Unlike bony marine fish, sharks don't drink water. Osmosis is a process in which water flows across a semi-permeable membrane from an area of higher concentration of water to an area of lower concentration of water, effectively moving to equalize concentrations. In the case of sharks vs. sea water, water flows from the sea water into the even-saltier shark via gill tissues. This system makes most sharks unable to live in fresh water. The exceptions are bull sharks and some stingrays, which can lower their salt concentrations to enter fresh water.
Sharks can secrete any extra salts through the rectal gland.
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