* Water enters through the mouth: The dogfish shark swims with its mouth slightly open, taking in water.
* Water passes over the gills: The water flows over the gills, which are located in five to seven pairs of gill slits on each side of the body.
* Oxygen is absorbed: The gills contain thin filaments with a large surface area. As the water passes over the gills, oxygen diffuses from the water into the shark's bloodstream.
* Carbon dioxide is released: At the same time, carbon dioxide from the shark's blood diffuses into the water.
* Water exits through the gill slits: The water, now depleted of oxygen and enriched with carbon dioxide, is expelled through the gill slits.
Key points:
* Countercurrent exchange: The flow of water over the gills is in the opposite direction to the flow of blood within the gills. This maximizes the diffusion of oxygen into the blood and carbon dioxide out of the blood, making the process highly efficient.
* Continuous breathing: Dogfish sharks constantly need to move water over their gills to stay alive. They can't breathe air, unlike some other fish species.
In addition to gills:
* Dogfish sharks also have a spiracle, a small opening behind their eyes. This allows them to take in water when they are resting on the bottom of the seafloor.
The dogfish shark's efficient breathing system allows them to survive in a variety of marine environments.