The extracellular fluid pertains to all body fluid outside the cell(s). In humans, the total body water composition is made up mainly of intracellular fluid (67%) and extracellular fluid (26%). The extracellular fluid, in turn, is composed of blood plasma, interstitial fluid, lymph and transcellular fluid (e.g. cerebrospinal fluid, synovial fluid, aqueous humour, serous fluid, gut fluid, etc.). The interstitial fluid and the blood plasma are the major components of the extracellular fluid.
Because of its composition, it, therefore, serves as a delivery medium for nutrients and waste products. It is also a crucial site for various homeostatic mechanisms. For instance, in humans, the normal glucose concentration of extracellular fluid that is regulated by homeostasis is approximately 5 mM. The pH of extracellular fluid is tightly regulated by buffers around 7.4. The volume of extracellular fluid is typically 15 L (i.e. 12 L is interstitial fluid and 3 L is plasma).
Word origin: Latin extrā ("outside of") + cellulāris, cellul(a), ("live cell")