"Osmoregulation is the control of the levels of water and mineral salts in the blood." (Link). Osmoregulation relates to homeostasis, which is where cells have the correct amount of water, mineral salts, glucose and temperature. In every organism, they are trying to create a perfect homeostatic environment, where they can live and thrive as a healthy living thing. Each different organism must do different things to create this homeostatic environment. Sharks are included in these organisms.
Sharks are special because their blood is naturally isotonic to where is lives. This means that the number of solutes inside is equal to the water around the shark. Because of this, sharks are considered osmoconformers, the opposite of osmoregulators. This is usual for marine organisms.
Sharks are able to achieve this isotonic state because of the high concentration of urea and trimethylamine N-oxide (TMAO) in them. Because of this concentration, most sharks are not able to survive in freshwater environments. An exception to this rule, however is the the Bull Shark. In sharks, their kidneys are used to release the salt into the ocean, and determine how much. If a shark were placed in a freshwater environment, their kidney would not be able to adapt to the change in saltiness, causing the shark to die because they can't keep an isotonic state of living. Bull Sharks are different because in a gradual change from ocean to freshwater (example: migrating), their kidneys are able to adapt to these changes, and keep an isotonic state.