Chronic obstructive pulmonary disease, or COPD, refers to a group of progressive diseases that make it hard to breathe. Progressive means the condition gets worse over time.
People with COPD have an airflow blockage, which means there's less air flowing in and out of the lungs. This happens for a number of reasons. In some cases, the bronchioles and alveoli, which are normally elastic or stretchy, become hardened.
Other times, the walls between the alveoli are destroyed, forming fewer and larger alveoli, or the walls of the bronchial tree become thick and inflamed, making them narrower.
The bronchi and bronchioles can also become narrowed and clogged if they produce more mucus than usual. Some people with COPD have just one of these problems, but for many people, a combination of two or more of these mechanisms causes their condition.