20.3: Conducting Zone

By the end of this section, you will be able to:

• List the organs that make up the conducting zone of the respiratory system
• Describe the structure and related functions of the respiratory epithelium
• Name the cartilages of the larynx and vocal cords and explain their role in voice production
• Compare and contrast the structure of the tracheobronchial tree regions

The major organs of the respiratory system function primarily to provide oxygen to body tissues for cellular respiration, remove the waste product carbon dioxide, and help to maintain acid-base balance. Portions of the respiratory system are also used for non-vital functions, such as sensing odors, speech production, and straining, such as during childbirth or coughing (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Respiratory System Organs. The organs of the respiratory system conduct air to the lungs for gas exchange. (Image credit: "Respiratory System Organs" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of "OLI - Drawing Lower respiratory tract and detail of alveoli - English labels" by KnowledgeWorks Global Ltd..)

Functionally, the respiratory system can be divided into a conducting zone and a respiratory zone. The conducting zone of the respiratory system includes the organs and structures not directly involved in gas exchange. Gas exchange occurs in the respiratory zone. The conducting zone delivers air to and from the respiratory zone.

Conducting Zone

The major functions of the conducting zone are to provide a route for incoming and outgoing air, remove debris and pathogens from the incoming air, and warm and humidify the incoming air. Several structures within the conducting zone perform other functions as well. The epithelium of the nasal passages, for example, is essential to sensing odors, and the larynx is our voice box.

The Nose and its Adjacent Structures

The primary entrance and exit for the respiratory system is through the nose. When discussing the nose, it is helpful to divide it into two major sections: the external nose, and the nasal cavity or internal nose.

The external nose consists of the surface and skeletal structures that result in the outward appearance of the nose and contribute to its numerous functions (Figure \(\PageIndex{2}\)). 

Figure \(\PageIndex{2}\): Nose. The major skeletal and cartilaginous features supporting the structure of the external nose. (Image credit: "Nasal Cartilages" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of Rinoplasti by https://www.ozanbalik.com.)

While the superior aspect of the nose consists of bone, most of the protruding portion of the nose is composed of hyaline cartilage. As a result, when looking at a skull much of the nose is missing. The nasal bone is one of a pair of bones that lies under the bridge of the nose. The nasal bone articulates superiorly with the frontal bone and laterally with the maxillary bones.

On either side of the midline, the alar cartilages and lateral cartilages, form the medial portion of the external nares, the narrow openings into each nasal cavity. The lateral portion of the external nares lack cartilage. The external nares open into the nasal cavities, which is separated into left and right sections by the nasal septum. The nasal septum is formed anteriorly by a portion of the septal cartilage, the flexible hyaline cartilage you can move with your fingers that forms the majority of the dorsum nasi, and posteriorly by the perpendicular plate of the ethmoid bone superiorly and the vomer bone inferiorly.

Each lateral wall of the nasal cavity has three bony projections, called the superior, middle, and inferior nasal conchae (Figure \(\PageIndex{3}\)). The inferior nasal conchae are separate bones, whereas the superior and middle nasal conchae are portions of the ethmoid bone. Several bones that help form the walls of the nasal cavity have hollowed air-containing spaces called the paranasal sinuses. The increased surface area of both the nasal cavities and the sinuses contribute to the richness of the tone of sounds produced, as evidenced by the change in your voice when you plug your nose or are congested.

Figure \(\PageIndex{3}\): Nasal Passages - Sagittal Section. Air passes through the external naris and enters the nasal cavity, which has raised nasal conchae protruding into the space. (Image credit: "Nasal Cavities" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of "Slagter - Drawing Head and neck: sagittal section - no labels" by Ron Slagter.)

