Sharks use their gills to absorb oxygen from the water. Most sharks have five gill slits on each side of their body, behind the mouth and above the pectoral fins. Water enters the mouth of the shark, enters a canal between the mouth and the gills (the orobranchial cavity), and then passes back to the outside through the gill openings. As the water passes over the gills, oxygen is absorbed into the blood across the thin skin of the gill surface, and carbon dioxide moves into the water.
Water can flow across the gills by two mechanisms. First, as the shark is swimming it may hold its mouth open, allowing water to flow in and then out through the gill slits as the fish moves forward. Some sharks, however, can get enough oxygen when they are not swimming by gulping water into their mouth, then forcing the water out through the gills with muscular contractions of the orobranchial cavity. It is not true that all sharks must always keep swimming to breathe.