Key Concepts
Introduction
Sodium (Na⁺) is the dominant extracellular cation, regulating extracellular fluid (ECF) osmolality and distribution. Sodium imbalances are among the most common electrolyte abnormalities in hospitalized patients. Because sodium determines serum osmolality, changes in Na⁺ directly cause water to shift between intracellular and extracellular compartments — explaining the neurological effects of both hypo and hypernatremia. Normal serum sodium: 135–145 mEq/L - Hyponatremia: Na⁺ < 135 mEq/L - Hypernatremia: Na⁺ > 145 mEq/L Why sodium = water regulation. Serum osmolality ≈ 2 × Na⁺ + glucose/18 + BUN/2.8 (normal: 280–295 mOsm/kg). ADH (antidiuretic hormone, vasopressin) is released when osmolality rises or blood volume drops → kidneys retain water → dilutes Na⁺ → osmolality falls. This system is the key to understanding both SIADH (too much ADH) and diabetes insipidus (too little ADH). On the exam, writers often pair stable-sounding options with unstable data—notice the mismatch before you commit. If the stem names a license or role, reread that line; scope errors are classic trap answers even when the clinical topic is familiar. Run a 60-second scan: breathing work and oxygenation, perfusion and...