In order to a Valve to control pressure, need to be able to sense the output pressure and adjust an orifice within the valve to maintain the desired pressure.  Although there are hundreds (or maybe thousands) of ways this can be done.

Most of us have seen these little valves with the handle on the top that seems to work backwards from the way we think it should.  But, have you ever really thought about what goes on inside?  Liquid (or gas) under pressure enters the valve through the inlet port.  It then goes through an adjustable orifice which is designed to have the minimum possible response to forward pressure (or else pressure on the inlet side of the valve would tend to hold the valve closed under high inlet pressure conditions).  Once the liquid passes through the orifice, it goes into a chamber, one wall of which is a diaphragm which is attached directly to the moving part of the adjustable orifice valve.  The diaphragm moves in response to the pressure on the outlet side of the adjustable orifice valve.  Two springs are used to temper the motion of the diaphragm and the attached moving part of the Valve.  Spring