Fluid balance is a foundational nursing competency and one of the most broadly tested topics on the NCLEX-RN. Maintaining homeostasis requires precise management of fluid intake and output, electrolyte balance, and intravascular volume. Fluid balance knowledge is applied in virtually every clinical specialization, from cardiac and renal nursing to surgical care, obstetrics, pediatrics, and critical care, because almost every acutely ill patient is either volume depleted, volume overloaded, or at risk of becoming so. Body fluid compartments: - Total body water (TBW) is about 60% of body weight in adult men and about 50% in women. - Intracellular fluid (ICF): two-thirds of TBW, located inside cells. - Extracellular fluid (ECF): one-third of TBW, located outside cells, subdivided into the intravascular plasma within blood vessels, the interstitial fluid between cells, and the transcellular fluids such as cerebrospinal, pleural, pericardial, peritoneal, and synovial fluid. Osmolality controls fluid movement. Water moves by osmosis from areas of lower osmolality toward areas of higher osmolality. Normal serum osmolality is 280-295 mOsm/kg, and sodium is the primary ECF osmole, so changes in serum sodium drive water...