Inhaled foreign particles that are not trapped by mucus can penetrate deeper into the respiratory system. Some may become lodged in the bronchi or bronchioles, causing irritation or infection. Others, particularly very fine particles, can reach the alveoli, potentially leading to serious lung diseases like chronic obstructive pulmonary disorder (COPD) or silicosis. The immune system also plays a role in responding to these particles, but repeated exposure can overwhelm the system and lead to chronic conditions.

When the nasal passages are blocked due to cold or nasal congestion, the body naturally compensates by breathing through the mouth. This allows for an adequate flow of air to the lungs even when the nasal passages are obstructed. However, mouth breathing bypasses the nasal cavity's warming, humidifying, and filtering functions, which may cause the inhaled air to be cooler and contain more particles and pathogens.

The diaphragm is a dome-shaped muscle that separates the thoracic and abdominal cavities. During inhalation, the diaphragm contracts and flattens, increasing the volume of the thoracic cavity. This decrease in pressure allows air to be drawn into the lungs. During exhalation, the diaphragm relaxes, returning to its dome shape, decreasing the volume of the thoracic cavity, and pushing air out of the lungs. While not part of the respiratory tract itself, the diaphragm plays a crucial role in the mechanical process of breathing.

The mucus in the respiratory system acts as a lubricant and a filter. It traps dust, pathogens, and other foreign particles that enter through the nasal cavity. The cilia, tiny hair-like structures, move the mucus toward the throat, where it can be swallowed or coughed up, thus eliminating the trapped debris. Without mucus, these foreign particles could reach the alveoli, potentially leading to infections or other respiratory issues. Mucus also helps in keeping the airways moist, preventing them from drying out.

The trachea is a larger, rigid tube supported by C-shaped cartilage rings, which keep it open and prevent collapse. It's lined with ciliated cells and mucus to filter the air. The bronchi are branches off the trachea that direct air into each lung. They contain some cartilage but less than the trachea. The bronchioles are smaller, lacking cartilage, and lead to the alveoli, where gas exchange occurs. Muscles in the walls of the bronchi and bronchioles allow them to constrict or dilate, regulating airflow. The gradual reduction in cartilage and increase in muscle control allow for more localized direction and control of airflow.