What prevents boost pressure from entering the pcv system?

[June]

I have read so many posts about the pcv systems on here and one thought came to mind I would like to understand. If the pcv connects to the intake and the oil box on the block, what prevents turbo boost pressure from the intake passing through the system to the crankcase causing positive pressure in the crankcase during boost? Or does the crankcase build psi during boost as a normal occurrence then go back to vacuum after boost? I am sure this is a silly question to most, but I sure would like to understand the working of this system better. Thank you, June

[ThommyKent]

Crankcase pressure results from from the pumping action of the underside of the pistons and also some small leakage past the piston rings. The piston rings prevent combustion chamber pressure from pressurizing the crankcase. You will notice an engine with low compression because of worn rings will pressurize the crankcase more and produces what is commonly referred to as blowby.