Clinical meaning
Electrolytes are charged ions essential for maintaining cellular membrane potentials, nerve impulse conduction, muscle contraction, acid-base balance, and enzymatic functions. The NP must understand the pathophysiology underlying each electrolyte disorder to prescribe appropriate correction and avoid iatrogenic complications. Potassium (K+, normal 3.5-5.0 mEq/L) is the principal intracellular cation; the Na+/K+-ATPase pump maintains the transmembrane potassium gradient that establishes the resting membrane potential (~-90 mV). Hyperkalemia reduces this gradient, making cells more excitable initially but eventually blocking depolarization entirely, producing the characteristic ECG progression: peaked T waves → prolonged PR → widened QRS → sine wave → VF/asystole. Hypokalemia increases the gradient, hyperpolarizing cells and producing muscle weakness, ileus, U waves, and predisposition to torsades de pointes. Sodium (Na+, normal 135-145 mEq/L) is the primary extracellular cation and the principal determinant of serum osmolality (calculated: 2×Na + glucose/18 + BUN/2.8). Hyponatremia is a water balance disorder, not a sodium deficit: excess free water dilutes serum sodium. Classification by volume status (hypovolemic, euvolemic, hypervolemic) and osmolality (hypotonic, isotonic, hypertonic) drives the diagnostic algorithm. The rate of correction is more dangerous than the abnormality itself in chronic hyponatremia -- overcorrection causes osmotic demyelination syndrome (ODS). Calcium (Ca²+, normal total 8.5-10.5 mg/dL, ionized 4.5-5.5 mg/dL) exists in three forms: ionized (physiologically active, 50%), albumin-bound (40%), and complexed (10%). For every 1 g/dL decrease in albumin below 4.0, add 0.8 mg/dL to measured total calcium to estimate corrected calcium. Hypocalcemia causes neuromuscular excitability (Chvostek sign, Trousseau sign, carpopedal spasm, laryngospasm, seizures) while hypercalcemia causes 'stones, bones, groans, and psychiatric moans.' Magnesium (Mg²+, normal 1.7-2.2 mg/dL) is critical for enzyme function and potassium channel stability; hypomagnesemia makes hypokalemia refractory to replacement because magnesium is required for Na+/K+-ATPase function and for the renal outer medullary potassium (ROMK) channel that recycles potassium. Phosphorus (PO₄³⁻, normal 2.5-4.5 mg/dL) has an inverse relationship with calcium regulated by PTH and vitamin D.