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Respiratory system

Author: Gordana Sendić, MD • Reviewer: Roberto Grujičić, MD Last reviewed: October 30, 2023 Reading time: 16 minutes

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 upper respiratory tract includes the nasal cavity , paranasal sinuses , pharynx and the portion of the larynx above the vocal cords.
• The lower respiratory tract includes the larynx below the vocal cords, the trachea , bronchi , bronchioles and the lungs.

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.

Nasal cavity

Paranasal sinuses, lower respiratory tract, microanatomy, upper respiratory tract infections, lower respiratory tract infections.

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.

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 choana e. 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. 

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!

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:

• Principal bronchus
• Pulmonary artery
• Two pulmonary veins
• Bronchial vessels
• Pulmonary autonomic plexus
• Lymph nodes and vessels

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 !

Clinical aspects

Upper respiratory tract infections are contagious infections that can be caused by a variety of bacteria and viruses. The most common causing agents are influenza virus (the flu), rhinoviruses and streptococcus bacteria. Depending on which part of the upper respiratory tract is affected, these infections may have different types, such as rhinitis , sinusitis , pharyngitis , epiglottitis , laryngitis and others. 

The common cold is the most common type of upper respiratory tract infection. It is a viral infection that usually involves the nose and throat, but other parts can be affected as well. The symptoms usually include sore throat,‎ coughing, sneezing, runny nose, headache, and fever.

Lower respiratory tract infections are infections that affect the parts of the respiratory tract below the vocal cords. These infections can affect the airways and manifest as bronchitis or bronchiolitis , or they can affect the lung alveoli and present as pneumonia . These can also occur in conjunction as bronchopneumonia. 

The most common cause of lower respiratory tract infections are bacteria, but they can also occur due to viruses, mycoplasma, rickettsiae and fungi. These agents invade the epithelial lining, causing inflammation, increased mucus secretion, and impaired mucociliary function. The inflammation and build-up of fluid in the lungs and airways may result in symptoms such as coughing, fever, sputum production, difficulty breathing or in severe cases, airway obstruction and impaired gas exchange.

References:

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Clinically Oriented Anatomy (7th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.
• Netter, F. (2019). Atlas of Human Anatomy (7th ed.). Philadelphia, PA: Saunders.
• Standring, S. (2016). Gray's Anatomy (41st ed.). Edinburgh: Elsevier Churchill Livingstone.
• Dasaraju PV, Liu C. Infections of the Respiratory System. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 93.
• Jeremy P. T. Ward; Jane Ward; Charles M. Wiener (2006). The respiratory system at a glance. Wiley-Blackwell.

Illustrators:

• The respiratory system (Systema respiratorium) - Begoña Rodriguez
• Organs of the respiratory system (overview) - Begoña Rodriguez
• Bronchi and alveoli (overview) - Paul Kim
• Medial view of the lung (overview) - Yousun Koh

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human respiratory system

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• Pressbooks Create - Human Biology - Respiratory System
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Trusted Britannica articles, summarized using artificial intelligence, to provide a quicker and simpler reading experience. This is a beta feature. Please verify important information in our full article.

This summary was created from our Britannica article using AI. Please verify important information in our full article.

human respiratory system , the system in humans that takes up oxygen and expels carbon dioxide .

The design of the respiratory system

The human gas-exchanging organ, the lung , is located in the thorax, where its delicate tissues are protected by the bony and muscular thoracic cage. The lung provides the tissues of the human body with a continuous flow of oxygen and clears the blood of the gaseous waste product, carbon dioxide . Atmospheric air is pumped in and out regularly through a system of pipes, called conducting airways, which join the gas-exchange region with the outside of the body. The airways can be divided into upper and lower airway systems. The transition between the two systems is located where the pathways of the respiratory and digestive systems cross, just at the top of the larynx .

The upper airway system comprises the nose and the paranasal cavities (or sinuses ), the pharynx (or throat), and partly also the oral cavity , since it may be used for breathing. The lower airway system consists of the larynx, the trachea , the stem bronchi , and all the airways ramifying intensively within the lungs, such as the intrapulmonary bronchi, the bronchioles, and the alveolar ducts. For respiration, the collaboration of other organ systems is clearly essential. The diaphragm , as the main respiratory muscle, and the intercostal muscles of the chest wall play an essential role by generating, under the control of the central nervous system , the pumping action on the lung. The muscles expand and contract the internal space of the thorax, the bony framework of which is formed by the ribs and the thoracic vertebrae. The contribution of the lung and chest wall (ribs and muscles) to respiration is described below in The mechanics of breathing . The blood, as a carrier for the gases, and the circulatory system (i.e., the heart and the blood vessels ) are mandatory elements of a working respiratory system ( see blood ; cardiovascular system ).

Morphology of the upper airways

The nose is the external protuberance of an internal space, the nasal cavity . It is subdivided into a left and right canal by a thin medial cartilaginous and bony wall, the nasal septum . Each canal opens to the face by a nostril and into the pharynx by the choana. The floor of the nasal cavity is formed by the palate , which also forms the roof of the oral cavity. The complex shape of the nasal cavity is due to projections of bony ridges, the superior, middle, and inferior turbinate bones (or conchae), from the lateral wall. The passageways thus formed below each ridge are called the superior, middle, and inferior nasal meatuses.

On each side, the intranasal space communicates with a series of neighbouring air-filled cavities within the skull (the paranasal sinuses ) and also, via the nasolacrimal duct , with the lacrimal apparatus in the corner of the eye . The duct drains the lacrimal fluid into the nasal cavity. This fact explains why nasal respiration can be rapidly impaired or even impeded during weeping: the lacrimal fluid is not only overflowing into tears, it is also flooding the nasal cavity.

