Marine sharks hold high concentrations of urea and trimethylamine oxide in their blood, which keeps them close to the salinity of seawater and stops water flooding in. In fresh water that same chemistry becomes a liability. A bull shark responds by excreting far more dilute urine, reducing blood urea substantially, and cutting salt loss through the rectal gland, a shark organ dedicated to secreting excess sodium chloride.
The result is real range. Bull sharks have been recorded roughly 1,100 kilometres up the Mississippi near Illinois, far up the Amazon, and in Lake Nicaragua, which was long thought to hold a landlocked species until tagging showed the sharks were moving through the San Juan River.
Osmoregulation is expensive, and bull sharks do not appear to complete their whole lives in fresh water. Adults return to the sea, and freshwater records are dominated by juveniles using low-salinity nurseries where large predators are scarce.