The problems faced by a fish in freshwater are the opposite of those faced in saltwater. The physiological strategies fish use to deal with both these problems are called osmoregulation. This is the clue to how it works – “osmo-” refers to osmosis, the absorption of chemical substances through a membrane. In the case of fishes, the substance is water, the membrane is skin, and on the microscopic level, the cell membrane. Water naturally tries to equalize itself – thus we see that a dry sponge placed in a dish of water will gradually absorb water; the water is trying to make the sponge as wet as the dish.
In saltwater, the ocean is saltier than the fish’s blood, so water tends to flow out of the fish, trying to dilute the sea down to the same concentration as the fish blood, even as salt tends to build up inside. Obviously, this is not good for the fish – it would dehydrate, just as we do if we drink seawater. Sharks prevent this by maintaining high levels of urea in their blood. For most organisms, including ourselves, this would be lethal, since urea is poisonous; but sharks also secrete trimetylamine oxide, which counteracts the toxic effect of urea. Sharks also have a rectal gland in the intestine which secretes excess sodium and chloride.
The bull shark faces a different problem when it moves into freshwater. In freshwater, water will flow into the fish, trying to dilute its blood down to the salinity of the river or lake, while salt is constantly being lost. Obviously, this is not good for the fish either – its cells would swell up and burst like overfilled water balloons. Most freshwater fishes counteract this by constantly producing urine, thus excreting the excess water, and by taking up the trace amounts of salt available through its chloride cells; but the bull shark, having evolved for a marine environment, does not have the adaptations of freshwater species. When a bull shark moves into freshwater, it changes the concentration of urea in its blood by about 50%, and increases its urine production by 20%. Its rectal gland shuts down.
As in any fish that moves back and forth from salt to freshwater, the changes happen gradually. A shark swimming into a river mouth will encounter gradually falling salt concentrations. Its body senses the change and begins the transition; over a period of several days, the transformation to freshwater life is complete, as the shark continues out of the estuary and into the river proper. In nature, the transition is always gradual like this; if a bull shark were to be captured in the ocean and immediately dumped into a lake, its body would not be prepared for the sudden change, and it would probably die before adjusting. But once the change has been made, the bull shark can go quite far upriver – there was a case of a bull shark captured in Alton, Illinois, 1200 kn up the Mississippi River, and the Lake Nicaragua sharks, once thought to be a different species, are now know to be a freshwater population of bull sharks, still able to return to the sea by the San Juan River.