Osmoregulation = When a fish, or a shark, or any other organism has to control, or regulate, the amount of osmosis going on in its body, it is called osmoregulation. That word is a combination of osmosis and regulate.

​

Most sharks live in the ocean, although some sharks, like the Bull Shark, can swim back and forth between salt and freshwater! When sharks are in the ocean, they have to deal with all that salt and find a way to keep the water they need inside their bodies. Bony fish deal with this by replacing all the water they lose, but sharks have evolved a trick that helps them avoid losing the water in the first place! Sharks make the water in their bodies just as salty as the ocean water around them.

​

Sharks have special chemicals in their bodies: urea and Trimethylamine-N-Oxide (TMAO)

Urea and TMAO help by making the inside of the shark’s body saltier. If the inside of the body is just as salty as the water around it, osmosis won’t take place. This means the shark won’t get dehydrated by losing too much water.

 

Another thing that make sharks good at osmoregulation is a special organ called the rectal gland. Rectal glands are only found in cartilaginous fish like sharks, skates, or stingrays. This organ is on the inside of the shark's body, located at the end of the intestine. The rectal gland gathers even more salt from the shark's blood than the kidneys and allows them to excrete excess salt that their kidneys don't filter out. This makes sharks well suited for life in salt water.