Pathophysiology
Clinical meaning
Potassium is the primary intracellular cation, with 98% of total body potassium (~3,500 mEq) residing inside cells and only 2% (~70 mEq) in the extracellular fluid, maintaining a normal serum concentration of 3.5-5.0 mEq/L. This steep concentration gradient across cell membranes (intracellular K+ ~150 mEq/L vs extracellular ~4 mEq/L) is maintained by the Na+/K+-ATPase pump, which actively transports 3 Na+ out and 2 K+ into cells with each cycle, and is essential for establishing the resting membrane potential (-70 to -90 mV) that governs cardiac, neural, and skeletal muscle excitability. Potassium homeostasis is regulated through two mechanisms: internal balance (transcellular shifting) and external balance (renal excretion). Internal balance is modulated by insulin (drives K+ into cells via Na+/K+-ATPase stimulation), beta-2 adrenergic catecholamines (shift K+ intracellularly), aldosterone (promotes both renal excretion and cellular uptake), and acid-base status (acidosis causes H+/K+ exchange pushing K+ out of cells, raising serum K+; alkalosis does the opposite). External balance is predominantly renal: 90% of daily potassium excretion occurs through the kidneys, primarily via secretion in the cortical collecting duct principal cells regulated by aldosterone. Aldosterone...