CBSE Class 10 Answered

As air passes down the tracheobronchial tree, it is warmed and water is added. Contaminants must also be removed. The air in the nasal cavity is filtered by nose  hairs and tiny microscopic hairs called cilia that line the passage. The passage is also lined with mucus which helps in this filtration. Cilia beat in a synchronized fashion brushing any collected dirt and mucus up toward the mouth. The accumulated material is then coughed out or swallowed. By the time the air reaches the alveoli, it is virtually sterile.