Fluid Volume Imbalances: Deficit, Excess And

Body fluid homeostasis depends on the balance between fluid intake and output, regulated by ADH (antidiuretic hormone), the RAAS (renin-angiotensin-aldosterone system), and atrial natriuretic peptide (ANP). Total body water is approximately 60% of body weight in adults, distributed between intracellular fluid (ICF, 40%) and extracellular fluid (ECF, 20% - composed of intravascular/plasma 5% and interstitial 15%). Fluid Volume Deficit (FVD/hypovolemia) results from loss of both water and sodium (isotonic loss from hemorrhage, vomiting, diarrhea, excessive diuresis) or water loss exceeding sodium loss (hypotonic loss from DI, sweating). Fluid Volume Excess (FVE/hypervolemia) results from excessive sodium and water retention (heart failure, kidney disease, excessive IV fluids, cirrhosis) or excessive water retention (SIADH). Third-spacing is the shift of fluid from the intravascular space into non-functional compartments (peritoneal cavity = ascites, pleural space = effusion, interstitial space = edema, pericardial space = effusion). Despite total body fluid excess, the patient may be intravascularly depleted. Edema formation depends on Starling forces: increased capillary hydrostatic pressure (HF, fluid overload), decreased plasma oncotic pressure (hypoalbuminemia from liver failure, nephrotic syndrome), increased capillary permeability (sepsis, burns, inflammation),...