Sodium (Na+) is the primary extracellular cation (normal 135-145 mEq/L) and the major determinant of serum osmolality. Sodium imbalances are fundamentally water balance disorders. Hyponatremia (<135 mEq/L) occurs when there is excess water relative to sodium. In dilutional hyponatremia (most common), total body water increases while sodium may be normal or even elevated. SIADH (syndrome of inappropriate ADH) is a classic cause - excessive ADH causes water retention, diluting sodium while maintaining euvolemia or mild hypervolemia. Other causes: heart failure (low effective circulating volume triggers ADH), cirrhosis, hypothyroidism, and polydipsia. Cerebral effects are critical: as serum osmolality drops, water moves into brain cells by osmosis, causing cerebral edema. Acute hyponatremia (<48 hours) is more dangerous than chronic because the brain has not had time to adapt (lose intracellular solutes). Hypernatremia (>145 mEq/L) occurs when water loss exceeds sodium loss or sodium intake exceeds water. Diabetes insipidus (central or nephrogenic) produces dilute urine despite high serum osmolality due to absent or ineffective ADH. Water moves OUT of brain cells, causing cellular shrinkage, tearing of bridging veins, and potential intracranial hemorrhage. Correction...