15.1: Organs and Structures of the Respiratory System

Learning Objectives

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

• List the structures that make up the respiratory system
• Describe how the respiratory system processes oxygen and CO₂
• Compare and contrast the functions of upper respiratory tract with the lower respiratory tract

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 for straining, such as during childbirth or coughing (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Major Respiratory Structures The major respiratory structures span the nasal cavity to the diaphragm.

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. The respiratory zone includes structures involved in gas exchange. Gas exchange is the process in which oxygen diffuses from our lungs into blood through inhalation, and carbon dioxide diffuses from our blood into our lungs to be exhaled.

The structures of the conducting zone broadly include:

• The nostrils and nasal cavity
• The pharynx and its subdivisions
• The larynx
• Trachea
• Bronchial tree
• Bronchi
• Bronchioles, until reaching the terminal bronchioles

The structures of the respiratory zone include:

• Respiratory bronchioles
• Alveolar ducts
• Alveolar sacs
• Alveoli

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. The conducting zone "conducts" 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 bronchial epithelium that lines the lungs can metabolize some airborne carcinogens.

The Nose and its Adjacent Structures

The major 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. We will briefly discuss the structures of the internal nose.

The nares (nostrils) open into the nasal cavity. Each lateral wall of the nasal cavity has three bony projections, called the superior, middle, and inferior nasal conchae. The inferior conchae are separate bones, whereas the superior and middle conchae are portions of the ethmoid bone. Conchae serve to increase the surface area of the nasal cavity and to disrupt the flow of air as it enters the nose, causing air to bounce along the epithelium, where it is cleaned and warmed. The conchae also conserve water and prevent dehydration of the nasal epithelium by trapping water during exhalation. Air exits the nasal cavities via the internal nares and moves into the pharynx.

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 and caption 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.)

The nares and anterior portion of the nasal cavities are lined with mucous membranes, containing sebaceous glands and hair follicles that serve to prevent the passage of large debris, such as dirt, through the nasal cavity. An olfactory epithelium used to detect odors is found deeper in the nasal cavity.

The conchae are lined by respiratory epithelium composed of pseudostratified ciliated columnar epithelium (Figure \(\PageIndex{4}\)). The 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. This moist epithelium functions to warm and humidify incoming air. Capillaries located just beneath the nasal epithelium warm the air by convection. Serous and mucus-producing cells also secrete the lysozyme enzyme and proteins called defensins, which have antibacterial properties. Immune cells that patrol the connective tissue deep to the respiratory epithelium provide additional protection.

Figure \(\PageIndex{4}\): Pseudostratified Ciliated Columnar Epithelium Respiratory epithelium is pseudostratified ciliated columnar epithelium. Seromucous glands provide lubricating mucus. LM × 680. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

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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 \(\PageIndex{5}\)).

Figure \(\PageIndex{5}\): Divisions of the Pharynx The pharynx is divided into three regions: the nasopharynx, the oropharynx, and the laryngopharynx.

The nasopharynx is flanked by the conchae of the nasal cavity, and it serves only as an airway. In addition, auditory (Eustachian) tubes that connect to each middle ear cavity open into the nasopharynx. This connection is why colds often lead to ear infections.

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 pseudostratified ciliated columnar epithelium to stratified squamous epithelium.

The laryngopharynx is inferior to the oropharynx and posterior to the larynx. It continues the route for ingested material and air until its inferior end, where the digestive and respiratory systems diverge. The stratified squamous epithelium of the oropharynx is continuous with the laryngopharynx. Anteriorly, the laryngopharynx opens into the larynx, which continues into the trachea, whereas posteriorly, it enters the esophagus, which continues into the stomach.

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 (Figure \(\PageIndex{6}\)). The structure of the larynx is formed by several pieces of cartilage. Three large cartilage pieces—the thyroid cartilage (anterior), epiglottis (superior), and cricoid cartilage (inferior)—form the major structure of the larynx. The thyroid cartilage is the largest piece of cartilage that makes up 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 ring, with a wide posterior region and a thinner anterior region. Three smaller, paired cartilages—the arytenoids, corniculates, and cuneiforms—attach to the epiglottis and the vocal cords and muscle that help move the vocal cords to produce speech.

