Basic Concepts of Fluid and Electrolyte Therapy

ever, contains large anions such as protein and glycogen, which cannot
escape and, therefore, draw in K + ions to maintain electrical neutrality
(Gibbs-Donnan equilibrium). These mechanisms ensure that
Na + and its balancing anions, Cl – and HCO 3–
, are the mainstay of ECF
osmolality, and K + has the corresponding function in the ICF.
Total body water (60% body weight)
Extracellular fluid (20% body weight)
Interstitial space
(14% body weight)
Na + 140 mmol/l
K + 4 mmol/l
Cell membrane
Capillary membrane
Intravascular space (6% body weight)
Na + 140 mmol/l
K + 4 mmol/l (Plasma) Red blood cells
Intracellular space
(40% body weight)
Na + 8 mmol/l
K + 151 mmol/l
Figure 1: Body fluid compartments with approximate electrolyte con centrations.
Red blood cells (haematocrit) account for approxi mately 45% of total
intravascular volume.
The ECF is further divided into the intravascular (within the circulation)
and the interstitial (extravascular fluid surrounding the cells)
fluid spaces. The intravascular space (blood volume = 5-7% of body
weight) has its own intracellular component in the form of red
(haematocrit = 40-45%) and white cells and an extracellular element
in the form of plasma (55-60% of total blood volume).
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