Intercellular and extracellular fluids are separated by the membranes of the body's cells. An example of such fluids is the fluid inside of blood cells (intercellular fluid) and that outside the blood cells (extracellular fluid). Blood cell membranes generally are not completely permeable. That is, they do not allow the unrestricted passage of any molecule back and forth across the membrane. Rather, the construction of the membranes makes them very permeable to the passage of water, but more selective in the other types of molecules and electrolytes (e.g. ions) that can move through the membrane. This difference in permeability results in a different chemical composition for intercellular and extracellular fluids.

The different ionic concentrations on one side of a membrane versus the other side can result in osmotic flow, the flow of water from a...