Every day, you breathe in just over 2,000 gallons of air—enough to almost fill up a normal-sized swimming pool. That's a lot of air. It's the amount needed to oxygenate approximately 2,000 gallons of blood pumped through your heart daily. Because lungs are constantly exposed to the external environment, they need some protection from dust, germs, and other unwanted matter. That's where mucus comes in. Your bronchial tubes are lined with cilia (they're like thin little hairs) that carry mucus up into your throat to trap those yucky intruders until you cough, sneeze, clear your throat, or swallow to get rid of them.

Source: American Lung Association