Low oil pressure?

The oil gets sucked up the pick up into the pump, and out of the pump through the engine and comes out the bearings, rocker arms and around the lifters and that's pretty much it. If the plug is in it goes through the filter and out to the galley's, and goes out the bearing's, if the plug is out the filter just fills with oil and the oil goes to the galley's and goes out the bearing's. Are you getting the picture! If you don't understand hydraulics you wont understand.