Each paranasal sinus is named for its associated bone: frontal sinus, maxillary sinus, sphenoidal sinus, and ethmoidal sinuses. The hollowed sinuses also lighten the weight of the skull.The frontal sinus is located just above the eyebrows, within the frontal bone (Figure \(\PageIndex{3}\) and Figure \(\PageIndex{4}\)). This irregular space may be divided at the midline into bilateral spaces, or these may be fused into a single sinus space. The frontal sinus is the most anterior of the paranasal sinuses. The largest sinus is the maxillary sinus (Figure \(\PageIndex{4}\)). These are paired and located within the right and left maxillary bones, where they occupy the area just below the orbits. The maxillary sinuses are most commonly involved during sinus infections. Because their connection to the nasal cavity is located high on their medial wall, they are difficult to drain. The sphenoid sinus is a single, midline sinus . It is located within the body of the sphenoid bone, just anterior and inferior to the sella turcica, thus making it the most posterior of the paranasal sinuses. The lateral aspects of the ethmoid bone contain multiple small spaces separated by very thin bony walls (Figure \(\PageIndex{4}\)). Each of these spaces is called an ethmoid air cell. These are located on both sides of the ethmoid bone, between the upper nasal cavity and medial orbit, just behind the superior nasal conchae.

C. Relative Locations

Figure \(\PageIndex{4}\): Paranasal Sinuses. The paranasal sinuses are hollow, air-filled spaces named for the skull bone that each occupies. The most anterior is the frontal sinus, located in the frontal bone above the eyebrows. The largest are the maxillary sinuses, located in the right and left maxillary bones below the orbits. The most posterior is the sphenoid sinus, located in the body of the sphenoid bone, under the sella turcica. The ethmoid air cells are multiple small spaces located in the right and left sides of the ethmoid bone, between the medial wall of the orbit and lateral wall of the upper nasal cavity. (Images A and B credit: "Paranasal Sinuses" by Mark Schaeffer are licensed under CC BY-NC-SA 4.0, modification of image from Anatomy Standard under CC BY-NC 4.0.  Image C credit: "Paranasal Sinuses" by OpenStax is licensed under CC BY 3.0)

The external nares and nasal vestibule at the anterior portion of each nasal cavity are composed of skin: lined with stratified squamous epithelium and contain sebaceous glands and hair follicles embedded in the dermis that serve to prevent the passage of large debris, such as dirt, through the nasal cavity.

Respiratory Epithelium

The conchae, meatuses, and paranasal sinuses are lined by a mucous membrane or mucosa called the respiratory epithelium composed of ciliated pseudostratified columnar epithelium (Figure \(\PageIndex{5}\) shows the epithelium lining in the trachea). (Mucous spelled with an "o" is the adjective form of the word, while mucus spelled without an "o" is the noun form used to describe the secretion produced by mucous membranes.) The respiratory epithelium contains goblet cells, one of the specialized, columnar epithelial cells that produce mucus to trap debris. The cilia of the respiratory epithelium help remove the mucus and debris from the nasal cavity with a constant beating motion, sweeping materials towards the throat to be swallowed. Interestingly, cold air slows the movement of the cilia, resulting in accumulation of mucus that may in turn lead to a runny nose during cold weather.

The tissue-covered nasal conchae and meatuses within the nasal cavities create turbulent airflow and provide an increased surface area to aid in filtering, warming, and humidifying air as it enters the body. Capillaries located just beneath the nasal epithelium warm the air by convection.  Immune cells that patrol the connective tissue deep to the respiratory epithelium provide additional protection.

A specialized olfactory epithelium used to detect odors (olfaction is the sense of smell) is found at the superior surface of the nasal cavity in the area of the olfactory foramina of the ethmoid bone and is covered in more detail in the chapter that includes the special senses of the nervous system.

Figure \(\PageIndex{5}\): Ciliated Pseudostratified Columnar Epithelium. The respiratory epithelium consists of a ciliated pseudostratified columnar epithelium featuring cilia at the apical surface facing the lumen of the airway and goblet cells that secrete mucus. Seromucous glands embedded in the dense irregular connective tissue of the submucosa provide additional lubricating mucus. LM × 680. (Image credit: "Pseudostratified Epithelium" by OpenStax is licensed under CC BY 3.0 / Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Pharynx

