From Normal Lung Function to COPD: what causes COPD?

Frequent coughing, excessive phlegm, shortness of breath are all symptoms of chronic obstructive pulmonary disease, or COPD. But COPD is actually not one disease, but two: chronic bronchitis and emphysema.

Both are long-term (chronic) problems that can obstruct breathing. Both also share a main cause—smoking. You do not have to have both conditions to have COPD, but many people with one disease also have the other.

Symptoms of both these conditions vary from one person to the next and usually grow worse as the condition advances. However, there are treatments and lifestyle changes that can greatly help.

What causes COPD?

COPD develops gradually over many years, as the airways become narrowed, and the lungs lose their ability to expand and contract effectively when you breathe. In most people with the disease, these problems stem from inflammation that occurs when something—most often cigarette smoke—irritates the respiratory tract intermittently over many years. The irritant can damage the cells that line the airways.

The irritant can also cause changes in the lining of the lung's airways that normally produce small amounts of mucus to lubricate the airway walls. Your body does not stand idly by while all this damage is being done. The immune system unleashes a flood of inflammatory cells called macrophages, neutrophils, and lymphocytes. Unfortunately, this defense mechanism ultimately makes the problem worse.

The "soldiers" of your body's defense system, the inflammatory cells carry out the attack on the irritants. In the course of their search-and-destroy mission, they infiltrate the walls of the airways and trigger the production of additional mucus inside the airways, leaving less room for air to pass.

The inflammatory cells also release a variety of chemicals, including enzymes called proteases. Proteases serve as a sort of molecular scissors, snipping apart proteins and tissues that the body wants to break down. In this manner, they aid in the fight against infection and injury.

An overabundance of these chemicals, however, is harmful and can contribute to emphysema and possibly to chronic bronchitis. In some people, a genetic defect is to blame for the particularly harmful effects of one type of destructive protease called neutrophil elastase.

For more information on the treatments, therapies, and everyday hints and tips to help you live well with COPD, check out Chronic Obstructive Pulmonary Disease (COPD), a Special Health Report from Harvard Medical School.

Image: AlexanderFord/Getty Images