Contrary to popular belief, acft aren't pressurized like a balloon, but more like a sink with constantly running water, the water is coming in at a pre-determined rate (the acft's pressurization system) and the level of water is controlled by regulating the stopper (the acft outflow valves). Using this method, fresh air is always being pumped in. There is a post prior to this one (Air filters /Heat Exchangers / ECS) in which NKP82 and Airplay provide an outstanding explanation of cooling/heating system operation. After this, the air is basically "pumped in" via the distribution system, and then regulated as it leaves the cabin.
To learn more about pressurization, first study the atmospheric pressure differences on increasing altitude, and the effect of the partial pressure of oxygen during this. Then move to the three methods of pressurization, Isobaric, Differential, and Iso-Differential.
I apologize, I don't have any good links for this subject, maybe someone else could help out? If you can't find any info, just post another question and I will reply in more detail.
"Never trust a clean Crew Chief"