At some point, you've probably looked under the bathroom sink and noticed the U-shaped pipe that runs from your sink drain (B) to a larger wastewater pipe in the wall. One end of that big wastewater pipe runs down to your sewer or septic system (E); the other end leads all the way up through the roof (D), letting fresh air in while allowing any smelly sewer gases to vent out to the sky. And although you can't see it, the same basic drain setup is at work behind or beneath your tub and shower (C).