Inhaled air moves deeper into the upper airway when it leaves the nasal cavities via the internal nares, also known as posterior nasal apertures, narrowings at the back of each nasal cavity, and moves into the pharynx.  The pharynx is a tube formed by skeletal muscle and lined by mucous membrane that is continuous with that of the nasal cavities. The pharynx is divided into three major regions: the nasopharynx, the oropharynx, and the laryngopharynx (Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{6}\): Divisions of the Pharynx. The pharynx a shared respiratory and digestive structure that is divided into three regions: the nasopharynx, the oropharynx, and the laryngopharynx.  (Image credit:"Regions of the Pharynx" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of "Slagter - Drawing Head and neck: sagittal section - no labels" by Ron Slagter.)

The nasopharynx is posterior to the conchae of the nasal cavity, and it is meant to serve only as an airway. The uvula is a small bulbous, teardrop-shaped structure located at the apex of the soft palate. Both the uvula and soft palate move like a pendulum during swallowing, swinging upward to close off the nasopharynx to prevent ingested materials from entering the nasal cavity. In addition, auditory (Eustachian) tubes that connect to each middle ear cavity open into the nasopharynx. This connection is why colds may lead to ear infections.

Unlike the nasopharynx which is a passageway for air only, the oropharynx is a passageway for both air and food. The oropharynx is bordered superiorly by the nasopharynx and anteriorly by the oral cavity.  As the nasopharynx becomes the oropharynx, the epithelium changes from ciliated pseudostratified columnar epithelium to a stratified squamous epithelium.

The laryngopharynx is inferior to the oropharynx. It continues the route for ingested material and air until its inferior end, where the digestive and respiratory systems diverge. It is posterior to the open epiglottis, allowing air to move in and out of the larynx. When you swallow ingested material, the epiglottis closes and the material moves into the esophagus. The stratified squamous epithelium of the oropharynx is continuous with the laryngopharynx.

Larynx

The larynx is a cartilaginous structure inferior to the laryngopharynx that connects the pharynx to the trachea and helps regulate the volume of air that enters and leaves the lungs, protects the respiratory passages from ingested materials, and produces sound (Figure \(\PageIndex{7}\)). The structure of the larynx is formed by several pieces of cartilage. With the exception of the epiglottis, the cartilages of the larynx are comprised of hyaline cartilage. Three large cartilage pieces—the thyroid cartilage (anterior), epiglottis (superior), and cricoid cartilage (inferior)—form the major structure of the larynx, which is also known as the voice box. The thyroid cartilage is the largest piece of cartilage that makes up the larynx and functions to form a protective shield across the anterior of the larynx. The thyroid cartilage consists of the laryngeal prominence, or “Adam’s apple,” which tends to be more prominent in males. The thick cricoid cartilage forms a continuous ring around the larynx to hold the airway open as pressure changes during ventilation. It has a taller posterior region and a shorter anterior region inferior to the thyroid cartilage. The cricoid cartilage is connected to the thyroid cartilage by the cricothyroid ligament across the midline anteriorly and is connected to the most superior cartilage of the trachea by the cricotracheal ligament. Three smaller, paired cartilages—the arytenoids, corniculates, and cuneiforms—attach to the epiglottis and the vocal cords as well as the muscle that helps move the vocal cords to produce sound.

Figure \(\PageIndex{7}\): Larynx. The larynx is comprised of several cartilaginous structures connected by ligaments. Within the larynx the vocal folds attach anteriorly to the thyroid cartilage and posteriorly to the arytenoid cartilage. (Image credit: "Larynx - Midsagittal View" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of "Slagter - Drawing Head and neck: sagittal section - no labels" by Ron Slagter.  "Larynx - Anterolateral View" by Jennifer Lange and John Polos is licensed under CC BY-NC-SA 4.0, modification of  "Slagter - Drawing Larynx anatomy: anterior lateral view - no labels" by Ron Slagter.)

The epiglottis, attached to the thyroid cartilage by a ligament, is a very flexible piece of elastic cartilage that is tethered to the body of the hyoid bone and remains open unless you are swallowing. When the hyoid bone moves during swallowing, the pharynx and larynx are lifted upward, allowing the pharynx to expand and the epiglottis to swing downward to cover the glottis. These movements produce a larger area for food to pass through, while preventing ingested food and beverages from entering the trachea.

