Fluid compartments

Body water compartments

In an adult total body water makes up approximately 60% by weight with some variation depending on age, sex and hydration status.

Water is the most important solvent in the human body with almost all biological reactions taking place in water. Membrane lipid is the other key solvent in which biological reactions take place. Water will move freely via osmosis between compartments where as the solutes dissolved within these fluids do not.


The total body water is distributed between the two main compartments as follows:

• 65% within the intracellular fluid (ICF) compartment.
• 35% within the extracellular fluid (ECF) compartments.

These compartments are separated by the plasma membranes of the cells

The extracellular fluid (ECF) compartment can be further subdivided into :

• Intravascular fluid (20-35% or ¼ of ECF): primary component of plasma
• Interstitial fluid (65-80% or ¾ of ECF): lies outside blood vessels

The barrier between these two flu­ids are the walls of the capillaries.