The upper respiratory tract consists of the nose, the nasal cavities and the oropharyngeal space. The surface area of the nasal cavity side walls is increased considerably by the superior, middle and inferior conchae. The inspiratory air is warmed, moistened and filtered inside the nasal cavity. To enable it to do this, its large surface area is covered with a mucous membrane (the mucosa), which contains mucous glands and ciliated epithelial cells. A dense network of blood vessels on the epithelial layer warms the inspiratory air to body temperature. In the nose, moisture is added to the inspiratory air by the action of the mucous glands, and the mucus is also able to filter out dust particles. The cilia then transport the mucus towards the throat, where it can be swallowed together with the trapped dust particles and pathogens. This nasal filter system supports the deposition of particles larger than 10µm, while smaller particles are able to pass through the nose and reach the lower respiratory tract.
When a person only breathes through his mouth, this cleaning and warming function is bypassed.
After the nose and throat, the inspiratory air passes though the larynx into the lower respiratory tract, which includes the trachea, the bronchi and the bronchioles. The trachea is still made up of rings of cartilage, but the bronchioles are entirely without cartilage. Consequently, they are particularly prone to constrictions, as happens in an asthma attack, for example. The bronchi divide into smaller and smaller branches until they terminate in about 300 million alveolar sacs. It is on the enormous surface of these alveoli that gas exchange actually takes place: oxygen (O2) is absorbed into the blood and carbon dioxide (CO2) is transferred to the expiratory air.
Diseases of the respiratory tract have conventionally been divided into two medical specialties - ENT and pulmonology - but the respiratory tract is one functional unit. It is unified particularly by the common mucous membrane, which covers all areas.
The functional connection between the upper and lower respiratory tracts becomes particularly apparent with the rhinitis-asthma link. If allergic rhinitis is not treated appropriately, the symptoms eventually spread to the lungs and asthma is triggered in as many as 20 to 50% of all cases.
The term "united airways" is a vivid way to express this anatomical and functional unity.