The glottis is composed of the vestibular folds, the true vocal cords, and the space between these folds through which air passes to and from the trachea (Figure \(\PageIndex{8}\)). A vestibular fold, or false vocal cord, is one of a pair of folded sections of mucous membrane that sit superior to the vocal folds and may function to protect the vocal folds and amplify sound. A true vocal cord (or vocal fold) is one of the white, membranous folds attached by muscle to the thyroid and arytenoid cartilages of the larynx on their outer edges. The inner edges of the true vocal cords contain an elastic vocal ligament covered in epithelial tissue that vibrates as air moves across it to produce sound. The size of the membranous folds of the true vocal cords differs between individuals, producing voices with different pitch ranges. Folds in males tend to be larger than those in females, which create a deeper voice. Movements of the muscles attaching the folds to the thyroid and arytenoid cartilages reposition the vocal folds to adjust the pitch of sounds produced.

Figure \(\PageIndex{8}\): Glottis. The glottis inside the larynx includes the vocal folds and the open space between them. A superior view shows the open epiglottis at the anterior and the trachea inferior to the glottis, visible beyond the opening of the V-shaped vocal folds.   (Image credit: "Glottis and Vocal Cords" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, based on illustration "Slagter - Drawing Larynx and vocal cords - no labels" by Ron Slagter and photo "Cuerdasvocales", via Wikimedia Commons.)

Continuous with the laryngopharynx, the superior portion of the larynx is lined with stratified squamous epithelium, transitioning into ciliated pseudostratified columnar epithelium that contains goblet cells. Similar to the nasal cavity and nasopharynx, this specialized epithelium produces mucus to trap debris and pathogens as they enter the trachea. Beginning in this region and continuing throughout much of the conducting zone, cilia beat the mucus upward towards the laryngopharynx, where it can be swallowed down the esophagus into the acidic environment of the stomach that functions to kill pathogens trapped in the mucus. The action of the cilia moving the mucus upward to be swallowed is referred to as the mucous escalator.

Trachea

The trachea (windpipe) extends from the larynx toward the lungs (Figure \(\PageIndex{9}\)). The mucosal layer of the wall of the trachea is lined with ciliated pseudostratified columnar epithelium featuring mucus-secreting goblet cells and covering a lamina propria of areolar connective tissue. The submucosa contains dense irregular connective tissue and houses seromucous glands that secrete a lubricating mucus that shares properties with serous fluid. The wall of the trachea contains 16 to 20 stacked, C-shaped pieces of hyaline cartilage that are positioned horizontally with the opening in the C at the back of the trachea. The incomplete rings of cartilage provide structural support that prevent the trachea from collapsing and protect it. The cartilages are connected to one another by dense connective tissue. The fibroelastic membrane consists of the trachealis muscle, made of smooth muscle, and elastic connective tissue. It is a flexible membrane that spans the gap across the the C-shaped cartilages at the posterior of the trachea, allowing the trachea to stretch and expand slightly during inhalation and exhalation. Its flexibility also allows the trachea to accommodate ingested materials passing through the esophagus that borders the trachea posteriorly. In addition, the smooth muscle of the trachealis muscle can be contracted to force air through the trachea during exhalation. The superficial wrapping of the trachea is an adventitia of dense irregular connective tissue.

Figure \(\PageIndex{9}\): Trachea. The mucosa lining the hollow lumen of the trachea is comprised of ciliated pseudostratified columnar epithelium that contains goblet cells covering a lamina propria of connective tissue. The submucosa houses seromucous tracheal glands. The C-shaped rings of hyaline cartilage are positioned deep to the mucosa with the opening at the posterior, which is spanned by the trachealis muscle and connective tissue. The superficial layer of the trachea is adventitia. The esophagus is positioned posterior to the trachea.  (Image credit: "Trachea" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of illustration by Open Stax College, slide @ 10x provided by Virginia Commonwealth University and slide @ 400x provided by the Regents of the University of Michigan Medical School © 2022.)

