Like other fish, sharks extract oxygen from seawater as it passes over their gills. Some sharks have a modified slit called a spiracle located just behind the eye, which is used in respiration. Due to their size and the nature of their metabolism, sharks have a higher demand for oxygen than most fish and they can not rely on ambient water currents to provide an adequate supply of oxygenated water. If a shark were to stop swimming, the water circulation would drop below the level necessary for respiration and the animal could suffocate. The process of ensuring an adequate flow of the gills by forward movement is known as ram ventilation. Some sharks, such as the blacktip reef shark, Carcharhinus melanopterus, and nurse shark, Ginglymostoma cirratum, can pump water over their gills as they rest. There are also recorded instances, as in certain caves along the Yucatan coast, where sharks rest on the cave floors and allow the fresh water outflow to pass over them. The outflow is strong enough to still allow for respiration; it is believed that the reason for this behaviour is that the fresh water helps remove parasites. The grey nurse shark, Carcharias taurus, is known to gulp air at the surface and store it in its stomach to provide additional buoyancy. Some sharks, if inverted, enter a natural state of tonic immobility - researchers use this condition for handling sharks safely.
In contrast to bony fishes, sharks do not drink seawater, instead they retain high concentrations of waste chemicals in their body to change the diffusion gradient so that they can absorb water direct from the sea. This adaptation prevents most sharks from surviving in freshwater, and they are therefore confined to a marine environment. A few exceptions to this occur, as with the bull shark, which has developed a way to change its kidney function to excrete large amounts of dilute urine.