The lipid bilayer is not miscible with either the extra- cellular fluid or the intracellular fluid. Therefore, it constitutes a barrier against movement of water molecules and watersoluble substances between the extracellular and intracellular fluid compartments. However, as demonstrated in Figure 4-2 by the leftmost arrow, a few sub- stances can penetrate this lipid bilayer, diffusing directly through the lipid substance itself; this is true mainly of lipid-soluble substances, as described later.