It is a lung disease that can be explained as the inability to easily expel the air taken into the lungs by breathing. Two processes that cause this condition are chronic bronchitis and emphysema.
Along with respiration, the place where the oxygen in the respiratory air passes into the blood and the carbon dioxide in the blood goes out is the alveoli at the end of the respiratory tract. Chronic bronchitis is the inflammation and narrowing of the airways, called bronchi, leading to the alveoli.

Emphysema means the breakdown and enlargement of these airways and vesicles. As a result, the inhaled air cannot be transmitted to the alveoli and remains restricted in the lung. This condition is called COPD.