Chronic bronchitis is a type of chronic obstructive pulmonary disease (COPD). In COPD, inflammation of the airways in the lungs results in obstruction to the flow of air. COPD develops due to exposure to harmful substances in the air, usually cigarette smoke. The disease can manifest in different ways, and 14% to 74% of patients with COPD will have symptoms consistent with the definition of chronic bronchitis.
How the Lungs Work
To understand chronic bronchitis, it is helpful to understand how the lungs work. The primary function of the lungs is to bring oxygen into the body and get rid of waste gases. When you inhale (breathe in), you pull air into your nose or mouth. This air passes into the space behind your nose and mouth (the pharynx) and then into the trachea, the large airway that leads down into your chest.
From here, air passes into smaller airways in each lung, called the bronchi. The bronchi divide many times into smaller and smaller airways, each finally terminating in grapelike clusters of sacs called alveoli. The thin walls of the alveoli are surrounded by tiny blood vessels (capillaries), and it is here that oxygen passes into the blood and carbon dioxide passes out.
A major function of the airways is to protect the alveoli from noxious substances you breathed in, and it does this in several ways. The walls of the airways are lined with a tissue layer called the epithelium. This layer consists of cells with tiny hairs called cilia. Interspersed among the ciliated cells are goblet cells. Along with deeper glands, the goblet cells produce mucus, which contains antibodies and other antimicrobial proteins. This airway mucus catches inhaled dust, particles, and germs. The cilia continuously beat upward, moving mucus into the throat, where it can be coughed up or swallowed.
Chronic bronchitis is a disease of airway inflammation, a complex process with multiple effects on the airways. One consequence of airway inflammation is an overproduction of mucus, which leads to the chronic productive cough that characterizes the disease.
What Causes Airway Inflammation?
Inflammation is a broad term that describes the processes the immune system uses to protect the body from harmful substances. Toxic particles and gases, such as those in cigarette smoke, cause an immune response in the airways, leading to inflammation of the airway epithelium.
Nearly all smokers develop inflammation in their lungs. Those with COPD, however, appear to have an abnormally robust immune response to inhaled toxins.
As part of this inflammatory response, white blood cells are attracted to the airways, and release substances that signal goblet cells to increase the production and release of mucus. Smokers also have increased numbers of goblet cells in their airway lining.
Compounding the problem, chronic inflammation reduces the function of the cilia, and the body is less able to get rid of excess mucus. A weak cough and obstruction of the small airways further impair mucus clearance.
In chronic bronchitis, the buildup of mucus in the airways obstructs airflow. As a result, the epithelial layer thickens, leading to airway narrowing and collapse. People with this condition develop worsening airflow obstruction, and may experience a progressive decline in lung function.
Acute vs. Chronic Bronchitis
Acute bronchitis is usually caused by a viral infection, though some bacteria, such as Mycoplasma pneumoniae, can also cause it. Acute bronchitis may occur year-round, but it is most common in the winter. Infection and the resulting immune response cause inflammation of the medium and large airways.
The illness often begins with typical symptoms of a common cold. Patients then develop a cough, which becomes the predominant symptom and can last up to three weeks in severe cases.
Acute bronchitis can affect people with or without underlying lung disease, and people with chronic bronchitis may develop superimposed acute bronchitis. An acute exacerbation of COPD is an episode of worsening respiratory symptoms (such as shortness of breath, cough, and sputum production), and is most often caused by a viral or bacterial infection. This is a major cause of morbidity and hospitalization among people with COPD.
A Word From Verywell
A description of smoking-related lung changes sounds scary, and there is no doubt that chronic bronchitis can be a serious, life-limiting disease. Fortunately, doctors have many treatments in their arsenal, including medications, pulmonary rehabilitation, and surgery in some cases. Cessation of smoking is vital to stopping further decline in lung function. Novel treatments for COPD are an active area of research, and therapeutic options may continue to increase.
