Fluid Balance

Total body water (TBW) comprises approximately 60% of body weight in adult males and 50% in females, distributed between intracellular fluid (ICF, 2/3 of TBW) and extracellular fluid (ECF, 1/3 of TBW). The ECF is further divided into intravascular (plasma, 25% of ECF) and interstitial (75% of ECF) compartments. Fluid movement between compartments is governed by Starling forces: capillary hydrostatic pressure pushes fluid out of vessels, while plasma oncotic pressure (primarily from albumin) pulls fluid in. Serum osmolality (normal 275-295 mOsm/kg) is tightly regulated by ADH from the posterior pituitary (released in response to increased osmolality or decreased blood volume) and the RAAS (activated by decreased renal perfusion). Sodium is the primary determinant of ECF osmolality and volume — hyponatremia represents relative water excess while hypernatremia represents relative water deficit. The clinician must differentiate between volume depletion (loss of isotonic fluid), dehydration (pure water loss), and fluid overload, as each requires different management strategies. Disorders of fluid balance frequently coexist with electrolyte and acid-base disturbances requiring systematic evaluation.