The upper airway (UAW) serves as the primary conduit for atmospheric air, functioning as the entry point for the entire respiratory system. Its purpose is to prepare inhaled air before it reaches the delicate tissues of the lungs. This anatomical region performs a series of modifications to the air, ensuring it is suitable for gas exchange within the alveolar sacs. The upper airway’s actions maintain lung health and the overall efficiency of respiration.
Defining the Upper Airway’s Scope
The upper airway encompasses the space extending from the outside world inward, beginning at the nostrils and the mouth. Functionally, it is the initial segment of the continuous air passage, spanning the face, head, and neck regions. This section of the respiratory tract acts as a funnel, directing air toward the lower structures.
The anatomical boundary that separates the upper airway from the lower airway is generally considered to be the larynx, or voice box. The line of demarcation is often identified at the level of the vocal folds, or the glottis. Structures above this point are part of the upper airway, while those below, such as the trachea and bronchi, constitute the lower airway.
The upper airway is a shared pathway, meaning it handles both air and food, unlike the purely respiratory lower airway. This dual-purpose design makes its protective mechanisms important for preventing foreign material from entering the lungs. The entirety of this passage is lined with a specialized mucous membrane.
Structural Components of the Upper Airway
The first major structures of the upper airway are the nasal cavity and the surrounding paranasal sinuses. The nasal cavity is divided by the nasal septum and contains three bony, shelf-like projections called conchae, or turbinates, which increase the surface area. The paranasal sinuses are air-filled cavities within the skull bones that connect to the nasal cavity:
- Frontal
- Maxillary
- Ethmoid
- Sphenoid
Air then passes into the pharynx, a muscular, funnel-shaped tube known as the throat. The pharynx is divided into three segments: the nasopharynx, oropharynx, and laryngopharynx. The nasopharynx is the uppermost part, located behind the nasal cavity, and serves exclusively as an air passage.
The oropharynx is positioned behind the oral cavity, extending from the soft palate down to the upper edge of the epiglottis. Both air and ingested substances pass through this region. The final segment, the laryngopharynx, sits just above the esophagus and the larynx.
The last structure of the upper airway is the larynx, a cartilaginous framework situated between the pharynx and the trachea. It is composed of nine cartilages, including the three unpaired ones: the thyroid cartilage (which forms the Adam’s apple), the cricoid cartilage, and the leaf-shaped epiglottis. Within the larynx are the vocal folds, which form the glottic opening, acting as the gateway into the lower airway.
Essential Air Conditioning Functions
The upper airway conditions the air by warming, humidifying, and filtering it for the lungs. The UAW modifies the air to match body conditions, as inhaling cold, dry air can damage alveolar surfaces. Air is warmed by passing over the mucous membranes in the nasal cavity, where capillaries transfer heat.
The air is humidified through contact with the moist mucosal lining of the nasal cavity and pharynx. This process adds water vapor, aiming to deliver air to the upper trachea at 30°C to 33°C with a relative humidity near 98%. This conditioning protects the lower respiratory tract from drying out and maintains lung tissue integrity.
Defense against foreign materials is another function, beginning with the filtration of large particles by nasal hairs, known as vibrissae, located inside the nostrils. For smaller contaminants, the respiratory passage employs the mucociliary escalator. This system uses a layer of mucus to trap dust, pathogens, and debris, while hair-like projections called cilia beat in a coordinated, wave-like motion.
The cilia continuously propel the mucus layer upward toward the pharynx. Once the mucus reaches the throat, it is swallowed or expelled, effectively clearing the respiratory tract. The conditioning process also functions in reverse during exhalation, where the upper airway reclaims heat and moisture from the outgoing breath, conserving the body’s resources.
Secondary Roles in Communication and Digestion
Beyond its respiratory functions, the upper airway enables human communication and contributes to the digestive process. The larynx contains the vocal folds, which are elastic ligaments that vibrate when air from the lungs passes across them. This vibration creates the sound of the human voice, a process known as phonation.
The sound produced in the larynx is modified and shaped into recognizable speech by the pharynx, oral cavity, and nasal cavity, which act as resonating chambers. Changes in the tension of the vocal cords and the shape of the mouth and throat allow for varied pitch and articulation. The tongue and lips further refine these sounds into distinct words.
The pharynx serves as a shared conduit, requiring a protective mechanism during deglutition, or swallowing. Swallowing temporarily converts the airway into a food passage. During this process, the tongue pushes the food bolus toward the back of the throat.
The leaf-shaped epiglottis, a cartilage attached to the larynx, plays a protective role by folding down and closing off the entrance to the trachea. This action diverts food and liquids into the esophagus, preventing them from entering the respiratory tract and causing aspiration. This coordination between breathing and swallowing highlights the dual nature of the upper airway.