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Respiratory system

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Main structures of the respiratory system.
Respiratory system (Systema respiratorum)

The respiratory system, also called the pulmonary system, consists of several organs that function as a whole to oxygenate the body through the process of respiration (breathing). This process involves inhaling air and conducting it to the lungs where gas exchange occurs, in which oxygen is extracted from the air, and carbon dioxide expelled from the body. The respiratory tract is divided into two sections at the level of the vocal cords; the upper and lower respiratory tract.

The lungs are most often considered as part of the lower respiratory tract, but are sometimes described as a separate entity. They contain the respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli

This article will discuss the anatomy and function of the respiratory system.

Key facts about the respiratory system
Upper respiratory tract Nasal cavity, paranasal sinuses, pharynx and larynx above the vocal cords
Lower respiratory airways Larynx below the vocal cords, trachea, bronchi, bronchioles and lungs
Functions Upper respiratory tract: conduction, filtration, humidification and warming of inhaled air
Lower respiratory tract: conduction and gas exchange
  1. Upper respiratory tract
    1. Nasal cavity
    2. Paranasal sinuses
    3. Pharynx
    4. Larynx
  2. Lower respiratory tract
  3. Tracheobronchial tree
    1. Lungs
    2. Microanatomy
  4. Function
  5. Clinical aspects
    1. Upper respiratory tract infections
    2. Lower respiratory tract infections
  6. Sources
+ Show all

Upper respiratory tract

The upper respiratory tract refers to the parts of the respiratory system that lie outside the thorax, more specifically above the cricoid cartilage and vocal cords. It includes the nasal cavity, paranasal sinuses, pharynx and the superior portion of the larynx. Most of the upper respiratory tract is lined with the pseudostratified ciliated columnar epithelium, also known as the respiratory epithelium. The exceptions are some parts of the pharynx and larynx.

Nasal cavity

The upper respiratory tract begins with the nasal cavity. The nasal cavity opens anteriorly on the face through the two nares, and posteriorly into the nasopharynx through the two choanae. The floor of the nasal cavity is formed by the hard palate, while the roof consists of the cribriform plate of the ethmoid bone posteriorly, and the frontal and nasal bones anteriorly. The nares and anterior portion of the nasal cavity contain sebaceous glands and hair follicles that serve to prevent any larger harmful particles from passing into the nasal cavity. 

The lateral walls of the nasal cavity contain three bony projections called nasal conchae (superior, middle and inferior), which increase the surface area of the nasal cavity. The nasal conchae also disrupt the laminar flow of air, making it slow and turbulent, thereby helping to humidify and warm up the air to body temperature. 

The roof of the nasal cavity contains the olfactory epithelium which consists of specialized sensory receptors. These receptors pick up airborne odorant molecules and transform them into action potentials that travel via the olfactory nerve to the cerebral cortex, allowing the brain to register them and provide a sense of smell.

Another pathway for the entry of air is the oral cavity. Although it is not classified as a part of the upper respiratory tract, the oral cavity provides an alternative route in the case of obstruction of the nasal cavity. The oral cavity opens anteriorly on the face through the oral fissure, while posteriorly, it opens into the oropharynx through a passage called the oropharyngeal isthmus. 

Paranasal sinuses

Several bones that form the walls of the nasal cavity contain air-filled spaces called the paranasal sinuses, which are named after their associated bones; maxillary, frontal, sphenoidal and ethmoidal sinuses.

The paranasal sinuses communicate with the nasal cavity via several openings, and thereby also receive the inhaled air and contribute to its humidifying and warming. In addition, the mucous membrane and respiratory epithelium that lines both the nasal cavity and the paranasal sinuses traps any harmful particles, dust or bacteria.


After passing through the nasal cavity and paranasal sinuses, the inhaled air exits through the choanae into the pharynx. The pharynx is a funnel-shaped muscular tube that contains three parts; the nasopharynx, oropharynx and laryngopharynx

  • The nasopharynx is the first and superiormost part of the pharynx, found posterior to the nasal cavity. This part of the pharynx serves only as an airway, and is thus lined with respiratory epithelium. Inferiorly, the uvula and soft palate swing upwards during swallowing to close off the nasopharynx and prevent food from entering the nasal cavity.
  • The oropharynx is found posterior to the oral cavity and communicates with it through the oropharyngeal isthmus. The oropharynx is a pathway for both the air incoming from the nasopharynx and the food incoming from the oral cavity. Thus, the oropharynx is lined with the more protective non-keratinizing stratified squamous epithelium.
  • The laryngopharynx (hypopharynx) is the most inferior part of the pharynx. It is the point at which the digestive and respiratory systems diverge. Anteriorly, the laryngopharynx continues into the larynx, whereas posteriorly it continues as the esophagus


Following the laryngopharynx, the next and last portion of the upper respiratory tract is the superior part of the larynx. The larynx is a complex hollow structure found anterior to the esophagus. It is supported by a cartilaginous skeleton connected by membranes, ligaments and associated muscles. Above the vocal cords, the larynx is lined with stratified squamous epithelium like the laryngopharynx. Below the vocal cords, this epithelium transitions into pseudostratified ciliated columnar epithelium with goblet cells (respiratory epithelium). 

Besides its main function to conduct the air, the larynx also houses the vocal cords that participate in voice production. The laryngeal inlet is closed by the epiglottis during swallowing to prevent food or liquid from entering the lower respiratory tract.

If you want to learn more about the anatomy and function of the larynx, take a look at the study unit below!

