Sodium Imbalance

Sodium (Na+) is the dominant extracellular cation and the principal determinant of extracellular fluid (ECF) osmolality and water distribution. Sodium imbalances are among the most common electrolyte abnormalities in hospitalized patients and a perennial NCLEX-RN focus. Because sodium controls serum osmolality, changes in serum Na+ directly drive water to shift between the intracellular and extracellular compartments, which explains why both hyponatremia and hypernatremia produce prominent neurological effects as brain cells swell or shrink. Normal serum sodium: 135-145 mEq/L. - Hyponatremia: Na+ < 135 mEq/L (cells swell as water moves in; cerebral edema). - Hypernatremia: Na+ > 145 mEq/L (cells shrink as water moves out; risk of intracranial bleeding). Why sodium equals water regulation. Serum osmolality is approximately 2 x Na+ + glucose/18 + BUN/2.8, with a normal value of 280-295 mOsm/kg. Antidiuretic hormone (ADH, vasopressin) is released when osmolality rises or effective blood volume falls; the kidneys then retain free water, which dilutes serum Na+ and lowers osmolality. This single feedback loop is the key to understanding the two most-tested sodium disorders: SIADH (inappropriately excessive ADH causing water retention and...