Bronchial Tree

The trachea branches into the right and left primary (main) bronchi (singular = bronchus) at the carina. The carina is a raised structure that contains specialized nervous tissue that induces violent coughing if a foreign body, such as food, is present. These bronchi are also lined by ciliated pseudostratified columnar epithelium containing mucus-producing goblet cells (Figure \(\PageIndex{10}\)). Rings of cartilage, similar to those of the trachea, support the structure of the bronchi and prevent their collapse. The primary bronchi branch to the secondary (lobar) bronchi that deliver air to the individual lobes of each lung. The secondary bronchi branch to the tertiary (segmental) bronchi that deliver air to the bronchopulmonary segments that comprise each lobe. 

Figure \(\PageIndex{10}\): Bronchi. The trachea branches at the carina into the right and left primary bronchi that deliver air to each lung. The primary bronchi branch into the secondary bronchi that deliver air to the lobes of each lung. The secondary bronchi branch into the tertiary bronchi that deliver air into the bronchopulmonary segments of each lobe.  (Image credit: "Bronchi" by Jennifer Lange is licensed under CC BY-NC-SA 4.0; contains modification of original by OpenStax College.)

The bronchial tree (or respiratory tree) is the collective term used for these highly branching bronchi, since they resemble branches of an upside-down tree (Figure \(\PageIndex{10}\) and Figure \(\PageIndex{11}\)). The main function of the bronchi, like other conducting zone structures, is to provide a passageway for air to move into and out of each lung. In addition, the mucous membrane traps debris and pathogens, cilia continue to work as a mucous escalator moving mucus up toward the laryngopharynx to be swallowed. Hyaline cartilage is featured within the wall of all the bronchi to reinforce the airways and help them stay open through pressure changes, but its quantity decreases with each branch point. The C-shaped rings of cartilage in the trachea become irregular plates of cartilage in the bronchi that are smaller and more sporadic in the tertiary bronchi than in the primary bronchi. A layer of smooth muscle is present in the bronchi that may bronchoconstrict, make the lumen diameter smaller by contracting the smooth muscle, or bronchodilate, make the lumen diameter larger by relaxing the smooth muscle, to adjust air flow in each passage. The quantity of smooth muscle in the wall relative to the lumen diameter increases with each branch point, meaning that the capability to adjust air flow is greater in the tertiary bronchi than in the primary bronchi.

Figure \(\PageIndex{11}\): Tracheobronchial Tree. As the airways branch and spread out within the lungs the histological structure changes and the total cross sectional area increases logarithmically.  *Values are intended to be illustrative because actual values differ based on the technique used for calculation (see Rao & Johncy, 2022). (Image credit: "Tracheobronchial Tree" by Jennifer Lange is licensed under CC BY-NC-SA 4.0; contains modification of original by OpenStax College.)

Bronchi continue to branch from the tertiary bronchi, getting smaller and smaller, until they become bronchioles. Bronchioles, which are 1 mm in diameter or less, further branch until they become the tiny terminal bronchioles, which lead to the structures of gas exchange. There are approximately 30,000 terminal bronchioles in each lung. As they branch, the epithelium of the bronchioles transitions from ciliated pseudostratified columnar epithelium to a thinner simple columnar epithelium to an even thinner simple cuboidal epithelium in the terminal bronchioles. Cilia and mucus-producing cells are sporadic in the largest bronchioles and then disappear as the bronchioles get smaller. The walls of the bronchioles do not contain cartilage like those of the bronchi, but they feature a significant layer of smooth muscle to change their diameter to adjust air flow.

DISORDERS OF THE...

Respiratory System: Asthma

Asthma is common condition that affects the lungs in both adults and children. Approximately 8.2 percent of adults (18.7 million) and 9.4 percent of children (7 million) in the United States suffer from asthma. In addition, asthma is the most frequent cause of hospitalization in children.

