Clinical meaning
Total body water comprises approximately 60% of adult body weight and is distributed between two major compartments: the intracellular fluid (ICF, approximately 40% of body weight, found inside cells) and the extracellular fluid (ECF, approximately 20% of body weight). The extracellular fluid is further divided into intravascular fluid (plasma, approximately 5% of body weight, contained within blood vessels), interstitial fluid (approximately 15% of body weight, surrounding cells in tissues), and transcellular fluid (a small volume found in specialized spaces such as cerebrospinal fluid, synovial fluid, and pleural fluid). Water movement between compartments is governed by osmotic pressure (determined by solute concentration, primarily sodium in the ECF and potassium in the ICF) and hydrostatic pressure (the mechanical force exerted by fluid against vessel or membrane walls). Osmosis drives water from areas of low solute concentration to high solute concentration across semipermeable membranes until equilibrium is reached. Normal serum osmolality ranges from 275 to 295 mOsm/kg and is primarily determined by sodium concentration. The body maintains fluid balance through several regulatory mechanisms. The hypothalamus contains osmoreceptors that detect changes in serum osmolality: when osmolality rises (indicating dehydration), the hypothalamus stimulates thirst and triggers the posterior pituitary to release antidiuretic hormone (ADH/vasopressin). ADH acts on the renal collecting ducts to increase water reabsorption, producing concentrated urine and reducing water loss. When blood volume or pressure decreases, the juxtaglomerular cells of the kidneys release renin, activating the renin-angiotensin-aldosterone system (RAAS). Angiotensin II causes vasoconstriction (raising blood pressure) and stimulates aldosterone release from the adrenal cortex. Aldosterone acts on the renal distal tubules and collecting ducts to increase sodium and water reabsorption while excreting potassium, thereby expanding blood volume. Atrial natriuretic peptide (ANP), released from atrial myocytes when the atria are stretched by excessive volume, opposes the RAAS by promoting sodium and water excretion. Fluid volume deficit (FVD/dehydration/hypovolemia) occurs when fluid output exceeds fluid intake or when fluid shifts from the intravascular space to other compartments. Causes include vomiting, diarrhea, hemorrhage, excessive diuresis, burns, and inadequate oral intake. Clinical findings include thirst, decreased skin turgor, dry mucous membranes, concentrated urine (high specific gravity), tachycardia, hypotension, and decreased urine output. Fluid volume excess (FVE/hypervolemia/fluid overload) occurs when fluid intake or retention exceeds output. Causes include heart failure, renal failure, excessive IV fluid administration, and conditions that increase aldosterone (liver cirrhosis). Clinical findings include weight gain, peripheral edema, pulmonary edema (crackles, dyspnea, orthopnea), jugular venous distension, and elevated blood pressure. Third-spacing is the pathological shift of fluid from the intravascular compartment into a non-functional interstitial space or body cavity (ascites, pleural effusion, tissue edema after burns or surgery). Third-spacing creates an intravascular deficit despite an overall increase in total body water, leading to the paradox of peripheral edema with concurrent dehydration and hypotension. Insensible fluid losses are water losses that occur continuously but are not easily measured: approximately 300-500 mL/day through the skin (evaporation, not sweat) and 300-400 mL/day through the lungs (exhaled water vapor). These losses increase with fever (add 100-150 mL per degree Celsius above 37 degrees), tachypnea, mechanical ventilation with dry gas, and low humidity environments. Accurate fluid balance monitoring through intake and output measurement and daily weights is essential for the practical nurse to identify fluid imbalances early and report them promptly.