sodium by osmosis, sodium concentration directly affects fluid distribution between intracellular and extracellular compartments. Hyponatremia (Na <135 mEq/L) causes water to shift into cells by osmosis, producing cellular edema — in the brain, this causes cerebral edema with headache, confusion, nausea, and potentially seizures and death. Hypernatremia (Na >145 mEq/L) causes water to shift out of cells by osmosis, producing cellular dehydration — in the brain, this causes cell shrinkage with restlessness, irritability, lethargy, hyperreflexia, and potentially seizures and coma. The kidneys regulate sodium balance through aldosterone (retains sodium, excretes potassium) and ADH/vasopressin (retains water). Common causes of hyponatremia for practical nurses to recognize: excessive water intake, thiazide diuretics, vomiting/diarrhea, SIADH. Common causes of hypernatremia: inadequate water intake (elderly, infants, NPO status), excessive insensible losses (fever, burns), diabetes insipidus.