Final answer:
The part of the SCBA that reduces high pressure in the cylinder to a usable lower pressure and controls the flow of air to the user is known as the Regulator.
Step-by-step explanation:
The part of the SCBA that reduces high pressure in the cylinder to a usable lower pressure and controls the flow of air to the user is known as the Regulator. The regulator operates based on the principles of Boyle's law, which states that the pressure of a gas is inversely proportional to its volume. In this case, the regulator adjusts the pressure of the air coming from the cylinder to a level that is safe and appropriate for the user.