Final answer:
Small particles are cleared from the airway by mucus that traps them, cilia that move the mucus upward, and coughing that expels the phlegm. This is part of the mucociliary escalator, the body's natural defence mechanism.
Step-by-step explanation:
Small particles, such as dust, dirt, viral particles, and bacteria, are cleared from the airway by the body's protective mechanisms. The bronchi and bronchioles of the lungs produce mucus, a sticky substance that traps particulates. This mucus is moved by cilia, which are small, hair-like projections lining the walls of the bronchi and bronchioles. These cilia beat in unison, pushing the mucus and trapped particles back up to the throat where they can be swallowed and eliminated via the esophagus. If necessary, coughing can also play a role in expelling phlegm, which contains the trapped particles, from the body.
When cilia move the trapped particles and mucus toward the throat, it can be disposed of by clearing the throat or by coughing. This process is known as the mucociliary escalator, a crucial part of the respiratory defence system. Trapped substances may either be expectorated or swallowed, where stomach acids destroy any potentially harmful content.