Sharks, skates, and rays generally elevate the concentration of solutes in their blood to levels slightly hyperosmotic to seawater. However, inorganic ions constitute only about one- third of the solute in shark's blood; the remainder consists of the organic molecules urea and trimethylamine oxide, or TMAO. As a consequence of being slightly hyperosmotic, sharks slowly gain water through osmosis, that is, Wo is slightly positive. The water that diffuses into the shark, mainly across the gills, is pumped out by the kidneys and exits as urine. Sodium, because it is maintained at approximately two-thirds its concentration in seawater, diffuses into sharks from seawater across the gill membranes and some sodium enters with food. Sharks excrete excess sodium mainly through a specialized gland associated with the rectum called the salt gland. The main point here is that sharks and their relatives reduce the costs of osmoregulation by decreasing the osmotic gradient between themselves and the external environment.
In contrast to most marine invertebrates and sharks, marine bony fish have body fluids that are strongly hypoosmotic to the surrounding medium. As a consequence, they lose water to the surrounding seawater, mostly across their gills. Marine bony fish make up these water losses by drinking seawater. However, drinking adds to salt influxes through their gills. The fish rid themselves of excess salts in two ways. Specialized "chloride" cells at the base of their gills secrete sodium and chloride directly to the surrounding seawater, while the kidneys excrete magnesium and sulfate. These ions exit with the urine. The urine, because it is hypoosmotic to the body fluids of the fish, represents a loss of water. However, the loss of water through the kidneys is low because the quantity of urine is low.
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