Body fluid compartments, illustration. An average healthy man is around 60% water by weight, while a healthy young woman is 50% water. About two thirds of the water is found in the intracellular compartment, inside cells, as intracellular fluid. The rest is found as extracellular fluid, either in blood plasma and lymph in the intravascular compartment, or as interstitial fluid which circulates through the interstitial compartment (the spaces between the cells in tissues). Fluids are constantly exchanged between these compartments, moving across capillary walls and cell membranes.