Key Concepts
Introduction
Fluid balance is a foundational nursing competency. Maintaining homeostasis requires precise management of fluid intake and output, electrolyte balance, and vascular volume. For NCLEX-RN Canada, fluid balance knowledge is tested in virtually every clinical specialization — from cardiac and renal nursing to surgical care, obstetrics, and critical care. Body fluid compartments: - Total body water (TBW): ~60% of body weight in adult men, ~50% in women - Intracellular fluid (ICF): 2/3 of TBW — inside cells - Extracellular fluid (ECF): 1/3 of TBW — outside cells - Intravascular: plasma (within blood vessels) - Interstitial: between cells and vessels (lymphatics) - Transcellular: CSF, pleural, pericardial, peritoneal, synovial fluid Osmolality controls fluid movement: - Water moves from areas of low osmolality → high osmolality (osmosis) - Normal serum osmolality: 280–295 mOsm/kg - Sodium is the primary ECF osmole — changes in Na⁺ drive water movement between ICF and ECF Normal daily I&O (adult): - Intake: ~2,500 mL/day (1,200 mL oral fluids, 1,000 mL food water, 300 mL metabolic water) - Output: ~2,500 mL/day (urine 1,200–1,500 mL, stool 200 mL, insensible 800–1,000 mL...