The lungs have several ways of protecting themselves from irritants.
The nose
First, the nose acts as a filter when breathing in, preventing large particles of pollutants from entering the lungs.
Mucus
If an irritant does enter the lung, it will get stuck in the thin layer of mucus (also called sputum or phlegm) that lines the inside of the breathing tubes. An average of 85 grams of mucus are secreted onto the lining of these breathing tubes every day. The mucus is “swept up” toward the mouth by little hairs called cilia that line the breathing tubes.
Hairs in the throat
Cilia move mucus from the lungs upward toward the throat to the epiglottis. The epiglottis is the gate, which opens allowing the mucus to be swallowed. This occurs without us even thinking about it.
Spitting up sputum is not “normal” and does not occur unless the individual has chronic bronchitis or there is an infection, such as a chest cold, pneumonia or an exacerbation of chronic obstructive pulmonary disease (COPD).
Coughing
Another protective mechanism for the lungs is the cough. While coughing is a common event, is not a normal event and is the result of irritation to the bronchial tubes. A cough can expel mucus from the lungs faster than cilia.
Bronchospasms
The last of the common methods used by the lungs to protect themselves can also create problems.The airways in the lungs are surrounded by bands of muscle.
When the lungs are irritated, these muscle bands can tighten, making the breathing tube narrower as the lungs try to keep the irritant out.
The rapid tightening of these muscles is called bronchospasm.
Some lungs are very sensitive to irritants and bronchospams may cause serious problems for people with COPD and they are often a major problem for those with asthma, because it is more difficult to breathe through narrowed airways.