Osmotic and Ionic Regulation

Osmoregulation refers to the processes by which the osmotic pressure of the body fluids and the water volume in an animal are held relatively constant. The osmotic pressure of fish blood is mostly provided by inorganic salts and is approximately one third that of seawater, with marine fish having slightly more salt in their blood than freshwater species. Because gills are permeable to water as well as oxygen, CO₂, etc., there is an osmotic flow of water out of the marine fish because the salt concentration of its blood is less than that of the ocean, and into the freshwater fish because of an opposite osmotic gradient. The diffusion gradient for sodium chloride (the primary salt) is in the direction opposite to the osmotic diffusion of water across the gill epithelium. Figure 1 illustrates these major ion and water fluxes in typical teleost fish. Figure 1 Comparison of ion and water fluxes in a marine and a freshwater fish. Ion fluxes are in m/W/kg/h. Thin arrows indicate passive processes, thick ones active transport. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780203718896/8cdb9437-27bb-4cf0-b11e-45c8380199ff/content/fig7_1.tif"/> (Data from Fletcher, C. R., J. Comp. Physiol., 124,157,1978; Eddy, F. B. and Bath, R. N., J. Exp. Biol., 83, 181, 1979).