sodium
Summary
Sodium (Na+) is the primary extracellular cation, critical for maintaining plasma osmolality, fluid balance, and neuromuscular function. Normal serum concentration is 135-145 mEq/L. Disorders of sodium (hyponatremia/hypernatremia) primarily reflect water balance abnormalities rather than sodium content alone, and are common high-yield topics on USMLE exams.
Detail
Sodium is regulated mainly through water homeostasis via ADH (vasopressin) and thirst mechanisms, along with the renin-angiotensin-aldosterone system (RAAS) which regulates sodium reabsorption in the kidney (primarily distal nephron via ENaC channels influenced by aldosterone). Sodium is essential for generating action potentials via voltage-gated Na+ channels, maintaining resting membrane potential, and driving secondary active transport (e.g., Na+/K+-ATPase, Na+/glucose cotransporters).
Hyponatremia (Na+ <135 mEq/L): Usually reflects excess free water relative to sodium, not sodium deficiency. Classified by volume status: hypovolemic (diarrhea, diuretics, Addison's), euvolemic (SIADH, hypothyroidism, psychogenic polydipsia), hypervolemic (heart failure, cirrhosis, nephrotic syndrome). Symptoms are primarily neurologic (cerebral edema): confusion, seizures, coma. Rapid correction risks osmotic demyelination syndrome (central pontine myelinolysis), so correction should not exceed 8-10 mEq/L per 24 hours.
Hypernatremia (Na+ >145 mEq/L): Reflects free water deficit relative to sodium. Causes include diabetes insipidus (central or nephrogenic), inadequate water intake (elderly, infants), excessive water loss (diarrhea, sweating), or iatrogenic sodium administration. Symptoms include lethargy, irritability, seizures due to cellular dehydration. Correction should also be gradual to avoid cerebral edema from overly rapid fluid shifts.
Clinical pearls: Calculate serum osmolality (2×Na + glucose/18 + BUN/2.8) to assess tonicity. Urine sodium and osmolality help differentiate causes of hyponatremia. Sodium is central to fluid/electrolyte management in nearly every organ system disease process, including renal, cardiac, endocrine, and neurologic disorders.
Sources
- First Aid for the USMLE Step 1
- Harrison's Principles of Internal Medicine
- Guyton and Hall Textbook of Medical Physiology
- UpToDate: Diagnostic evaluation of adults with hyponatremia
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