The paranasal sinuses are sets of paired single or multiple cavities of variable size. Most of their development takes place after birth, and they reach their final size toward age 20. The sinuses are located in four different skull bones—the maxilla, the frontal, the ethmoid, and the sphenoid bones. Correspondingly, they are called the maxillary sinus , which is the largest cavity; the frontal sinus; the ethmoid sinuses ; and the sphenoid sinus , which is located in the upper posterior wall of the nasal cavity. The sinuses have two principal functions: because they are filled with air, they help keep the weight of the skull within reasonable limits, and they serve as resonance chambers for the human voice.

The nasal cavity with its adjacent spaces is lined by a respiratory mucosa . Typically, the mucosa of the nose contains mucus-secreting glands and venous plexuses; its top cell layer, the epithelium , consists principally of two cell types, ciliated and secreting cells. This structural design reflects the particular ancillary functions of the nose and of the upper airways in general with respect to respiration. They clean, moisten, and warm the inspired air, preparing it for intimate contact with the delicate tissues of the gas-exchange area. During expiration through the nose, the air is dried and cooled, a process that saves water and energy.

Two regions of the nasal cavity have a different lining. The vestibule , at the entrance of the nose, is lined by skin that bears short thick hairs called vibrissae . In the roof of the nose, the olfactory bulb with its sensory epithelium checks the quality of the inspired air. About two dozen olfactory nerves convey the sensation of smell from the olfactory cells through the bony roof of the nasal cavity to the central nervous system .

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Course: high school biology   >   unit 8.

• Meet the heart!
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The respiratory system

Common mistakes and misconceptions.

• Incorrect : Physiological respiration and cellular respiration are the same thing.
• Correct : People sometimes use the word "respiration" to refer to the process of cellular respiration, which is a cellular process in which carbohydrates are used to generate usable energy. Physiological respiration and cellular respiration are related processes, but they are not the same.
• Incorrect : We breathe in only oxygen and breathe out only carbon dioxide.
• Correct : Often the terms "oxygen" and "air" are used interchangeably. It is true that the air we breathe in has more oxygen than the air we breathe out, and the air we breathe out has more carbon dioxide than the air that we breathe in. However, oxygen is just one of the gases found in the air we breathe. (In fact, the air has more nitrogen than oxygen!)
• Incorrect : The respiratory system works alone in transporting oxygen through the body.
• Correct : The respiratory system works directly with the circulatory system to provide oxygen to the body. Oxygen taken in from the respiratory system moves into blood vessels that then circulate oxygen-rich blood to tissues and cells.

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16.2: Structure and Function of the Respiratory System

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• Suzanne Wakim & Mandeep Grewal
• Butte College

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Seeing Your Breath

Why can you “see your breath” on a cold day? The air you exhale through your nose and mouth is warm, like the inside of your body. Exhaled air also contains a lot of water vapor because it passes over moist surfaces from the lungs to the nose or mouth. The water vapor in your breath cools suddenly when it reaches the much colder outside air. This causes the water vapor to condense into a fog of tiny droplets of liquid water. You release water vapor and other gases from your body through the process of respiration.

What is Respiration?

Respiration is the life-sustaining process in which gases are exchanged between the body and the outside atmosphere. Specifically, oxygen moves from the outside air into the body; and water vapor, carbon dioxide, and other waste gases move from inside the body into the outside air. Respiration is carried out mainly by the respiratory system. It is important to note that respiration by the respiratory system is not the same process as cellular respiration that occurs inside cells, although the two processes are closely connected. Cellular respiration is the metabolic process in which cells obtain energy, usually by “burning” glucose in the presence of oxygen. When cellular respiration is aerobic, it uses oxygen and releases carbon dioxide as a waste product. Respiration by the respiratory system supplies the oxygen needed by cells for aerobic cellular respiration and removes the carbon dioxide produced by cells during cellular respiration.

Respiration by the respiratory system actually involves two subsidiary processes. One process is ventilation or breathing. This is the physical process of conducting air to and from the lungs. The other process is gas exchange. This is the biochemical process in which oxygen diffuses out of the air and into the blood while carbon dioxide and other waste gases diffuse out of the blood and into the air. All of the organs of the respiratory system are involved in breathing, but only the lungs are involved in gas exchange.

Respiratory Organs

The organs of the respiratory system form a continuous system of passages called the respiratory tract, through which air flows into and out of the body. The respiratory tract has two major divisions: the upper respiratory tract and the lower respiratory tract. The organs in each division are shown in Figure \(\PageIndex{2}\). In addition to these organs, certain muscles of the thorax (the body cavity that fills the chest) are also involved in respiration by enabling breathing. Most important is a large muscle called the diaphragm, which lies below the lungs and separates the thorax from the abdomen. Smaller muscles between the ribs also play a role in breathing. You can learn more about breathing muscles in the concept of Breathing .

Upper Respiratory Tract

All of the organs and other structures of the upper respiratory tract are involved in the conduction or the movement of air into and out of the body. Upper respiratory tract organs provide a route for air to move between the outside atmosphere and the lungs. They also clean, humidity, and warm the incoming air. However, no gas exchange occurs in these organs.

Nasal Cavity

The nasal cavity is a large, air-filled space in the skull above and behind the nose in the middle of the face. It is a continuation of the two nostrils. As inhaled air flows through the nasal cavity, it is warmed and humidified. Hairs in the nose help trap larger foreign particles in the air before they go deeper into the respiratory tract. In addition to its respiratory functions, the nasal cavity also contains chemoreceptors that are needed for the sense of smell and that contribute importantly to the sense of taste.

