Your home's provide and drainage procedure have to normally be two distinct subsystems, without any overlapping. For the fixtures (bridges between the two systems), the air admitted via the vent stack and vent pipes retains the traps sealed and helps prevent sewer gases from backing up through the drains. https://1701-b.s3-ap-northeast-1.amazonaws.com/streamline-plumbing-solutions/DrainageContractor.html