Clearly, osmoregulation is a vital function affecting all aspects of fish health. If we look at the structure of the gill or gut under a powerful microscope we would see the tissue that lines these structures, the epithelium, is very thin, usually only one cell thick. Such areas are rich in blood vessels with very thin walls. The combined thickness of epithelium and blood vessels is less than the size of a full stop. A substance will diffuse from an area where it is more concentrated to an area where it is less concentrated.
Osmosis is a special case of diffusion.
If the gill and gut surfaces were unfolded and laid out flat we would have a massive area, much greater than the combined surface area of the skin and scales. At least half of the exposed surface of a fish is permeable to small particles. It is this permeability that leads us to describe a fish as an open system, constantly exchanging particles between its inside and the surrounding water. Now, perhaps, it is possible to understand why changes in the surrounding water have such an important effect on fish. Diff and 'Oz' The permeability of the gills and gut and, to a lesser extent, the skin, which are all in close contact with the water, leads to a further challenge for aquatic animals.