The pharynx is divided into three sections: the nasopharynx, oropharynx, and laryngopharynx. The structures within these regions include the epiglottis, vocal cords, and arytenoid cartilage, which is attached to the vocal cords. The nasopharynx, located behind the nose, is connected to the mouth through an isthmus that allows for both oral and nasal breathing. It also contains small masses of lymphoid tissue called adenoids or pharyngeal tonsils, which help fight infections. The nasopharynx is connected to the middle ear cavity by the eustachian tubes, which help equalize pressure between the throat, nose, and ears and serve as a drainage pathway for lymphatic fluid. Although these structures play a role in fighting infections, they are also commonly affected by infections in the head.

The soft palate separates the nasopharynx from the oropharynx, which extends to the epiglottis. The laryngopharynx begins at the epiglottis and ends at the esophagus. Its main function is to regulate the passage of food into the esophagus and air into the lungs. Infections can cause swelling in these structures, leading to increased airflow resistance. During swallowing, the epiglottis and arytenoid cartilage cover or act as a hood over the vocal cords, preventing the aspiration of food and liquid into the lower respiratory tract. This coordinated swallowing mechanism is closely synchronized with muscle reflexes that control the opening and closing of the airway. However, patients with certain neuromuscular diseases may have altered muscle reflexes, increasing their risk of aspiration and pneumonia.

Moving on to the lower airways, we have the trachea, bronchi, bronchioles, and respiratory unit. The right lung, located in the right hemithorax, is divided into three lobes (upper, middle, and lower) by two interlobular fissures (oblique and horizontal). On the other hand, the left lung, found in the left hemithorax, is divided into two lobes (upper, including the lingula, and lower) by an oblique fissure.
The pharynx is divided into three sections: the nasopharynx, oropharynx, and laryngopharynx. Important • Fig. 20.1 Illustrations of Upper Airway Anatomy.A, Anterior view of the paranasal sinuses. B, Lateral view of the nasal passage structures demonstrating the superior, middle, and inferior turbinates and the sinus ostia. C, Lateral midsagittal section view of the head and neck, showing the three divisions of the pharynx (nasopharynx, oropharynx, and laryngopharynx) and surrounding upper airway structures.
CHAPTER 20 Introduction to the Respiratory System 435 structures within these regions include the epiglottis, vocal cords, and arytenoid cartilage which is attached to the vocal cords (see
Fig. 20.1C ). The nasopharynx (2 to 3 cm wide and 3 to 4 cm long) is the most anterior and lies behind the nose. In this region, the nose and mouth are connected via an isthmus (canals) that enables both oral and nasal breathing. Also, the nasopharynx contains small masses of lymphoid tissue (adenoids), also known as pharyngeal tonsils, which fight infections. The nasopharynx is connected to the middle ear cavity via  the eustachian tubes, which aid in equalizing pressure in  the ear to atmospheric pressure; thus they represent a drainage pathway of lymphatic fluid between the throat, nose, and ears. This network of structures provides a means of fighting infections but also is a common location for infections in the head.
The soft palate separates the nasopharynx and the oropharynx, which ends at the epiglottis. The laryngopharynx begins at the epiglottis and ends at the esophagus. Its major role is to help regulate the passage of food into the esophagus and air into the lungs. With some infections, these structures can become edematous and contribute significantly to airflow resistance. The epiglottis and arytenoid cartilage (attached to the vocal cords) cover or act as a hood over the vocal cords during swallowing. Thus under normal circumstances, the epiglottis and arytenoid cartilage function to prevent aspiration of food and liquid into the lower respiratory tract. The act of swallowing food after mastication (chewing) usually occurs within 2 seconds, and it is closely synchronized with muscle reflexes that coordinate opening and closing of the airway. Hence, air is allowed to enter the lower airways, and food and liquids are kept out. Patients with some neuromuscular diseases have altered muscle reflexes and can lose this coordinated swallowing mechanism. Such patients may become susceptible to aspiration of food and liquid, which poses a risk for pneumonia.
Lower Airways: Trachea, Bronchi, Bronchioles, and Respiratory Unit
The right lung, located in the right hemithorax, is divided into three lobes (upper, middle, and lower) by two interlobular fissures (oblique, horizontal), whereas the left lung, located in the left hemithorax, is divided into two lobes (upper, including the lingula, a tongue-like projection of the anterior aspect of the upper lobe, and lower) by an oblique fissure (