ABSTRACT
Describe the major fluid compartments of the body in the adult.
Total body water (TBW) varies depending on age, size, gender and fat content. It is approximately 60% of body weight (BW) in the average adult male (i.e. 42 L) and 50% in the average adult female.
The remainder of the weight is made up of protein, minerals and fat.
The main components of TBW are the extracellular and intracellular compartments.
Intracellular fluid (ICF) makes up two-thirds of TBW (i.e. 28 L), and is contained within the phospholipid bilayer of the cell membrane.
Extracellular fluid (ECF) makes up one-third of TBW (i.e. 14 L). This is divided into:
interstitial fluid (ISF), which makes up 75% of the ECF (i.e. 9.5 L) and lies between cells, but outside the cell membrane
plasma, making up 25% of the ECF (i.e. 3.5 L), contained within the vasculature
transcellular fluids (TCF) (i.e.1 L), which are secreted fluids that are separated from the plasma by an epithelial layer (pleural, peritoneal, gastrointestinal fluids, CSF, intra-ocular fluids, sweat, saliva and bile), the so-called ‘third space’
Total blood volume (TBV) consists of plasma and red cell volume, and is 5–6 L.
Fluid compartments for a 70 kg maleCompartment
% BW
% TBW
% ECF
Volume (L)
TBW
±60
42
ICF
40
67
28
ECF
20
33
14
• ISF
15
10.5
75
9.5
• plasma
5
3.5
25
3.5
• TCF
<1
1.0
