Asthma and chronic obstructive pulmonary disease (COPD), which includes chronic bronchitis and emphysema, are obstructive diseases of the lungs. These diseases narrow the airways and make breathing difficult. However, they affect the airways in slightly different ways.

What happens when you breathe

Air enters the body through the nose and mouth. It then passes down the pharynx (throat), through the larynx (voice box), and into the trachea (windpipe), the body’s largest airway. The entrance to the larynx is covered by a flap of cartilage called the epiglottis, which prevents food from entering the trachea.

The trachea branches into two main airways (the right and left mainstem bronchi) that deliver air into your lungs. These bronchi branch into smaller bronchi and eventually into even smaller airways called bronchioles. At the end of each bronchiole are clusters of dozens of alveoli (small air-filled sacs surrounded by a thick layer of tiny blood vessels called capillaries).

The lungs have the job of supplying oxygen to the body while removing carbon dioxide from the blood. This process, called gas exchange, occurs in the alveoli.

Oxygen from inhaled air travels through the thin walls of the alveoli and into the bloodstream via the capillaries. At the same time, carbon dioxide (a waste product that accumulates in the blood) moves from the capillaries to the alveoli and is exhaled through the nose and mouth.

Asthma and COPD

In asthma, the bronchial lining becomes inflamed and swells, and the lungs produce too much mucus, which can obstruct the airways. Also, the smooth muscle around the bronchi can tighten.

Chronic bronchitis also involves inflammation of the bronchial lining and an overproduction of mucus; the smooth muscle around the bronchi is often unaffected, although some people with chronic bronchitis may also have some smooth muscle tightening.

In emphysema, the walls between the alveoli are destroyed, causing the airways—which are normally held open by the elastic elements in the walls of the alveoli—to collapse.