The pharynx is a tube-like structure that connects the nasal cavity and the back of the mouth to other structures lower in the throat, including the larynx. The pharynx has dual functions: both air and food (or other swallowed substances) pass through it, so it is part of both the respiratory and digestive systems. Air passes from the nasal cavity through the pharynx to the larynx (as well as in the opposite direction). Food passes from the mouth through the pharynx to the esophagus.

The larynx connects the pharynx and trachea and helps to conduct air through the respiratory tract. The larynx is also called the voice box because it contains the vocal cords, which vibrate when air flows over them, thereby producing sound. You can see the vocal cords in the larynx in Figure \(\PageIndex{3}\). Certain muscles in the larynx move the vocal cords apart to allow breathing. Other muscles in the larynx move the vocal cords together to allow the production of vocal sounds. The latter muscles also control the pitch of sounds and help control their volume.

A very important function of the larynx is protecting the trachea from aspirated food. When swallowing occurs, the backward motion of the tongue forces a flap called the epiglottis to close over the entrance to the larynx. You can see the epiglottis in Figure \(\PageIndex{3}\). This prevents swallowed material from entering the larynx and moving deeper into the respiratory tract. If swallowed material does start to enter the larynx, it irritates the larynx and stimulates a strong cough reflex. This generally expels the material out of the larynx and into the throat.

Lower Respiratory Tract

The trachea and other passages of the lower respiratory tract conduct air between the upper respiratory tract and the lungs. These passages form an inverted tree-like shape (Figure \(\PageIndex{4}\)), with repeated branching as they move deeper into the lungs. All told, there are an astonishing 1,500 miles of airways conducting air through the human respiratory tract! It is only in the lungs, however, that gas exchange occurs between the air and the bloodstream.

The trachea, or windpipe, is the widest passageway in the respiratory tract. It is about 2.5 cm (1 in.) wide and 10-15 cm (4-6 in.) long. It is formed by rings of cartilage, which make it relatively strong and resilient. The trachea connects the larynx to the lungs for the passage of air through the respiratory tract. The trachea branches at the bottom to form two bronchial tubes.

Bronchi and Bronchioles

There are two main bronchial tubes, or bronchi (singular, bronchus) , called the right and left bronchi. The bronchi carry air between the trachea and lungs. Each bronchus branches into smaller, secondary bronchi; and secondary bronchi branch into still smaller tertiary bronchi. The smallest bronchi branch into very small tubules called bronchioles. The tiniest bronchioles end in alveolar ducts, which terminate in clusters of minuscule air sacs, called alveoli (singular, alveolus), in the lungs.

The lungs are the largest organs of the respiratory tract. They are suspended within the pleural cavity of the thorax. In Figure \(\PageIndex{5}\), you can see that each of the two lungs is divided into sections. These are called lobes, and they are separated from each other by connective tissues. The right lung is larger and contains three lobes. The left lung is smaller and contains only two lobes. The smaller left lung allows room for the heart, which is just left of the center of the chest.

Lung tissue consists mainly of alveoli (Figure \(\PageIndex{6}\)). These tiny air sacs are the functional units of the lungs where gas exchange takes place. The two lungs may contain as many as 700 million alveoli, providing a huge total surface area for gas exchange to take place. In fact, alveoli in the two lungs provide as much surface area as half a tennis court! Each time you breathe in, the alveoli fill with air, making the lungs expand. Oxygen in the air inside the alveoli is absorbed by the blood in the mesh-like network of tiny capillaries that surrounds each alveolus. The blood in these capillaries also releases carbon dioxide into the air inside the alveoli. Each time you breathe out, air leaves the alveoli and rushes into the outside atmosphere, carrying waste gases with it.

The lungs receive blood from two major sources. They receive deoxygenated blood from the heart. This blood absorbs oxygen in the lungs and carries it back to the heart to be pumped to cells throughout the body. The lungs also receive oxygenated blood from the heart that provides oxygen to the cells of the lungs for cellular respiration.

Protecting the Respiratory System

You may be able to survive for weeks without food and for days without water, but you can survive without oxygen for only a matter of minutes except under exceptional circumstances. Therefore, protecting the respiratory system is vital. That’s why making sure a patient has an open airway is the first step in treating many medical emergencies. Fortunately, the respiratory system is well protected by the ribcage of the skeletal system. However, the extensive surface area of the respiratory system is directly exposed to the outside world and all its potential dangers in inhaled air. Therefore, it should come as no surprise that the respiratory system has a variety of ways to protect itself from harmful substances such as dust and pathogens in the air.

The main way the respiratory system protects itself is called the mucociliary escalator. From the nose through the bronchi, the respiratory tract is covered in the epithelium that contains mucus-secreting goblet cells. The mucus traps particles and pathogens in the incoming air. The epithelium of the respiratory tract is also covered with tiny cell projections called cilia (singular, cilium), as shown in Figure \(\PageIndex{7}\). The cilia constantly move in a sweeping motion upward toward the throat, moving the mucus and trapped particles and pathogens away from the lungs and toward the outside of the body.

What happens to the material that moves up the mucociliary escalator to the throat? It is generally removed from the respiratory tract by clearing the throat or coughing. Coughing is a largely involuntary response of the respiratory system that occurs when nerves lining the airways are irritated. The response causes air to be expelled forcefully from the trachea, helping to remove mucus and any debris it contains (called phlegm) from the upper respiratory tract to the mouth. The phlegm may spit out (expectorated), or it may be swallowed and destroyed by stomach acids.

