How does an isotonic/ hyperotic or hypotonic fish regulate its water balance (=maintain homeostatsis)?
Then isosmtic animal does n have to expend energy, overcoming an osmotic gradient. this startegy is not witho costs, however. thought the total concentration of solutes is the same inside and outside the animal, there are differences in the concentrations of some individual solutes.these concentration differentials can only be determined by active transport, which comsumes some energy
as a consequence of being slightly hyperosmotic, sharks slowly gain water though osmosis, that is Wo
is slightly paotive . the water that diffusee into the shark mainly across the gills, is pumped out by the kidneys and exits as urine. soduim, because it is maintained at approximately 2/3 its concentration n salt water, diffuses into the shark from seawater across the gill memebranes and some soduim enter with food. sharks excrete excess soduim maily through a specialized gland associated with the rectum called the salt gland. the main point here is that the sharks and thier relavites reduce the costof osmoregulation, regulation of internal salt and water concentration, by decreasing the osomtic gradient between themselves and the external enviroment.
sharks and marine bony have body fluids that are stongly hypoosmotic to the surronding medium. as a consequence, they lose water to the surrouding sea water, mostly across their gills. marine bony fish make uo these water losses by drinking seawater. however drinking icreases salt gain. the fish rid themselves if excess salts in 2 ways. specialized "chloride" cells at the base of their gills secrect soduim ans chloride directly to the surrounding seawater , while the kidneys excrete magnesium and sulfate. howere the loss of water through the kidnesys is low because the volume of urine is low.
freshwater fish are hyperosomtic: they have body fluids that contain more salt and less water than the surrounding medium. as a conqunece, water floods inward and salt diffuse outward across their gills. fressh water fish excrete water as large quantities of dulite urine. they replace the salts they lose to the external environement in 2 ways. chloride cells at teh base of the gill flaments absorbs sodium and chloride from the water, while other salts are ingested with food.