A flow control system includes a substantially rigid vessel with a process fluid reservoir situated in the rigid vessel is in fluid communication with an outlet. A movable member and/or a working fluid is situated in the rigid vessel so as to selectively expel process fluid from the outlet. Where a movable member is employed, movement of the movable member in a first direction causes process fluid to be drawn into an inlet and movement of the movable member in a second direction causes process fluid to be expelled from the outlet. In other implementations, the working fluid is metered into a working fluid reservoir to compress a flexible process fluid reservoir and expel process fluid therefrom.