Chronic bronchitis

In chronic bronchitis, the bronchi — the tubes that carry air in and out of the lungs — have an inner lining that stays irritated all the time. That ongoing irritation does two things at once: it thickens the wall of the tube, which swells, and it drives the glands inside to make more mucus, the thick stuff we know as phlegm. Picture a garden hose whose inner wall has swollen and that is also coated on the inside with a sticky layer: the channel is still there, but it is no longer the clean tube it used to be.

That is why chronic bronchitis isn't identified by an X-ray or a lab test, but by the calendar. There is a classic definition doctors have used for decades, and it comes down to counting: a cough that brings up phlegm for three months or more, and not in one isolated year but in two years in a row. And there is a required step before the name gets applied, which is ruling out other things that could cause that same cough.