Sodium Imbalance

Sodium (Na⁺) is the dominant extracellular cation, but the serum sodium concentration reflects the relationship between sodium and water more than the body's total sodium content. When plasma osmolality changes, water shifts across cell membranes. Brain cells are especially sensitive: water moving into them produces cerebral swelling, while water leaving them produces cellular dehydration. A typical serum sodium reference range is 136–145 mmol/L. Hyponatremia is below 136 mmol/L; mild hyponatremia is 130–135 mmol/L, moderate is 125–129 mmol/L, and severe is below 125 mmol/L. Hypernatremia is generally above 145 mmol/L. Symptoms depend not only on the value but also on how quickly it changed. Acute hyponatremia develops in less than 48 hours. Chronic hyponatremia persists longer than 48 hours, allowing the brain to adapt; that adaptation makes rapid correction dangerous because it can cause osmotic demyelination syndrome (ODS). ADH, also called vasopressin, is released when osmolality rises or effective circulating volume falls. It makes the renal collecting ducts retain water. Excess ADH can therefore dilute serum sodium, as in SIADH; inadequate ADH effect can cause large free-water losses, as in diabetes...