Anatomical structures of the upper respiratory tract include nose, nasal cavity and pharynx. They belong to the conducting zones of the respiratory tract. Nasal cavity and nasopharynx are lined with ciliated pseudostratified epithelium, rich in mucus secreting goblet cells, while the oral part of the pharynx is lined by stratified squamous epithelium.
The nasal respiratory epithelium includes an extensive network of blood vessels. It warms the incoming air adjusting it to body temperature. The mucus lining of this area evaporates water, which humidifies the inspiratory air. Sticky secretions of the mucus cells entrap dust and other small particles including microorganisms. Cilia of the respiratory epithelium beat in order to move mucus from the nasal cavity towards the pharynx, where it can be swallowed. Normally, the nose and paranasal sinuses produce approximately 1 quart (about 1100 ml) of mucus in 24 hours. The amount of mucus increases considerably when the nose and/or sinuses are inflamed.