(a.k.a. zona occludens) Tight junctions are most important in restricting transport across an epithelial membrane, for instance in the wall of a capillary. They vary somewhat in how "tight" they are. Depending on the extent and complexity of the fusions,  some allow water and small molecules to pass through gaps between them (e.g. in the kidneys), and others allow very little to pass. In this way tight junctions allow these epithelial sheets to act as semipermeable membranes. [Diagram]