49 Introduction: The Respiratory System

Heather Ketchum and Eric Bright

The Respiratory System4

Hold your breath. Really! See how long you can hold your breath as you continue reading…How long can you do it? Chances are you are feeling uncomfortable already. A typical human cannot survive without breathing for more than 3 minutes, and even if you wanted to hold your breath longer, your autonomic nervous system would take control. This is because every cell in the body needs to run the oxidative stages of cellular respiration, the process by which energy is produced in the form of adenosine triphosphate (ATP). For oxidative phosphorylation to occur, oxygen is used as a reactant and carbon dioxide is released as a waste product. You may be surprised to learn that although oxygen is a critical need for cells, it is actually the accumulation of carbon dioxide that primarily drives your need to breathe. Carbon dioxide is exhaled and oxygen is inhaled through the respiratory system, which includes muscles to move air into and out of the lungs, passageways through which air moves, and microscopic gas exchange surfaces covered by capillaries. The circulatory system transports gases from the lungs to tissues throughout the body and vice versa. A variety of diseases can affect the respiratory system, such as asthma, emphysema, chronic obstruction pulmonary disorder (COPD), and lung cancer. All of these conditions affect the gas exchange process and result in labored breathing and other difficulties.

The Organs and Structures of the Respiratory System

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).

Major Respiratory Structures

This figure shows the upper half of the human body. The major organs in the respiratory system are labeled.

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 gas exchange occurs in 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. The following structures are part of the conducting zone and carry out the functions of the conducting zone: larynx, trachea, primary bronchi, secondary bronchi, and bronchioles. Several of these structures perform other functions as well.

Nasal Cavity

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. Here we will only focus on the nasal cavity.

Upper Airway

This figure shows a cross section view of the nose and throat. The major parts are labeled.

The 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.

Most of the nasal cavity is lined by the respiratory epithelium. This epithelium is composed of pseudostratified ciliated columnar epithelium (Figure). 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.

Pseudostratified Ciliated Columnar Epithelium

This figure shows a micrograph of pseudostratified 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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View the University of Michigan WebScope at to explore the tissue sample in greater detail.


The pharynx is a tube formed by skeletal muscle and lined by mucous membrane that is continuous with that of the nasal cavities (see Figure). The pharynx is essentially the back of the throat.


The larynx is a cartilaginous structure that connects the pharynx to the trachea and helps regulate the volume of air that enters and leaves the lungs (see [link]). Structures associated with the larynx also prevent food from entering the respiratory tract.

The epiglottis is a very flexible piece of elastic cartilage that covers the opening of the trachea (see Figure). When in the “closed” position, the unattached end of the epiglottis rests on the glottis. The glottis is the opening of the larynx. It is composed of the true vocal cords (see Figure). The act of swallowing causes the pharynx and larynx to lift upward, allowing the pharynx to expand and the epiglottis of the larynx to swing downward, closing the opening to the trachea. These movements produce a larger area for food to pass through, while preventing food and beverages from entering the trachea.

The upper 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 back of throat, where it can be swallowed down the esophagus.


The trachea (windpipe) extends from the larynx toward the lungs (Figurea). The trachea is formed by 16 to 20 stacked, C-shaped pieces of hyaline cartilage that are connected by dense connective tissue. The connective tissue allows the trachea to stretch and expand slightly during inhalation and exhalation, whereas the rings of cartilage provide structural support and prevent the trachea from collapsing. Similar to the larynx, the trachea is lined with goblet cells and cilia. The goblet cells produce mucus that traps debris while the cilia move the trapped debris into the back of the throat where it can be swallowed and enter the esophagus.


The top panel of this figure shows the trachea and its organs. The major parts including the larynx, trachea, bronchi, and lungs are labeled.

(a) The tracheal tube is formed by stacked, C-shaped pieces of hyaline cartilage. (b) The layer visible in this cross-section of tracheal wall tissue between the hyaline cartilage and the lumen of the trachea is the mucosa, which is composed of pseudostratified ciliated columnar epithelium that contains goblet cells. LM × 1220. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Bronchial Tree

The trachea branches into the right and left primary bronchi at the carina. These bronchi are also lined by pseudostratified ciliated columnar epithelium containing mucus-producing goblet cells (Figureb). The carina is a raised structure that conRings of cartilage, similar to those of the trachea, support the structure of the bronchi and prevent their collapse. The primary bronchi enter the lungs at the hilum, a concave region where blood vessels, lymphatic vessels, and nerves also enter the lungs. The right and left primary bronchi branch into smaller diameter tubes with many branches called secondary bronchi (also called lobar bronchi). The secondary bronchi branch into even smaller diameter branches called tertiary bronchi (also called segmental bronchi) and finally those branch into even smaller branches called bronchioles. A bronchial tree (or respiratory tree) is the collective term used for these multiple-branched bronchi. 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.