Figure \(\PageIndex{6}\): 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, is a very flexible piece of elastic cartilage that covers the opening of the trachea (see Figure \(\PageIndex{6}\)). When in the “closed” position, the unattached end of the epiglottis rests on the glottis. 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. 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.

Continuous with the laryngopharynx, the superior portion of the larynx is lined with stratified squamous epithelium, transitioning into pseudostratified ciliated 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. The cilia beat the mucus upward towards the laryngopharynx, where it can be swallowed down the esophagus.

Trachea

The trachea (windpipe) extends from the larynx toward the lungs (Figure \(\PageIndex{8}\)). 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.

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 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.

Respiratory Zone

In contrast to the conducting zone, the respiratory zone includes structures that are directly involved in gas exchange. The respiratory zone begins where the terminal bronchioles transition into a respiratory bronchiole, the smallest type of bronchiole (Figure \(\PageIndex{9}\)), which then leads to an alveolar duct, opening into a cluster of alveoli. A respiratory bronchiole is distinguished from a terminal bronchiole by the presence of alveoli within the walls of a respiratory bronchiole.

Figure \(\PageIndex{13}\): Respiratory Zone Bronchioles lead to alveolar sacs in the respiratory zone, where gas exchange occurs.

Alveoli

Respiratory bronchioles divide distally to form alveolar ducts. An alveolar duct does not have its own walls, but are channels created by the walls of the alveoli that line it. Each alveolar duct opens into a cluster of alveoli. An alveolar duct and all of the alveoli connected to it form a cluster called an alveolar sac. There are an average of 480 million alveoli in the lungs, yielding an incredibly large surface area for gas exchange.

Each alveolar sac is surrounded by a capillary bed with a network of capillaries that distribute several capillaries to route along the outside of each alveolus in the alveolar sac \(\PageIndex{14}\). Deoxygenated blood is delivered to the capillary bed via an arteriole branching from the pulmonary artery and oxygenated blood is collected by a venule that drains to the pulmonary vein. An alveolus is approximately 200 μm in diameter with elastic walls that allow the alveolus to stretch during air intake like a balloon inflating, which greatly increases the surface area available for gas exchange, and then return to a smaller size to assist with expiration.

Figure \(\PageIndex{14}\): Structures of the Respiratory Zone (a) The alveolus is responsible for gas exchange. (b) A micrograph shows the alveolar structures within lung tissue. LM × 178. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

The alveolar wall consists of three major cell types: type I alveolar cells, type II alveolar cells, and alveolar macrophages. A type I alveolar cell is a squamous epithelial cell of the alveoli, which constitute up to 97 percent of the alveolar surface area. These cells are about 25 nm thick and are highly permeable to gases. A type II alveolar cell is interspersed among the type I cells and secretes pulmonary surfactant, a substance composed of phospholipids and proteins that reduces the surface tension of the alveoli. Roaming around the alveolar wall is the alveolar macrophage, a phagocytic cell of the immune system that removes debris and pathogens that have reached the alveoli.

The simple squamous epithelium formed by type I alveolar cells is attached to a thin, elastic basement membrane. This epithelium is extremely thin and borders the endothelial membrane of capillaries. Taken together, the alveoli and capillary membranes form a respiratory membrane that is approximately 0.5 μm (micrometers) thick. The respiratory membrane allows gases to cross by simple diffusion, allowing oxygen to be picked up by the blood for transport and CO₂ to be released into the air of the alveoli.

Diseases 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 mononuclear cells, 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 \(\PageIndex{11}\) Normal and Bronchial Asthma Tissues (a) Normal lung tissue does not have the characteristics of lung tissue during (b) an asthma attack, which include thickened mucosa, increased mucus-producing goblet cells, and eosinophil infiltrates.

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.

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