How does reversing gear rotation change the discharge direction of a gear pump?

In a gear pump, fluid is moved by the rotation of two meshing gears within a casing. As the gears unmesh, a partial vacuum is created that draws fluid into the pump, which is then carried around the outer edges of the gears toward the discharge side.12

When the gears rotate, they trap fluid between the gear teeth and the fixed casing, transporting it to the opposite side of the pump. As the gears mesh together again at the discharge point, the volume decreases, creating an increase in pressure that forces the fluid out into the outlet line. Because this mechanism relies on the path created by the rotating gears against the casing, the pump can discharge fluid in either direction; reversing the direction of the gear rotation effectively swaps the intake and discharge sides of the pump.2