Bronchioles, which are about 1 mm in diameter, further branch until they become the tiny terminal bronchioles. The terminal bronchioles are the last structure of the conducting zone and will lead to the structures of gas exchange. There are more than 1000 terminal bronchioles in each lung. The muscular walls of the bronchioles do not contain cartilage like those of the bronchi. This muscular wall can change the size of the tubing to increase or decrease airflow through the tube.

Respiratory Zone

In contrast to the conducting zone, the respiratory zone includes structures that are directly involved in gas exchange. These structures include the respiratory structures include the respiratory bronchioles, alveolar ducts, and alveoli. The respiratory zone begins where the terminal bronchioles join a respiratory bronchiole, the smallest type of bronchiole (Figure), which then leads to an alveolar duct, opening into a cluster of alveoli.

Respiratory Zone

This image shows the bronchioles and alveolar sacs in the lungs and depicts the exchange of oxygenated and deoxygenated blood in the pulmonary blood vessels.

Bronchioles lead to alveolar sacs in the respiratory zone, where gas exchange occurs.


An alveolar duct is a tube composed of smooth muscle and connective tissue, which opens into a cluster of alveoli. An alveolus is one of the many small, grape-like sacs that are attached to the alveolar ducts.

An alveolar sac is a cluster of many individual alveoli that are responsible for gas exchange. An alveolus is approximately 200 µm in diameter with elastic walls that allow the alveolus to stretch during air intake, which greatly increases the surface area available for gas exchange. Alveoli are connected to their neighbors by alveolar pores, which help maintain equal air pressure throughout the alveoli and lung (Figure).

Structures of the Respiratory Zone

This figure shows the detailed structure of the alveolus. The top panel shows the alveolar sacs and the bronchioles. The middle panel shows a magnified view of the alveolus, and the bottom panel shows a micrograph of the cross section of a bronchiole.

(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. Type I cells form the wall of the alveoli. 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 by breaking hydrogen bonds in water molecules. Pulmonary surfactant allows the lungs to expand easily during respiration. 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 mm thick. The respiratory membrane allows gases to cross by simple diffusion, allowing oxygen to be picked up by the blood for transport and COto be released into the air of the alveoli.


Respiratory System: AsthmaAsthma 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). 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.

Normal and Bronchial Asthma Tissues

The top panel of this figure shows normal lung tissue, and the bottom panel shows lung tissue inflamed by asthma.(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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Visit this site to learn more about what happens during an asthma attack. What are the three changes that occur inside the airways during an asthma attack?

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Watch this video to learn more about the bronchial tree.


alveolar duct
small tube that leads from the terminal bronchiole to the respiratory bronchiole and is the point of attachment for alveoli
alveolar macrophage
immune system cell of the alveolus that removes debris and pathogens
alveolar pore
opening that allows airflow between neighboring alveoli
alveolar sac
cluster of alveoli
small, grape-like sac that performs gas exchange in the lungs
bronchial tree
collective name for the multiple branches of the bronchi and bronchioles of the respiratory system
branch of bronchi that are 1 mm or less in diameter and terminate at alveolar sacs
tube connected to the trachea that branches into many subsidiaries and provides a passageway for air to enter and leave the lungs
conducting zone
region of the respiratory system that includes the organs and structures that provide passageways for air and are not directly involved in gas exchange
leaf-shaped piece of elastic cartilage that is a portion of the larynx that swings to close the trachea during swallowing
opening between the vocal folds through which air passes when producing speech
cartilaginous structure that produces the voice, prevents food and beverages from entering the trachea, and regulates the volume of air that enters and leaves the lungs
region of the conducting zone that forms a tube of skeletal muscle lined with respiratory epithelium; located between the nasal conchae and the esophagus and trachea
pulmonary surfactant
substance composed of phospholipids and proteins that reduces the surface tension of the alveoli; made by type II alveolar cells
respiratory bronchiole
specific type of bronchiole that leads to alveolar sacs
respiratory epithelium
ciliated lining of much of the conducting zone that is specialized to remove debris and pathogens, and produce mucus
respiratory membrane
alveolar and capillary wall together, which form an air-blood barrier that facilitates the simple diffusion of gases
respiratory zone
includes structures of the respiratory system that are directly involved in gas exchange
tube composed of cartilaginous rings and supporting tissue that connects the lung bronchi and the larynx; provides a route for air to enter and exit the lung
type I alveolar cell
squamous epithelial cells that are the major cell type in the alveolar wall; highly permeable to gases
type II alveolar cell
cuboidal epithelial cells that are the minor cell type in the alveolar wall; secrete pulmonary surfactant

Heather Ketchum & Eric Bright, OU Human Physiology Textbook. OpenStax CNX. Jun 18, 2015. Download for free at http://cnx.org/contents/e4f804ec-103f-4157-92e1-71eed7aa8584@1


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Introduction to Exercise Science for Fitness Professionals by Heather Ketchum and Eric Bright is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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