2.1.1 Mucous Blanket and Mucociliary Clearance

Nasal mucosa functions to protect the lower airways from inhalation of particular matter, pathogens, and allergens through both barrier and biochemical protective mechanisms. The pseudostratified ciliated columnar epithelium of the nasal airway, also referred to as respiratory epithelium, is made up of ciliated and nonciliated columnar cells, goblet cells, and basal cells. This epithelium is found in the posterior two-thirds of the nasal cavity (posterior to the limen nasi) and produces a mucous blanket both characteristic and vital to function of the upper respiratory tract (Fig. 2‑1). ¹

Particle characteristics including size, shape, and aerodynamic qualities determine the degree of deposition. Particles smaller than 0.5 µm will make it past the filter of the nasal airways and into the lower respiratory system. Larger particles are trapped within the multiple filtering mechanisms of the nasal airway. Of particles 1 µm in size, studies have shown that approximately 60% will be deposited in the nasal cavities. This percentage increases further with increasing particle size. ¹ Multiple studies have looked at the distribution of caught particles within the nasal cavity, and a site of concentrated deposition has been found to be just posterior to the nasal valve and the anterior aspect of the middle turbinate. The internal valve is the site of transition of airflow from laminar to turbulent after which the flow is directed posteriorly toward the middle turbinate.

Mucin production by goblet cells must be appropriately regulated. Inadequate production can lead to impaired trapping of particles for clearance, whereas overproduction can lead to airway obstruction and impair clearance that can ultimately promote recurrent and persistent respiratory infections. This epithelium lies over a basement membrane that unique to its location in the nasal cavity is permeated by capillaries, which allows fluid to pass directly through these vessels onto the mucosal surface and alter its consistency. ²

The mucous blanket is driven by ciliary movement toward the lateral pharyngeal walls towards the esophageal inlet for swallowing. Cilia beat at a frequency of 1,000 strokes per minute and the mean velocity of particle transport are estimated at 6 mm/min under normal physiologic conditions. ³ Mucous flow is a barrier to microorganisms, irritants, and allergens. The mucous blanket is described as having two separate layers, the inner, or sol, layer that is thin and driven forward by the beating cilia that it surrounds, and the outer gel layer that is viscous and rich in glycoproteins. This thin inner layer surrounds the cilia and propels the overlying viscous layer posteriorly over its surface. The mucous blanket exits the nasal cavity and is replaced by fresh mucous secretions every 10 to 15 minutes via the activity of this ciliary movement. ^{3 ,​ 4}

2.1.2 Innate Mucosal Immunity

Tight junctions and secretions produced by the respiratory epithelium of the nose provide mechanical protection from pathogens and debris. ^{5 ,​ 6} These foreign materials are typically cleared by respiratory cilia. However, when the physical defenses provide insufficient protection, the nasal cavity relies on an inflammatory reaction. Innate mucosal immunity is composed of trigger molecules such as toll-like receptors that recognize pattern-associated molecular patterns present on pathogens. ⁶ The activation of toll-like receptors triggers cytokine release with subsequent induction of the adaptive immune response, as well as the production of nitric oxide (NO). ⁶ Antimicrobial peptides, including defensins and cathelicidins, are then activated to directly kill bacteria, viruses, and fungi, and further potentiate tissue inflammation. ^{6 ,​ 7} Innate mucosal immunity is important especially when the nasal mucosa is physiologically abnormal and cannot clear foreign substances through ciliary clearance.

2.1.3 Nasal Sensation/Innervation

The mimetic muscles covering the external nose are innervated by the facial nerve while its sensory function is provided by branches from the ophthalmic and maxillary divisions of the trigeminal nerve. Sensation of the nasal cavity and septum is provided largely by the maxillary division of the trigeminal nerve, namely the nasopalatine nerve, anterior ethmoidal branch of the nasociliary nerve, as well as the anterior superior alveolar nerve. ¹ Branches of the maxillary division have free nerve endings diffusely within the nasal mucosa, which also provides the sensory input from inhalation of irritants that may reflexively lead to protective mechanisms such as sneezing, tearing, or increased secretions. ⁸ Also important to note is the contribution of autonomic control on nasal blood flow. Sympathetic stimulation leads to reduction in blood flow and decongestion whereas parasympathetic activity increases blood flow and congestion. ¹

2.1.4 Nasal Airflow

Inspired air flows across the nasal mucosa that is thus exposed to a varying amount of particulate matter. The intricate and tortuous anatomy of the nasal cavity serves to increase the surface area available for this exposure, which optimizes olfaction, heating, humidification, and filtering of the inspired air. Airflow reaches its maximum velocity when passing through the internal nasal valve at which point collapse may be seen to varying degrees based on Bernoulli’s theorem of airflow showing increased velocity and decreased transmural pressure at areas of reduced caliber. ^{4 ,​ 8 ,​ 9}

Turbulence further increases contact of the estimated 14,000 L of inspired air that on average passes through the nasal cavity, with the nasal mucosa. ^{8 ,​ 10}

The nasal cycle, first described by Kayser in 1895, has been shown to be present in approximately 80% of the population. The cycle refers to changes in airway resistance and in nasal passage caliber that alter nasal airflow; submucosal vascular engorgement on one side balances with decongestion on the other side. The cycle occurs every 0.5 to 3 hours. ¹¹ During changes leading to cyclically altered cross-sectional area, the overall combined nasal airway resistance importantly remains unchanged. At times, the nasal cycle may lead to near total obstruction, both subjectively and on clinical and radiologic examination. This comes into play when examining patients reporting nasal obstruction. ⁴

Importantly, nasal airflow is also affected by postural changes, exertion, and sex hormones. NO, acting as a neurotransmitter, also significantly contributes to changes in nasal airflow, ciliary beat frequency, and mucous production. Increased airflow leads to a decreased NO concentration in the nasal cavity, an increased NO concentration in the lower respiratory tract, and decreased ciliary beat frequency. ¹