Asthma is a chronic disease characterized by inflammation and edema of the airway, and bronchospasms (that is, constriction of the bronchioles), which can inhibit air from entering the lungs. In addition, excessive mucus secretion can occur, which further contributes to airway occlusion (Figure \(\PageIndex{11}\)). Cells of the immune system, such as eosinophils and macrophages, may also be involved in infiltrating the walls of the bronchi and bronchioles.

Bronchospasms occur periodically and lead to an “asthma attack.” An attack may be triggered by environmental factors such as dust, pollen, pet hair, or dander, changes in the weather, mold, tobacco smoke, and respiratory infections, or by exercise and stress.

Figure \(\PageIndex{12}\): Normal and Asthmatic Bronchial Tissues. The picture shows a closeup of normal and asthmatic lung tissue.  Normal lung tissue (B) does not have the characteristics of lung tissue during an asthma attack (C), which include thickened mucosa, increased mucus-producing goblet cells, increased numbers of macrophages and mast cells, and eosinophil infiltrates. (Image credit: "Asthma by NIH is licensed under CC BY 2.0)

Symptoms of an asthma attack involve coughing, shortness of breath, wheezing, and tightness of the chest. Symptoms of a severe asthma attack that requires immediate medical attention would include difficulty breathing that results in blue (cyanotic) lips or face, confusion, drowsiness, a rapid pulse, sweating, and severe anxiety. The severity of the condition, frequency of attacks, and identified triggers influence the type of medication that an individual may require. Longer-term treatments are used for those with more severe asthma. Short-term, fast-acting drugs that are used to treat an asthma attack are typically administered via an inhaler. For young children or individuals who have difficulty using an inhaler, asthma medications can be administered via a nebulizer.

In many cases, the underlying cause of the condition is unknown. However, recent research has demonstrated that certain viruses, such as human rhinovirus C (HRVC), and the bacteria Mycoplasma pneumoniae and Chlamydia pneumoniae that are contracted in infancy or early childhood, may contribute to the development of many cases of asthma.

Concept Review

The respiratory system is responsible for obtaining oxygen and getting rid of carbon dioxide, and aiding in speech production and in sensing odors. From a functional perspective, the respiratory system can be divided into two major areas: the conducting zone and the respiratory zone.

The conducting zone consists of all of the structures that provide passageways for air to travel into and out of the lungs:

• nasal cavities - contain the conchae and meatuses that expand the surface area of the cavity, which helps to warm and humidify incoming air, while removing debris and pathogens;
• pharynx - composed of three major sections:
  • the nasopharynx, which is continuous with the nasal cavity
  • the oropharynx, which borders the nasopharynx and the oral cavity and
  • the laryngopharynx, which connects the oropharynx to the larynx and esophagus;
• larynx - a cartilaginous structure involved in passing air to and from the trachea (while blocking food from entering) and sound production;
• trachea - long tube lined with respiratory epithelium and supported by cartilage that conducts air from the cervical region to the thoracic region;
• bronchi - three levels of cartilage supported tubes lined with respiratory epithelium:
  • primary bronchi lead to each lung,
  • secondary bronchi lead to each lobe of the lung,
  • tertiary bronchi lead to each segment of each lobe;
• most bronchioles - conduct air to the respiratory zone. 

The structure of the airway wall progressively changes as the branches go deeper into the lungs and air is distributed into smaller but more numerous tubes.  Changes include:

• the diameter of the individual tubes decreases
• the amount of hyaline cartilage supporting the tube wall decreases
• the extent of the of smooth muscle surrounding the airways increases
• the epithelial tissue becomes progressively thinner.

Review Questions

Query \(\PageIndex{1}\)

 

Critical Thinking Questions

Query \(\PageIndex{2}\)

Query \(\PageIndex{3}\)

Query \(\PageIndex{4}\)

 

References

Ananda Rao, A., & Johncy, S. (2022). Tennis Courts in the Human Body: A Review of the Misleading Metaphor in Medical Literature. Cureus, 14(1), e21474. https://doi.org/10.7759/cureus.21474.

Glossary

Query \(\PageIndex{5}\)

Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology