Extracellular Fluid

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The extracellular fluid [extra-, outside of] is the watery internal environment of multicellular organisms. Extracellular fluid surrounds the cells and acts as a transition between an organism's external environment and the environment inside their cells[1], the intracellular fluid. Because of this, the extracellular fluid must maintain relatively stable conditions, for example pH and concentrations of water, ions and glucose. These conditions are regulated by the mechanism of homeostasis.

The extracellular fluid is sub-divided into two:

  1. the blood and plasma and
  2. the interstitial fluid.

The plasma of the body has the same composition as the blood, with the exception of erythrocytes. The composition of the blood and the interstitial fluid is also very similar.

The amount of each type of body fluid varies. 33% of body fluid is made up of extracellular fluid, 25% of which is interstitial fluid and the other 8% is plasma. The remaining 67% present in the intracellular fluid.


  1. Silverthorn D., Johnson B., Ober W., Garrison C., Silverthorn A. (2009) Human Physiology, An Integrated Approach, 5th edition, San Francisco: Pearson Benjamin Cummings
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