Buoyancy control device with an inflatable air bladder that aids to establish neutral buoyancy underwater and positive buoyancy on the surface.
Pressure regulator that reduces pressurized breathing gas to ambiet pressure, and delivers it to the diver.
You cannot breathe directly out of a scuba tank because of the high pressure. A regulator reduces pressure to safe a level that can be inhaled by the diver. A diving regulator has two parts: the First Stage reduces the tank pressure and connects it to the Second Stage which contains the mouthpiece for breathing