Sneezing is a similar involuntary response that occurs when nerves lining the nasal passage are irritated. It results in forceful expulsion of air from the mouth, which sprays millions of tiny droplets of mucus and other debris out of the mouth and into the air, as shown in Figure \(\PageIndex{8}\). This explains why it is so important to sneeze into a sleeve rather than the air to help prevent the transmission of respiratory pathogens.

How the Respiratory System Works with Other Organ Systems

The amount of oxygen and carbon dioxide in the blood must be maintained within a limited range for the survival of the organism. Cells cannot survive for long without oxygen, and if there is too much carbon dioxide in the blood, the blood becomes dangerously acidic (pH is too low). Conversely, if there is too little carbon dioxide in the blood, the blood becomes too basic (pH is too high). The respiratory system works hand-in-hand with the nervous and cardiovascular systems to maintain homeostasis in blood gases and pH.

It is the level of carbon dioxide rather than the level of oxygen that is most closely monitored to maintain blood gas and pH homeostasis. The level of carbon dioxide in the blood is detected by cells in the brain, which speed up or slow down the rate of breathing through the autonomic nervous system as needed to bring the carbon dioxide level within the normal range. Faster breathing lowers the carbon dioxide level (and raises the oxygen level and pH); slower breathing has the opposite effects. In this way, the levels of carbon dioxide and oxygen, as well as pH, are maintained within normal limits.

The respiratory system also works closely with the cardiovascular system to maintain homeostasis. The respiratory system exchanges gases between the blood and the outside air, but it needs the cardiovascular system to carry them to and from body cells. Oxygen is absorbed by the blood in the lungs and then transported through a vast network of blood vessels to cells throughout the body where it is needed for aerobic cellular respiration. The same system absorbs carbon dioxide from cells and carries it to the respiratory system for removal from the body.

Feature: My Human Body

Choking is the mechanical obstruction of the flow of air from the atmosphere into the lungs. It prevents breathing and may be partial or complete. Partial choking allows some though inadequate airflow into the lung—prolonged or complete choking results in asphyxia, or suffocation, which is potentially fatal.

Obstruction of the airway typically occurs in the pharynx or trachea. Young children are more prone to choking than are older people, in part because they often put small objects in their mouths and do not appreciate the risk of choking that they pose. Young children may choke on small toys or parts of toys or on household objects in addition to food. Foods that can adapt their shape to that of the pharynx, such as bananas and marshmallows, are especially dangerous and may cause choking in adults as well as children.

How can you tell if a loved one is choking? The person cannot speak or cry out or has great difficulty doing so. Breathing, if possible, is labored, producing gasping or wheezing. The person may desperately clutch at his or her throat or mouth. If breathing is not soon restored, the person’s face will start to turn blue from lack of oxygen. This will be followed by unconsciousness if oxygen deprivation continues beyond a few minutes.

If an infant is choking, turning the baby upside down and slapping on the back may dislodge the obstructing object. To help an older person who is choking, first, encourage the person to cough. Give them a few hardback slaps to help force the lodged object out of the airway. If these steps fail, perform the Heimlich maneuver on the person. You can easily find instructional videos online to learn how to do it. If the Heimlich maneuver also fails, call for emergency medical care immediately.

• What is respiration, as carried out by the respiratory system? Name the two subsidiary processes it involves.
• Describe the respiratory tract.
• Identify the organs of the upper respiratory tract, and state their functions.
• List the organs of the lower respiratory tract. Which organs are involved only in conduction?
• Where does gas exchange take place?
• How does the respiratory system protect itself from potentially harmful substances in the air?
• Explain how the rate of breathing is controlled.
• Why does the respiratory system need the cardiovascular system to help it perform its main function of gas exchange?

trachea; nasal cavity; alveoli; bronchioles; larynx; bronchi; pharynx

D. Bronchus

• Describe two ways in which the body prevents food from entering the lungs.
• True or False. The lungs receive some oxygenated blood.
• True or False. Gas exchange occurs in both the upper and lower respiratory tracts.

B. food particles

D. All of the above

• What is the relationship between respiration and cellular respiration?

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Attributions.

• Snowboarders breath on a cold day by Alain Wong via Unsplash License
• Conducting Passages by Lord Akryl , Jmarchn, public domain via Wikimedia Commons
• Larynx by Alan Hoofring , National Cancer Institute, public domain via Wikimedia Commons
• Lung Diagram by Patrick J. Lynch ; CC BY 2.5 via Wikimedia Commons
• Lung Structure by National Heart Lung and Blood Institute, public domain via Wikimedia Commons
• Alveoli by helix84 licensed CC BY 2.5 , via Wikimedia Commons
• Ciliated Epithelium by Blausen.com staff (2014). " Medical gallery of Blausen Medical 2014 ". WikiJournal of Medicine 1 (2). DOI : 10.15347/wjm/2014.010 . ISSN 2002-4436 . licensed CC BY 3.0 via Wikimedia Commons
• Sneeze by James Gathany, CDC , public domain via Wikimedia Commons
• Abdominal Thrusts by Amanda M. Woodhead, public domain via Wikimedia Commons
• Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0

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Top 5 Functions of the Respiratory System: A Look Inside Key Respiratory Activities

Through breathing, inhalation and exhalation, the respiratory system facilitates the exchange of gases between the air and the blood and between the blood and the body’s cells. The respiratory system also helps us to smell and create sound. The following are the five key functions of the respiratory system.

Breathing In and Speaking Out: How the Structures of the Upper Respiratory System Work

The structures of the upper respiratory system, or respiratory tract, allow us to breathe and speak.

• The nose and nasal cavities provide airways for respiration.
• The paranasal sinuses surround the nasal cavities.
• The pharynx connects the nasal and oral cavities to the larynx and esophagus.
• The larynx and vocal cords allow us to breathe and talk and sing.
• Structures that produce sound depend on the hyoid bone.

Drawing In and Processing Air: Functions of the Trachea, Bronchi, Lungs, and Alveoli

• Trachea: the main airway to the lungs
• Bronchi: passageways that bring air in and out of the lungs
• Lungs: structures responsible for gas exchange between the air we breathe and our bodies
• Alveoli: microscopic air sacs that are the site of external respiration
• Diaphragm: the muscle that is key to the physical process of breathing

Common Respiratory Diseases and Disorders: COPD, Asthma, Sinusitis, Influenza, and Pneumothorax

• Most respiratory diseases and disorders can be described as either infectious or chronic.
• Inflamed airways become irritated during inhalation during an asthma attack.
• Sinusitis is the inflammation of mucous membranes in the nasal sinuses.
• The flu virus can pass through the air from one person to another.
• Chest trauma can cause pneumothorax, a collapsed lung.

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22.1 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 CO 2
• 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 22.2 ).

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. 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.

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 22.3 ). The root is the region of the nose located between the eyebrows. The bridge is the part of the nose that connects the root to the rest of the nose. The dorsum nasi is the length of the nose. The apex is the tip of the nose. On either side of the apex, the nostrils are formed by the alae (singular = ala). An ala is a cartilaginous structure that forms the lateral side of each naris (plural = nares), or nostril opening. The philtrum is the concave surface that connects the apex of the nose to the upper lip.

Underneath the thin skin of the nose are its skeletal features (see Figure 22.3 , lower illustration). While the root and bridge of the nose consist of bone, the protruding portion of the nose is composed of cartilage. As a result, when looking at a skull, the nose is missing. The nasal bone is one of a pair of bones that lies under the root and bridge of the nose. The nasal bone articulates superiorly with the frontal bone and laterally with the maxillary bones. Septal cartilage is flexible hyaline cartilage connected to the nasal bone, forming the dorsum nasi. The alar cartilage consists of the apex of the nose; it surrounds the naris.

The nares open into the nasal cavity, which is separated into left and right sections by the nasal septum ( Figure 22.4 ). The nasal septum is formed anteriorly by a portion of the septal cartilage (the flexible portion you can touch with your fingers) and posteriorly by the perpendicular plate of the ethmoid bone (a cranial bone located just posterior to the nasal bones) and the thin vomer bones (whose name refers to its plough shape). 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 and meatuses also conserve water and prevent dehydration of the nasal epithelium by trapping water during exhalation. The floor of the nasal cavity is composed of the palate. The hard palate at the anterior region of the nasal cavity is composed of bone. The soft palate at the posterior portion of the nasal cavity consists of muscle tissue. Air exits the nasal cavities via the internal nares and moves into the pharynx.

Several bones that help form the walls of the nasal cavity have air-containing spaces called the paranasal sinuses, which serve to warm and humidify incoming air. Sinuses are lined with a mucosa. Each paranasal sinus is named for its associated bone: frontal sinus, maxillary sinus, sphenoidal sinus, and ethmoidal sinus. The sinuses produce mucus and lighten the weight of the skull.

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, meatuses, and paranasal sinuses are lined by respiratory epithelium composed of pseudostratified ciliated columnar epithelium ( Figure 22.5 ). 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.

Interactive Link

View the University of Michigan WebScope 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 22.4 ). The pharynx is divided into three major regions: the nasopharynx, the oropharynx, and the laryngopharynx ( Figure 22.6 ).

The nasopharynx is flanked by the conchae of the nasal cavity, and it serves only as an airway. At the top of the nasopharynx are the pharyngeal tonsils. A pharyngeal tonsil , also called an adenoid, is an aggregate of lymphoid reticular tissue similar to a lymph node that lies at the superior portion of the nasopharynx. The function of the pharyngeal tonsil is not well understood, but it contains a rich supply of lymphocytes and is covered with ciliated epithelium that traps and destroys invading pathogens that enter during inhalation. The pharyngeal tonsils are large in children, but interestingly, tend to regress with age and may even disappear. 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 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. The fauces is the opening at the connection between the oral cavity and the oropharynx. As the nasopharynx becomes the oropharynx, the epithelium changes from pseudostratified ciliated columnar epithelium to stratified squamous epithelium. The oropharynx contains two distinct sets of tonsils, the palatine and lingual tonsils. A palatine tonsil is one of a pair of structures located laterally in the oropharynx in the area of the fauces. The lingual tonsil is located at the base of the tongue. Similar to the pharyngeal tonsil, the palatine and lingual tonsils are composed of lymphoid tissue, and trap and destroy pathogens entering the body through the oral or nasal cavities.

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, whereas posteriorly, it enters the esophagus.

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 22.7 ). 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.

The epiglottis , attached to the thyroid cartilage, is a very flexible piece of elastic cartilage that covers the opening of the trachea (see Figure 22.4 ). 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 ( Figure 22.8 ). A vestibular fold , or false vocal cord, is one of a pair of folded sections of mucous membrane. A true vocal cord 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 are free, allowing oscillation 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. 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.

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.

The trachea (windpipe) extends from the larynx toward the lungs ( Figure 22.9 a ). The trachea is formed by 16 to 20 stacked, C-shaped pieces of hyaline cartilage that are connected by dense connective tissue. The trachealis muscle and elastic connective tissue together form the fibroelastic membrane , a flexible membrane that closes the posterior surface of the trachea, connecting the C-shaped cartilages. The fibroelastic membrane 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. In addition, the trachealis muscle can be contracted to force air through the trachea during exhalation. The trachea is lined with pseudostratified ciliated columnar epithelium, which is continuous with the larynx. The esophagus borders the trachea posteriorly.

