The upper airway contributes to respiratory function by providing air conditioning, filtering, and sensory monitoring of the ambient environment. These same structures are vulnerable to the effects of inhaled irritants and allergens. A growing body of evidence links the development of rhinitis with that of asthma, making the prevention (and early recognition) of upper airway inflammation a priority.
FUNCTIONAL ANATOMY OF THE UPPER AIRWAY
Anatomy of the Upper Airway
The upper airway extends from the nares to the larynx (Figure 18–1). The surface area of the upper airway is increased by the presence of the nasal turbinates, which enhance the nose’s air conditioning and filtering ability. The anterior nasal cavity is lined with a squamous epithelium; posterior to the tip of the inferior turbinate, it transitions to a ciliated epithelium, complete with secretory cells, submucous glands, and venous capacitance vessels. The nasal vasculature responds to a variety of humoral and neural factors which, by changing the nasal mucosal thickness, affect upper airway patency. These stimuli also affect glandular secretion, giving rise to the two main symptoms associated with nasal disease: rhinorrhea and airflow obstruction. An area at the superior margin of each nasal cavity is dedicated to the olfactory (cranial nerve I) neuroepithelium, the only portion of the central nervous system (CNS) exposed directly to the environment, and the only portion of the CNS which continuously regenerates throughout one’s lifespan. The entire nasal and oral cavities (as well as the cornea and conjunctivae) are also innervated by the trigeminal nerve (cranial nerve V), which gives rise to sensations of temperature, mechanical stimulation, and chemical irritation (Figure 18–2).
Anatomy of the upper airway.
Innervation of the nasal cavity. The olfactory epithelium connects, via perforations in the cribriform plate, with the olfactory bulbs. The ethmoid and infraorbital nerves arise from the ophthalmic (first) division of the trigeminal nerve; the maxillary nerve constitutes the second division of the trigeminal.
Functions of the Upper Airway
The upper respiratory tract performs several essential physiologic functions. These include air conditioning, filtering, microbial defense, sensation, and phonation (Table 18–1). During the fraction of a second that inspired air travels through the upper airway, its temperature is adjusted to near body temperature, and its relative humidity regulated between 75% and 80%. These physical alterations to inspired air help minimize thermal and osmotic stresses on the tracheobronchial tree. The major fraction of particulate matter larger than 1 μm in diameter is deposited in the upper airway (Figure 18–3). The majority of impacted material—captured in the mucous blanket—is transported posteriorly via ciliary action until it empties into the nasopharynx and then is ...