Respiration

Bull Sharks are very unique when it comes to respiration, also known as the process of exchanging gases O2 and CO2. Most sharks are only able to do this in salt water, however, Bull Sharks have the ability to breath in saltwater and freshwater, in other words, they are Diadromous. Their talent relies on 4 key organs: Rectal gland, kidney, liver and gills. Since sharks need salt in order to survive, these organs work together to maintain and preserve salt levels needed in the body.

Bull Sharks are able to breath in freshwater because of a process called Osmoregulation, which is where an organism regulates internal water balance to maintain homeostasis. In simple terms, its having a balance of salt and water so your cells dont consume too less or to much salt. Other sharks dont have to worry about it because they stay in saltwater, their organs don't have to adapt to changing amounts of salt in-take. But Bull Sharks have organs that work in a special way to make sure the concentration of salts inside their cells matches the surrounding water to prevent dehydration/overhydration. Here's how they work:

Rectal Glands: Naturally excretes excess salt that accumulates from being in salt water, when in fresh water the shark stops excreting it to maintain sodium and chloride levels in its body. 

Kidneys: Since kidneys produce and remove waste from urine, they absorb any salt from it when in freshwater. The salt then goes to the blood stream for re-absorption. They also work harder to get rid of diluted urine to maintain osmotic balance. 

Liver: The liver produces urea, a waste product from metabolizing protein, in response to the lower salinity of freshwater. Urea helps the shark maintain osmotic balance by increasing the solute concentration of its blood relative to the surrounding freshwater. The liver of the bull shark is also unusually large, taking up roughly 25% of its body volume. This liver is filled with oils that are less dense than water. The decreased density helps with the negative buoyancy (tendency of an object to float/rise in fluid) that bull sharks experience in the lower density of freshwater, basically helping them swim without using too much energy in freshwater.

Gills: All sharks take O2 and salt in through gills, by moving and swimming they will naturally flow in. The gills of bull sharks are adapted to actively take up sodium and chloride from freshwater. They essentially act as a protein channel using active transport, except they try not to let any salt out when in freshwater.