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 ( Figure 22.9 b ). The carina is a raised structure that contains specialized nervous tissue that induces violent coughing if a foreign body, such as food, is present. Rings 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 bronchi continue to branch into a bronchial tree. 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.

A bronchiole branches from the tertiary bronchi. Bronchioles, which are about 1 mm in diameter, further branch until they become the tiny terminal bronchioles, which 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. The respiratory zone begins where the terminal bronchioles join a respiratory bronchiole , the smallest type of bronchiole ( Figure 22.10 ), which then leads to an alveolar duct, opening into a cluster of alveoli.

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 22.11 ).

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 CO 2 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 22.12 ). 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.

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.

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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Module 6: The Respiratory System

Introduction to the respiratory system, learning objectives.

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

• List the structures of the respiratory system
• List the major functions of the respiratory system
• Outline the forces that allow for air movement into and out of the lungs
• Outline the process of gas exchange
• Summarize the process of oxygen and carbon dioxide transport within the respiratory system
• Create a flow chart illustrating how respiration is controlled
• Discuss how the respiratory system responds to exercise
• Describe the development of the respiratory system in the embryo

Figure 1. The thin air at high elevations can strain the human respiratory system. (credit: “bortescristian”/flickr.com)

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.

• Anatomy & Physiology. Provided by : OpenStax CNX. Located at : http://cnx.org/contents/[email protected] . License : CC BY: Attribution . License Terms : Download for free at http://cnx.org/contents/[email protected]

Respiratory System Anatomy and Physiology

Breathe life into your understanding with our guide on the respiratory system anatomy and physiology. Nursing students, immerse yourself in the intricate dance of inhalation and exhalation that fuels every living moment.

Table of Contents

Functions of the respiratory system, main bronchi, the respiratory membrane, respiration, mechanics of breathing, respiratory volumes and capacities, respiratory sounds, external respiration, gas transport, and internal respiration, control of respiration, age-related physiological changes in the respiratory system.

The functions of the respiratory system are:

• Oxygen supplier.  The job of the respiratory system is to keep the body constantly supplied with oxygen.
• Elimination.  Elimination of carbon dioxide.
• Gas exchange.  The respiratory system organs oversee the gas exchanges that occur between the blood and the external environment.
• Passageway.  Passageways that allow air to reach the lungs.
• Humidifier.  Purify, humidify, and warm incoming air.

Anatomy of the Respiratory System

The organs of the respiratory system include the nose, pharynx , larynx, trachea, bronchi, and their smaller branches, and the lungs, which contain the alveoli.

The nose is the only externally visible part of the respiratory system.

• Nostrils.  During breathing, air enters the nose by passing through the nostrils, or nares.
• Nasal cavity. The interior of the nose consists of the nasal cavity, divided by a midline nasal septum .
• Olfactory receptors. The olfactory receptors for the sense of smell are located in the mucosa in the slitlike superior part of the nasal cavity, just beneath the ethmoid bone .
• Respiratory mucosa. The rest of the mucosal lining, the nasal cavity called the respiratory mucosa, rests on a rich network of thin-walled veins that warms the air as it flows past.
• Mucus.  In addition, the sticky mucus produced by the mucosa’s glands moistens the air and traps incoming bacteria and other foreign debris, and lysozyme enzymes in the mucus destroy bacteria chemically.
• Ciliated cells. The ciliated cells of the nasal mucosa create a gentle current that moves the sheet of contaminated mucus posteriorly toward the throat, where it is swallowed and digested by stomach juices.
• Conchae.  The lateral walls of the nasal cavity are uneven owing to three mucosa-covered projections, or lobes called conchae, which greatly increase the surface area of the mucosa exposed to the air, and also increase the air turbulence in the nasal cavity.
• Palate. The nasal cavity is separated from the oral cavity below by a partition, the palate; anteriorly, where the palate is supported by bone, is the hard palate; the unsupported posterior part is the soft palate .
• Paranasal sinuses. The nasal cavity is surrounded by a ring of paranasal sinuses located in the frontal, sphenoid, ethmoid, and maxillary bones ; theses sinuses lighten the skull , and they act as a resonance chamber for speech.

• Size. The pharynx is a muscular passageway about 13 cm (5 inches) long that vaguely resembles a short length of red garden hose.
• Function.  Commonly called the throat , the pharynx serves as a common passageway for food and air.
• Portions of the pharynx. Air enters the superior portion, the nasopharynx , from the nasal cavity and then descends through the oropharynx and laryngopharynx to enter the larynx below.
• Pharyngotympanic tube. The pharyngotympanic tubes, which drain the middle ear open into the nasopharynx.
• Pharyngeal tonsil. The pharyngeal tonsil, often called adenoid is located high in the nasopharynx.
• Palatine tonsils . The palatine tonsils are in the oropharynx at the end of the soft palate.
• Lingual tonsils . The lingual tonsils lie at the base of the tongue.

The larynx or voice box routes air and food into the proper channels and plays a role in speech.

• Structure.  Located inferior to the pharynx, it is formed by eight rigid hyaline cartilages and a spoon-shaped flap of elastic cartilage, the epiglottis .
• Thyroid cartilage. The largest of the hyaline cartilages is the shield-shaped thyroid cartilage, which protrudes anteriorly and is commonly called Adam’s apple .
• Epiglottis.  Sometimes referred to as the “guardian of the airways” , the epiglottis protects the superior opening of the larynx.
• Vocal folds. Part of the mucous membrane of the larynx forms a pair of folds, called the vocal folds, or true vocal cords , which vibrate with expelled air and allows us to speak.
• Glottis.  The slitlike passageway between the vocal folds is the glottis.

