Part of the show Avian Flu, How Flu Spreads and How to Avoid It
Molly from Hartford asked:
Why do we cough up phlegm?
Actually, not a lot of people do. Most of the phlegm that you produce ends up in your stomach. On the way to your lungs, all of your airways are lined with a sticky substance called mucus. It's there to mop up things like bacteria, viruses and particles of dust and dirt from the air. The linings of your lungs have tiny thread-like projections on their that beat. They all beat in the same direction which creates a current which pushes the mucus out of your lungs and up to your throat. Now if you get a lot of mucus, sometimes you cough and it can come flying out of your mouth. But most of it goes down into your stomach. That's very important, because as it has all the nasty things you've breathed in locked up in it. These are carried down into your stomach where stomach acid kills all the bugs. What's left then goes into your intestines. In the walls of your intestines the dead bugs and dirt meet special patches called Peyer's patches, and that's where your immune system learns what's doing the rounds in the air around you, what sorts of things it has to mount an attack against, and what sorts of things it has to ignore. So normally you swallow most of your phlegm, but if you have excess production, coughing it up helps to get rid of it.