Like Jerry says, the two are inter-related, not independent. Every pump will have a pump curve, which is where the pump performs relative to volume and pressure - increasing volume on the X axis (vertical), increasing pressure on the Y axis (horizontal) with the curve falling off from left (0 pressure, highest volume) to the right as pressure increases. The pump will have a point of maximum efficiency, but will operate anywhere on the curve - at any given pressure (system resistance) you will have a specific volume flow for that pump in new and clean condition.