Pathophysiology
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:...