Lower respiratory tract

The lower respiratory tract refers to the parts of the respiratory system that lie below the cricoid cartilage and vocal cords, including the inferior part of the larynx, tracheobronchial tree and lungs.

Tracheobronchial tree

The tracheobronchial tree is a portion of the respiratory tract that conducts the air from the upper airways to the lung parenchyma. It consists of the trachea and the intrapulmonary airways (bronchi and bronchioles).The trachea is located in the superior mediastinum and represents the trunk of the tracheobronchial tree. The trachea bifurcates at the level of the sternal angle (T5) into the left and right main bronchi, one for each lung.

  • The left main bronchus passes inferolaterally to enter the hilum of the left lung. On its course, it passes inferior to the arch of the aorta and anterior to the esophagus and thoracic aorta.
  • The right main bronchus passes inferolaterally to enter the hilum of the right lung. The right main bronchus has a more vertical course than its left counterpart and is also wider and shorter. This makes the right bronchus more susceptible to foreign body impaction.

As they reach the lungs, the main bronchi branch out into increasingly smaller intrapulmonary bronchi. The left main bronchus divides into two secondary lobar bronchi, while the right main bronchus divides into three secondary lobar bronchi that supply the lobes of the left and right lung, respectively.

Each of the lobar bronchi further divides into tertiary segmental bronchi that aerate the bronchopulmonary segments. The segmental bronchi then give rise to several generations of intrasegmental (conducting) bronchioles, which end as terminal bronchioles. Each terminal bronchiole gives rise to several generations of respiratory bronchioles. Respiratory bronchioles extend into several alveolar ducts, which lead into alveolar sacs, each of which contains many grape-like outpocketings called alveoli. Since they contain alveoli, these structures mark the site where gas exchange begins to occur.


The lungs are a pair of spongy organs located within the thoracic cavity. The right lung is larger than the left lung and consists of three lobes (superior, middle and inferior), which are divided by two fissures; oblique and horizontal fissure. The left lung has only two lobes (superior and inferior), divided by one oblique fissure. 

Each lung has three surfaces, an apex and a base. The surfaces of the lung are the costal, mediastinal and diaphragmatic surface, which are named after the adjacent anatomical structure which that surface faces. The mediastinal surface connects the lung to the mediastinum via its hilum. The apex of the lung is where the mediastinal and costal surfaces meet. It is the most superior portion of the lung, that extends into the root of the neck. The base is the lowest concave part of the lung that rests upon the diaphragm.

Each hilum of the lung contains the following:


On the microscopic level, the lower respiratory tract is characterized by several changes of epithelial lining, serving different purposes. Beginning from the inferior part of the larynx to the tertiary segmental bronchi, the lower respiratory tract is lined with pseudostratified ciliated columnar epithelium with goblet cells. The goblet cells produce mucus that lubricates and protects the airway by trapping any inhaled harmful particles. These trapped particles are then propelled towards the upper respiratory tract by the cilia of the epithelial cells and eventually expelled by coughing. 

As the larger tertiary segmental bronchi divide into smaller bronchi, the epithelium begins to change from respiratory epithelium to a simple columnar ciliated epithelium. This epithelium is continued in the larger terminal bronchioles, and transitions into a simple cuboidal epithelium in smaller terminal bronchioles. The epithelium of the terminal bronchioles contains exocrine bronchiolar cells called club cells, formerly known as Clara cells. These are non-ciliated cuboidal cells that contribute to the production of surfactant. In addition, the terminal bronchioles contain smooth muscle in their walls, that allows for bronchoconstriction and bronchodilation to occur. 

Terminal bronchioles then branch into respiratory bronchioles, which are also lined by simple cuboidal epithelium. The walls of the respiratory bronchioles extend into alveoli, and the epithelium changes into a simple squamous epithelium composed of type I and type II pneumocytes. Type I pneumocytes are thin, squamous cells that carry out the gas exchange, while type II pneumocytes are larger cuboidal cells that produce surfactant.


The main function of the respiratory system is pulmonary ventilation, which is the movement of air between the atmosphere and the lung by inspiration and expiration driven by the respiratory muscles. The respiratory system works as a whole to extract the oxygen from the inhaled air and eliminate the carbon dioxide from the body by exhalation. The upper respiratory mainly has an air-conducting function, while the lower respiratory tract serves both the conducting and respiratory functions. 

Besides its main function to conduct the air to the lower respiratory tract, the upper respiratory also performs several other functions. As mentioned earlier, the nasal cavity and paranasal sinuses change the properties of the air by humidifying and warming it in order to prepare it for the process of respiration. The air is also filtered from dust, pathogens and other particles by the nasal hair follicles and the ciliary epithelium.  

The portion of the lower respiratory tract, starting from the respiratory bronchioles, is the place where gas exchange begins to occur. This process is also known as external respiration, in which the oxygen from the inhaled air diffuses from the alveoli into the adjacent capillaries, while the carbon dioxide diffuses from the capillaries into the alveoli to be exhaled. The newly oxygenated blood then goes on to supply all the tissues in the body and undergoes internal respiration. This is the process in which the oxygen from the systemic circulation exchanges with carbon-dioxide from the tissues. Overall, the difference between external and internal respiration is that the former represents gas exchange with the external environment and takes place in the alveoli, while the latter represents gas exchange within the body and takes place in the tissues.

Test your knowledge on the respiratory system with this quiz.

To learn more about the complex respiratory system and solidify what you already learned in this article, head over to our respiratory system quizzes and labeled diagrams!

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