The body fluids found outside the cells, such as plasma (the liquid portion of blood and lymph), and interstitial fluid, are generically termed extracellular fluid. Three-fourths of all extracellular fluid is stored as interstitial (between cells) fluid, and one-fourth as plasma. Cells in the tissues are separated from one another by spaces called interstitium, usually filled with a fibrous complex termed extracellular matrix. This structure is formed by collagen and elastin fibers and elongated glycoproteins, also containing proteoglycan filaments that form a hydrophilic gel. Whereas collagen and elastin fibers have a structural function, proteoglycan form a network of coiled thin filaments, mainly constituted by hyaluronic acid, which entrap a viscous fluid known as interstitial fluid, rich in nutrients, ions, hormones, and other molecules necessary to cellular function. The interstitial fluid has almost the same composition of plasma, except for having much lower protein concentrations, and it...