Health care providers use the terms chronic bronchitis and emphysema to differentiate two basic forms of chronic obstructive pulmonary disease (COPD), but they are really two sides of a coin and often coexist. If you have chronic bronchitis, you probably have a persistent cough. Whereas if you have emphysema, you probably have signs of lung tissue damage on computed tomography (CT, or CAT) scanning.

Bronchitis is inflammation of the cells that line the bronchial tubes, the airways that move air within the lungs.

The term "bronchitis" sometimes refers to "acute bronchitis," in which a viral infection of the airways causes fever, cough and phlegm. This kind of bronchitis is "acute" because it gets better in several days to a week or two. Most people develop acute bronchitis at some time during their lives.

Long-term inflammation is called chronic bronchitis, defined by health care providers as a daily cough with phlegm or mucus for at least three months each year for two years in a row. Chronic bronchitis is not purely an infection, although an infection is a common cause of a flare-up. Instead of being caused by infection, chronic bronchitis begins after long-term exposure to the pollutants and nicotine in cigarette smoke. The airways then become thickened and produce more than the normal amount of mucus.

The inflammation causes the airway lining to scar and thicken, and the mucus can narrow or block (obstruct) the airways. After many years of smoking, the airways become permanently thickened Mucus and airway changes from chronic bronchitis can cause a persistent, annoying cough (sometimes called smoker's cough) and may cause wheezing. If mucus plugs one or more airways in the lung, this can prevent an area of the lung from delivering fresh air and oxygen to the blood. The oxygen level in the blood can decrease abruptly and severely during a bronchitis flare-up.

Some of the cells that line the airways have tiny hairlike structures called cilia on their surface. Cilia move back and forth in a beating motion much like the movement of legs on a millipede. The motion of the cilia helps to push mucus from deep in the lung airways up towards the throat, where most mucus is swallowed (some is coughed up). Since mucus traps bacteria, it is healthiest for you to be able to eliminate it from your lungs. For a while after exposure to cigarettes, irritants in the smoke cause cilia to stop moving. This causes mucus to collect in the airways, where it can block an airway or promote infection. Many smokers develop a morning cough from mucus that collects in the lungs while cilia are moving poorly. After a night of sleep with no cigarettes, cilia may begin to move better and may mobilize mucus in the lungs, so mornings may be times with more frequent coughing.

Emphysema describes the damage COPD causes to the air sacs (also known as alveoli). When you inhale, air enters your mouth or nose and moves into the trachea in the throat and then into the largest of the bronchial tubes. Air passes through smaller and smaller tubes until it gets to the ends, where tiny, balloonlike air sacs (the alveoli) are surrounded by small blood vessels. At the surface of each air sac, oxygen from your breath enters the blood, and carbon dioxide moves from the blood into the air sac to be exhaled.

Cigarette smoking and airway inflammation damage the thin, elastic walls of the air sacs. As walls between air sacs break down, the air sacs combine to form fewer, larger air sacs. But bigger isn't better.

Large air sacs don't absorb as much oxygen as normal ones. Small airways that deliver breaths into the air sacs recoil to a narrower shape because they are no longer well supported by the lung tissue that surrounds them. Over time, the damaged lung tissue surrounding the air sacs loses its natural elasticity, and the tissue becomes scarred and stiff. It becomes harder to breathe in and out. Stale, oxygen-depleted air gets trapped inside the stiffened lungs. (In cases of severe emphysema, this trapped air enlarges the chest cavity so the chest can have a barrel shape.)