A system including the method employed, for controlling the flow of a pressurized gas through a variable orifice flow control valve in a pressurized gas conduit system, such as a ventilator, in response to either a desired flow rate/volume input signal, a desired pressure signal or both. With respect to the flow rate input signal, the system generates a flow signal indicative of the actual flow rate of gas through the flow control valve by measuring the pressure drop across the valve and using the actual orifice area based on a control valve position signal given to the valve. This flow signal is compared to the flow rate input signal to give a correction signal which is used to form a volume flow reference signal. With respect to the desired pressure signal, a signal indicative of the effective pressure at a set location in the system is compared to the desired pressure signal and processed to give a pressure flow reference signal. Depending on the mode desired, either the volume flow reference signal or the pressure flow reference signal or the greater of the two becomes a flow reference signal that is used to generate a control valve position signal based on a predetermined relationship. This signal is then used to vary the valve orifice area and correct the flow rate.