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Ok while watching a show on the Discovery channel this is what I found out:
Salt water fish have a lower salt concentration in their body than the
surrounding ocean. Because of this salt enters their body to equalize. They
are built so that they can excrete high levels of salt through their urine
and through their gills. This is how they get rid of salt.
Salt water fish that can swim in fresh water (i.e. salmon when they spawn)
have a higher level of salt in their bodies than the surrounding fresh
water. They have mechanisms to hold in the salt and also constantly excrete
the excess water that their body absorbs via urine.
The reasons humans die when they drink sea water is because the body can
only excrete urine with a concentration of 2% salt. Sea water is at a
concentration of 3% salt. So as you drink sea water you add 1% salt to your
body because you can only get rid of 2% of it via your urine. I would think
that fresh water plants are built the same way.
Crazy fact is that sharks store urea (a type of salt) in their body to
equalize their salt content against the ocean.