If they work against gravity, the fluid may not flow.

For example, consider a pipe with a fluid flowing inside.

Yet now she could taste it, the warm fluid flowing into her mouth.

If the effect of the pressures work with gravity, the fluid will flow.

After the vacuum is released, the fluid flows into the part.

Once again, the dark fluid flowed ahead with renewed strength.

Once the bulb is full the fluid will flow without further effort from the higher to the lower container.

This, in turn, encourages the fluids to flow more easily to the surface.

The condensed working fluid then flows back to the hot end of the pipe.

The negative sign is needed because fluid flows from high pressure to low pressure.