Every soil-waste-vent system is really made of two components: the waste lines and the vent lines. The drain line is what removes waste liquid from every fixture, whether it is a sink, tub, toilet, dishwasher or washing machine and brings it the main sewer. The vent line is responsible for normalizing pressure within the system. The vent line does not carry any waste from the fixtures. Instead, it is responsible for maintaining proper pressure within the system as well as providing an outlet for sewer gases.