• Length.  Air entering the trachea or windpipe from the larynx travels down its length (10 to 12 cm or about 4 inches) to the level of the fifth thoracic vertebra , which is approximately midchest.
• Structure.  The trachea is fairly rigid because its walls are reinforced with C-shaped rings of hyaline cartilage; the open parts of the rings abut the esophagus and allow it to expand anteriorly when we swallow a large piece of food, while the solid portions support the trachea walls and keep it patent, or open, in spite of the pressure changes that occur during breathing.
• Cilia.  The trachea is lined with ciliated mucosa that beat continuously and in a direction opposite to that of the incoming air as they propel mucus, loaded with dust particles and other debris away from the lungs to the throat, where it can be swallowed or spat out.
• Structure.  The right and left main (primary) bronchi are formed by the division of the trachea.
• Location.  Each main bronchus runs obliquely before it plunges into the medial depression of the lung on its own side.
• Size.  The right main bronchus is wider, shorter, and straighter than the left.

• Location.  The lungs occupy the entire thoracic cavity except for the most central area, the mediastinum , which houses the heart, the great blood vessels, bronchi, esophagus, and other organs.
• Apex.  The narrow, superior portion of each lung, the apex, is just deep into the clavicle .
• Base.  The broad lung area resting on the diaphragm is the base.
• Division.  Each lung is divided into lobes by fissures; the left lung has two lobes , and the right lung has three .
• Pleura.  The surface of each lung is covered with a visceral serosa called the pulmonary , or visceral pleura, and the walls of the thoracic cavity are lined by the parietal pleura .
• Pleural fluid. The pleural membranes produce pleural fluid, a slippery serous secretion that allows the lungs to glide easily over the thorax wall during breathing movements and causes the two pleural layers to cling together.
• Pleural space. The lungs are held tightly to the thorax wall, and the pleural space is more of a potential space than an actual one.
• Bronchioles .  The smallest of the conducting passageways are the bronchioles.
• Alveoli.  The terminal bronchioles lead to the respiratory zone structures, even smaller conduits that eventually terminate in alveoli or air sacs.
• Respiratory zone. The respiratory zone, which includes the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli, is the only site of gas exchange .
• Conducting zone structures. All other respiratory passages are conducting zone structures that serve as conduits to and from the respiratory zone.
• Stroma.  The balance of the lung tissue, its stroma, is mainly elastic connective tissue that allows the lungs to recoil passively as we exhale.
• Wall structure. The walls of the alveoli are composed largely of a single, thin layer of squamous epithelial cells.
• Alveolar pores. Alveolar pores connect neighboring air sacs and provide alternative routes for air to reach alveoli whose feeder bronchioles have been clogged by mucus or otherwise blocked.
• Respiratory membrane. Together, the alveolar and capillary walls, their fused basement membranes, and occasional elastic fibers construct the respiratory membrane (air-blood barrier), which has gas (air) flowing past on one side and blood flowing past on the other.
• Alveolar macrophages . Remarkably efficient alveolar macrophages sometimes called “dust cells” , wander in and out of the alveoli picking up bacteria, carbon particles, and other debris.
• Cuboidal cells. Also scattered amid the epithelial cells that form most of the alveolar walls are chunky cuboidal cells, which produce a lipid (fat) molecule called surfactant , which coats the gas-exposed alveolar surfaces and is very important in lung function.

Physiology of the Respiratory System

The major function of the respiratory system is to supply the body with oxygen and to dispose of carbon dioxide. To do this, at least four distinct events, collectively called respiration, must occur.

• Pulmonary ventilation . Air must move into and out of the lungs so that gasses in the air sacs are continuously refreshed, and this process is commonly called breathing.
• External respiration. Gas exchange between the pulmonary blood and alveoli must take place.
• Respiratory gas transport. Oxygen and carbon dioxide must be transported to and from the lungs and tissue cells of the body via the bloodstream.
• Internal respiration. At systemic capillaries, gas exchanges must be made between the blood and tissue cells.
• Rule.  Volume changes lead to pressure changes, which lead to the flow of gasses to equalize pressure.
• Inspiration.  Air is flowing into the lungs; the chest is expanded laterally, the rib cage is elevated, and the diaphragm is depressed and flattened; lungs are stretched to the larger thoracic volume, causing the intrapulmonary pressure to fall and air to flow into the lungs.
• Expiration.  Air is leaving the lungs; the chest is depressed and the lateral dimension is reduced, the rib cage is descended, and the diaphragm is elevated and dome-shaped; lungs recoil to a smaller volume, intrapulmonary pressure rises, and air flows out of the lung.
• Intrapulmonary volume. Intrapulmonary volume is the volume within the lungs.
• Intrapleural pressure. The normal pressure within the pleural space, the intrapleural pressure, is always negative, and this is the major factor preventing the collapse of the lungs.
• Nonrespiratory air movements. Nonrespiratory movements are a result of reflex activity, but some may be produced voluntarily such as coughing , sneezing, crying, laughing, hiccups, and yawning.

