The ciliated, pseudostratified columnar epithelium of the respiratory mucosa is composed of five cell types:  Ciliated cells, tall columnar cells with cilia that project into the mucus covering the surface of the epithelium  Goblet cells that synthesize and secrete mucus  Brush cells, a general name for those cells in the respira tory tract that bear short, blunt microvilli  Small granule cells (Kulchitsky cells) that resemble basal cells but contain secretory granules. These are enteroendocrine cells of the APUD system  Basal cells, stem cells from which the other cell types arise
The epithelium of the respiratory region of the nasal cavity is essentially the same as the epithelium lining most of the parts that follow in the conducting system. Because the respiratory epithelium of the trachea is studied and examined in preference to that of the nasal cavity, the above cell types are discussed in the section on the trachea (page 672).
The mucosa of the respiratory region warms, moistens, and filters inspired air.
The lamina propria of the respiratory mucosa has a rich, vascular network that includes a complex set of capillary loops. The arrangement of the vessels allows the inhaled air to be warmed by blood flowing through the part of the loop closest to the surface. The capillaries that reside near the surface are arranged in rows; the blood flows perpendicular to the airflow, much as one would find in a mechanical heat exchange system. These same vessels may become engorged and leaky during allergic reactions or viral infections such as the common cold. The lamina propria then becomes distended with ﬂuid, resulting in marked swelling of the mucous membrane with consequent restriction of the air passage, making breathing difficult. The lamina propria also contains mucous glands, many exhibiting serous demilunes. Their secretions supplement that of the goblet cells in the respiratory epithelium.
By increasing surface area, the conchae (turbinates) increase the efficiency with which the inspired air is warmed. The turbinates also increase the efficiency of filtration of inspired air through the process of turbulent precipitation. The air stream is broken into eddies by the turbinates. Particulate matter suspended in the air stream is thrown out of the stream and adheres to the mucus-covered wall of the nasal cavity. Particles trapped in this layer of mucus are transported to the pharynx by means of coordinated sweeping movements of cilia and are then swallowed.
Olfactory Region of the Nasal Cavity
The olfactory region is located on part of the dome of each nasal cavity and, to a variable extent, the contiguous lateral and medial nasal walls. It is lined with a specialized olfactory mucosa. In living tissue, this mucosa is distinguished by its slight yellowish brown color caused by pigment in the olfactory epithelium and the associated olfactory glands. In humans, the total surface area of the olfactory mucosa is only about 10 cm2; in animals with an acute sense of smell, the total surface area of the olfactory mucosa is considerably more extensive. For instance, certain dogs species have more than 150 cm2.
The lamina propria of the olfactory mucosa is directly contiguous with the periosteum of the underlying bone (Plate 69, page 688). This connective tissue contains numerous blood and lymphatic vessels, unmyelinated olfactory nerves, myelinated nerves, and olfactory glands.
The olfactory epithelium, like the epithelium of the respiratory region, is also pseudostratified, but it contains very different cell types. Also, it lacks goblet cells (Fig. 19.3 and Plate 69, page 688).
The olfactory epithelium is composed of the following cell types:  Olfactory receptor cells are bipolar neurons that span the thickness of the epithelium and enter the central nerve system.