Body Fluids

Life support, inside and out

Body fluids are aqueous liquids containing the cellular and ionic components necessary for proper body function while also providing transport for a vast array of solutes and products of metabolism. Body fluids can be most generally categorized according to whether they reside inside the cell membrane or surrounding it.

Intracellular fluid (ICF) or cytosol accounts for two-thirds of all body fluid and is found within cells, separated into compartments by various organelle membranes. This fluid is mostly water, containing a complex set of dissolved and suspended proteins and other substances. Ionic concentrations (such as sodium and potassium) are critical to proper ICF function, as it is the difference between these levels and ion concentrations found outside the cell that drive several essential cellular processes, including:

  • Osmoregulation
  • Cell signaling
  • Creation of action potentials in endocrine, muscle and nerve cells

Extracellular fluid (ECF) accounts for about one-third of the body’s total fluid content, immersing cells and providing a medium for the movement of nutrients and metabolic waste products throughout the body. ECF can be sub-categorized according to composition, function and body location, and includes:

Medical analysis of body fluid samples, particularly of ECFs, is commonly done to test for the presence of microorganisms, inflammation, cancers, and a number of other conditions diagnosable or at least implicated by abnormal colors, consistencies, or detected levels of various enzymes, metabolites or other substances.


Extracellular Fluid. Wikipedia, the Free Encyclopedia. Available from URL:

Fluid. Encyclopedia Britannica, Available from URL:

The Water in You: Water and the Human Body. USGS. Available from URL:

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