In some animals, including mammals, the extracellular fluid can be divided into two major subcompartments, interstitial fluid and blood plasma. The extracellular fluid also includes the transcellular fluid; making up only about 2.5 percent of the ECF.
The volume of ECF is typically 15L (of which 12L is interstitial fluid and 3L is plasma).
Interstitial Fluid makes up 16% of your body weight and blood plasma 4% of your body weight.
Extracellular Fluid, in conjunction with intracellular fluid, helps control the movement of water and electrolytes throughout the body. In order to maintain osmotic balance, the extracellular compartments of a mammal's body must be able to excrete and absorb water to and from the environment. Inorganic ions must also be exchanged between ECF and the external environment to maintain homeostasis. These exchanges occur across specialized epithelial cells and through a filtration process in the kidneys. Extracellular Fluid allows a solute balance between the outside and the inside of the cell, or to produce a gradient (which is commonly used to initiate diffusion, osmosis, facilitated diffusion, active transport, etc.). Blood flow delivers oxygen to arterioles, capillaries, and venules and clears waste products of metabolism. A second flow of fluid important to organ function is filtration of plasma across the microcirculation into the interstitial lymphatic compartment of the ECF. Multiple mechanisms regulate this flow and the distribution of ECF between the plasma volume and interstitial lymphatic fluid compartments.
- Main cations:
- Main anions:
It is poorer in proteins, as compared to intracellular fluid.
- Ionic Composition
Na+ 150 (mM)
K+ 5 (mM)
Cl- 110 (mM)
Extracellular Fluid is the body fluid that is not contained in cells. It is often secreted by cells to provide a constant environment for cellular operation.
ECF is found in blood, lymph, body cavities lined with serous (moisture-exuding) membrane, cavities and channels of the brain and spinal cord, and in muscle and other body tissues.
- Interstitial Fluid
the extracellular fluid bathing most tissues, excluding the fluid within the lymph and blood vessels.
- Blood Plasma
the yellow or gray-yellow, protein-containing fluid portion of blood in which the blood cells and platelets are normally suspended.
See also 
1. http://www.thefreedictionary.com/extracellular+fluid 2. http://medical-dictionary.thefreedictionary.com/extracellular+fluid 3. http://www.britannica.com/EBchecked/topic/199041/extracellular-fluid 4. http://www.biology-online.org/dictionary/Extracellular_fluid 5. http://www.expertconsultbook.com/expertconsult/op/book.do?method=display&type=bookPage&decorator=none&eid=4-u1.0-B978-1-4160-3675-3..50009-5--cesec54&isbn=978-1-4160-3675-3
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