Answer: Sharks store enough urea to match the total solute concentration of the surrounding seawater
Explanation: Sharks (along with amphibians and coelocanths) are osmoconformers and ion regulators.
Urea is one of the ions that sharks utilize. The majority of species already produce urea, which is quite simple to create and, from an osmotic standpoint, functions perfectly as an ion. The main drawback of urea is that it destabilizes many enzymes, however this is remedied by using another ion, tri-methyl amine oxide (TMAO).
When a shark dies, the urea in their bodily fluids turns into the deadly and foul-smelling ammonia.
Although sharks are frequently considered to be "primitive" creatures, they actually have a sophisticated and successful way of surviving in salt water. Another astonishing characteristic of sharks is their usage of a waste product to maintain osmotic balance.