The intracellular fluid makes up over two thirds of the body's water. This is located in the cytoplasm of the cells. The volume of this fluid compartment MUST be kept within precise limits, along with the blood plasma (which as described above is the intravascular fluid). The fluid in the cytoplasm is directly controlled by the concentration of solutes in the interstitial fluid (most often salts) that surrounds the cells.
When we describe the interstitial fluid as isotonic with the intracellular fluid in the cytoplasm, what could we possibly meaning? Going back to the greek roots of the words isos, "equal," and tonos, "tension," we can see that water will not flow in or out of the cell membranes because the concentration of the solute is the same on both sides.