About 60 percent of the adult human body is fluid, mainly
a water solution of ions and other substances. Although
most of this fluid is inside the cells and is called intracellular
fluid, about one third is in the spaces outside the cells
and is called extracellular fluid. This extracellular fluid is
in constant motion throughout the body. It is transported
rapidly in the circulating blood and then mixed between
the blood and the tissue fluids by diffusion through the
capillary walls.