Ok, so I have the Medela Pump-In-Style Advanced breastpump. I bought it from someone on Craigslist. Anyway, I have not had my baby yet, but am curious as to what the parts actually do. Ok, not all the parts, just the membranes. What do they do? How often should they be replaced? I have looked all over Medela's website and googled it as well, but havent really gotten any clear answers other than they have something to do with suction. Im planning on trying to breastfeed exclusively as I wont be going back to work after maternity leave runs out, but I also want to pump so that my husband can feed him as well. Does anyone know or have any experience working with these things? This is our first chile, so ANY help or advice or guidance is really appreciated!! Thanks!!