Why don Fishes face OSMOSIS in water?
Osmoregulation is the regulation of water and ion concentrations in the body. Keeping this regulation precise is critical in maintaining life in a cell. Balance of water and ions is partly linked to excretion, the removal of metabolic wastes from the body. Freshwater fishes excrete copious amount of hypotonic urine. Marine water fishes drink sea water and excrete salts through the chloride cells present in the gill epithelium. Cartilaginous fishes such as sharks, rays, and skates, have plasma that is approximately isosmotic to seawater. This unusually high osmotic concentration (compared to that of other vertebrates) is maintained by high levels of urea and trimethylamine oxide (TMAO) in the blood. In most vertebrates, levels of urea this high would damage proteins, but the presence of the TMAO helps to stabilize these protein molecules against the adverse effects of urea. Excess inorganic electrolytes, such as Na+ and Cl- which diffuse into the blood at the gills, are excreted by way
