Perioperative electrolyte and fluid balance

Total body water (TBW) is consistently 70% of the lean body weight across age and sex ranges but varies as a percentage of actual bodyweight between groups and individuals owing to varied deposition of adipose tissue, which contains less water than muscle. TBW is 75% of total body weight in neonates, 70% in infants and 45% in the elderly. The average adult male (70 kg) has 60% of total body weight (42 litres) as water and an adult female (50 kg) has 55% (27 litres). This water is distributed between the extracellular (ECF) and intracellular (ICF) compartments. Extracellular fluid is, by definition, fluid outside the cells and comprises one-third (14 litres) of TBW. It is further subdivided into: (i) intravascular fluid (i.e. plasma), which fills the vascular system and, together with the red blood cells, constitutes the total blood volume, which is about 5% (3.5 litres) of body weight; and (ii) interstitial fluid, which is fluid outside the vascular system. It is mostly found in tissues adjacent to the microvascular circulation. More distant connective tissues, the so-called ‘third space’, remain relatively dry. Intracellular fluid is contained within cells and comprises about two-thirds of TBW. Transcellular fluids are fluids (total 0.6% of TBW) contained in body cavities, for example CSF, ocular, synovial, peritoneal and pleural fluids.