Pathophysiology
Clinical meaning
The human body is approximately 60% water by weight (higher in infants, lower in elderly and obese individuals), distributed between two major compartments: intracellular fluid (ICF, approximately 40% of body weight -- the fluid inside cells) and extracellular fluid (ECF, approximately 20% of body weight -- subdivided into intravascular/plasma at 5% and interstitial fluid at 15%). The movement of water between compartments is governed by osmotic and hydrostatic pressures. Osmolality (the concentration of dissolved particles) determines water movement -- water moves from areas of lower osmolality to higher osmolality by osmosis. Normal serum osmolality is 275-295 mOsm/kg. Electrolytes are minerals that carry electrical charges and are essential for cellular function: sodium (Na+, 135-145 mEq/L -- the primary ECF cation, regulates fluid volume and osmolality), potassium (K+, 3.5-5.0 mEq/L -- the primary ICF cation, critical for cardiac conduction, muscle contraction, and nerve impulse transmission), calcium (Ca2+, 2.1-2.6 mmol/L -- essential for bone formation, muscle contraction, blood clotting, and nerve function), magnesium (Mg2+, 0.75-1.05 mmol/L -- cofactor for enzymatic reactions, neuromuscular function), and phosphate (PO4, 0.81-1.45 mmol/L -- inverse relationship with calcium)....