Suggest you sketch out a typical open system and a closed system and look at what is different between them - then think about how a pump would operate on each system. It's a fairly basic concept - once you understand the differences, come back and we can discuss the operation of each system.
Of course you have searched the net for open and closed pumping systems, yes?
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)