What is the primary function of a drain in a plumbing system?

The primary function of a drain in a plumbing system is to carry wastewater or water-borne waste away from fixtures and out of the building to a sewer or septic system. Drains are essential for maintaining hygiene and sanitation, as they facilitate the removal of dirty water and waste, preventing potential buildups that could lead to clogs, unpleasant odors, or contamination of clean water supplies. This function is crucial for the overall efficiency and safety of plumbing systems, making it imperative that drains are properly designed, installed, and maintained.

Other choices represent different aspects of plumbing but do not encompass the primary purpose of a drain. The supply of clean water to fixtures pertains to the role of supply lines, which bring fresh water into the system. Regulating water pressure is handled through various plumbing components such as pressure regulators and is not a function of drains. Ventilation for pipes helps to prevent negative pressure and ensure proper airflow, which aids in drainage but is not the primary purpose of a drain itself. Therefore, understanding that drains specifically and primarily handle the expulsion of wastewater clarifies their critical function in plumbing systems.