Figure 182

Diagram of the relationship of the pharynx to the respiratory and digestive systems. The pharynx is divided into three parts: nasopharynx, oropharynx, and laryngopharynx. It is located posterior to the nasal and oral cavities and extends interiorly past the larynx. The pharynx serves both respiratory and digestive systems. This mid-sagittal section also transects the cartilages forming the skeleton of

- oropharynx laryngopharynx ventricular fold vocal fold esophagus the larynx (i.e., epiglottis, thyroid cartilage, and cricoid cartilage). Note the ventricular and vocal folds in the middle of the larynx, approximately at the level of the thyroid cartilage. This part of the larynx represents the narrowest portion of the respiratory system and is responsible for producing sound by audible vibration of the vocal folds.

• Small granule cells that resemble basal cells but contain secretory granules

• Basal cells, stem cells from which the other cell types arise

The epithelium of the respiratory segment 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 575).

The mucosa of the respiratory segment warms, moistens, and filters inspired air

The lamina propria of the respiratory segment 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 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.

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