All sharks have five to seven pairs of gills on the side of the head.  Gas exchange occurs at the gills and oxygenated water must always be flowing over the gill filaments for respiration to occur.  Water enters through the mouth of the animal, into the pharynx, over the gills and exits through the gill slits.  Respiratory gas exchange takes place on the surface of the gill filaments as the water passes over and out the gills.