Methods, apparatuses, and systems for controlling mass flow rates and pressures in passageways coupled to reaction chambers are disclosed herein. In one embodiment, a method includes controlling a mass flow rate in a passageway in response to a first condition by modulating a valve of a mass flow and pressure control unit, and controlling a pressure in the passageway in response to a second condition different than the first condition by modulating the valve of the mass flow and pressure control unit. In another embodiment, an apparatus includes a mass flow measurement device, a pressure sensor, a modulating valve in the passageway, and a controller operably coupled to the mass flow measurement device, the pressure sensor, and the modulating valve. The controller has a computer-readable medium containing instructions to perform the above-mentioned method.