Extracellular fluid is composed of blood plasma and interstitial fluid, or the fluid between cells. The total composition of extracellular fluid is 20 percent blood plasma and 80 percent interstitial fluid.
Extracellular fluid is found outside of cells throughout the body in enclosed compartments. It differs from the fluid that is found inside of cells and tissues in composition. In total, extracellular fluid comprises roughly one-third of the body's fluids; the other two-thirds is intercellular. The interstitial fluid and the blood plasma found in extracellular fluid are almost identical in composition, but blood plasma contains larger amounts of dissolved proteins than interstitial fluid.