endocrine and metabolic disorders
Water and Sodium Balance
Introduction
Body fluid volume and electrolyte concentration are normally maintained within very narrow limits despite wide variations in dietary intake, metabolic activity, and environmental stresses. Homeostasis of body fluids is preserved primarily by the kidneys. Water and sodium balance are closely interdependent. Total body water (TBW) is about 60% of body weight in men (ranging from about 50% in obese people to 70% in lean people) and about 50% in women. Almost two thirds of TBW is in the intracellular compartment (intracellular fluid, or ICF); the other one third is extracellular (extracellular fluid, or ECF). Normally, about 25% of the ECF is in the intravascular compartment; the other 75% is interstitial fluid (see figure Fluid compartments in an average 70-kg man ). Total body water=70 kg×0.60=42 L (280 mOsm/kg [280 mmol/kg]). The major intracellular cation is potassium. The major extracellular cation is sodium. Concentrations of intracellular and extracellular cations are as follows: Intracellular potassium concentration averages 140 mEq/L (140 mmol/L). Extracellular potassium concentration is 3.5 to 5 mEq/L (3.5 to 5 mmol/L). Intracellular sodium concentration is 12 mEq/L (12 mmol/L). Extracellular sodium concentration averages 140 mEq/L (140 mmol/L). 