• Tidal volume. Normal quiet breathing moves approximately 500 ml of air into and out of the lungs with each breath.
• Inspiratory reserve volume. The amount of air that can be taken in forcibly over the tidal volume is the inspiratory reserve volume, which is normally between 2100 ml to 3200 ml.
• Expiratory reserve volume. The amount of air that can be forcibly exhaled after a tidal expiration, the expiratory reserve volume, is approximately 1200 ml.
• Residual volume. Even after the most strenuous expiration, about 1200 ml of air still remains in the lungs and it cannot be voluntarily expelled; this is called residual volume, and it is important because it allows gas exchange to go on continuously even between breaths and helps to keep the alveoli inflated.
• Vital capacity. The total amount of exchangeable air is typically around 4800 ml in healthy young men, and this respiratory capacity is the vital capacity, which is the sum of the tidal volume, inspiratory reserve volume, and expiratory reserve volume.
• Dead space volume. Much of the air that enters the respiratory tract remains in the conducting zone passageways and never reaches the alveoli; this is called the dead space volume and during a normal tidal breath, it amounts to about 150 ml.
• Functional volume. The functional volume, which is the air that actually reaches the respiratory zone and contributes to gas exchange , is about 350 ml.
• Spirometer.  Respiratory capacities are measured with a spirometer, wherein as a person breathes, the volumes of air exhaled can be read on an indicator, which shows the changes in air volume inside the apparatus.
• Bronchial sounds. Bronchial sounds are produced by air rushing through the large respiratory passageways (trachea and bronchi).
• Vesicular breathing sounds. Vesicular breathing sounds occur as air fills the alveoli, and they are soft and resemble a muffled breeze.
• External respiration. External respiration or pulmonary gas exchange involves oxygen being loaded and carbon dioxide being unloaded from the blood.
• Internal respiration. In internal respiration or systemic capillary gas exchange , oxygen is unloaded and carbon dioxide is loaded into the blood.
• Gas transport. Oxygen is transported in the blood in two ways: most attaches to hemoglobin molecules inside the RBCs to form oxyhemoglobin, or a very small amount of oxygen is carried dissolved in the plasma ; while carbon dioxide is transported in plasma as bicarbonate ion, or a smaller amount (between 20 to 30 percent of the transported carbon dioxide) is carried inside the RBCs bound to hemoglobin.

Neural Regulation

• Phrenic and intercostal nerves . These two nerves regulate the activity of the respiratory muscles, the diaphragm, and external intercostals.
• Medulla and pons . Neural centers that control respiratory rhythm and depth are located mainly in the medulla and pons; the medulla, which sets the basic rhythm of breathing, contains a pacemaker , or self-exciting inspiratory center, and an expiratory center that inhibits the pacemaker in a rhythmic way; pons centers appear to smooth out the basic rhythm of inspiration and expiration set by the medulla.
• Eupnea.  The normal respiratory rate is referred to as eupnea, and it is maintained at a rate of 12 to 15 respirations/minute .
• Hyperpnea.  During exercise, we breathe more vigorously and deeply because the brain centers send more impulses to the respiratory muscles, and this respiratory pattern is called hyperpnea.

Non-neural Factors Influencing Respiratory Rate and Depth

• Physical factors. Although the medulla’s respiratory centers set the basic rhythm of breathing, there is no question that physical factors such as talking, coughing, and exercising can modify both the rate and depth of breathing, as well as an increased body temperature, which increases the rate of breathing.
• Volition (conscious control). Voluntary control of breathing is limited, and the respiratory centers will simply ignore messages from the cortex (our wishes) when the oxygen supply in the blood is getting low or blood pH is falling .
• Emotional factors. Emotional factors also modify the rate and depth of breathing through reflexes initiated by emotional stimuli acting through centers in the hypothalamus .
• Chemical factors. The most important factors that modify respiratory rate and depth are chemical- the levels of carbon dioxide and oxygen in the blood; increased levels of carbon dioxide and decreased blood pH are the most important stimuli leading to an increase in the rate and depth of breathing, while a decrease in oxygen levels become important stimuli when the levels are dangerously low.
• Hyperventilation.  Hyperventilation blows off more carbon dioxide and decreases the amount of carbonic acid, which returns blood pH to the normal range when carbon dioxide or other sources of acids begin to accumulate in the blood.
• Hypoventilation.  Hypoventilation or extremely slow or shallow breathing allows carbon dioxide to accumulate in the blood and brings blood pH back into normal range when blood starts to become slightly alkaline.

Respiratory efficiency is reduced with age. They are unable to compensate for increased oxygen need and are significantly increasing the amount of air inspired. Therefore, difficulty in breathing is usually common especially during activities.  Expiratory muscles become weaker so their cough efficiency is reduced and the amount of air left in the lungs is increased. Health promotion teaching can include smoking cessation, preventing respiratory infections through handwashing , and ensuring up to date influenza and pneumonia vaccinations.

Craving more insights? Dive into these related materials to enhance your study journey!

• Anatomy and Physiology Nursing Test Banks . This nursing test bank includes questions about Anatomy and Physiology and its related concepts such as: structure and functions of the human body, nursing care management of patients with conditions related to the different body systems.

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Respiratory System

Jan 01, 2020

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Respiratory System. Parts. The respiratory system is divided into two parts: Upper respiratory tract Lower respiratory tract. Major Organs and Functions. Nose: The only Externally visible part of the respiratory system.

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• carbon dioxide
• respiratory tract
• intrapulmonary pressure
• expiration letting air
• alveoli tiny air sacs

Presentation Transcript

Parts • The respiratory system is divided into two parts: • Upper respiratory tract • Lowerrespiratory tract

Major Organs and Functions • Nose:The only Externally visible part of the respiratory system. • During the process of breathing, air passes through the external nares (nostrils) • The interior of the nose is called the nasal cavity, which is divided by the midline/nasal septum. • The Respiratory Mucosa lining the nasal cavity contains olfactory receptors which provide for the sense of smell. • Beneath the mucosa lies a network of thin walled veins, which function to warm and moisten air as it enters the nose. Also the mucosa traps incoming bacteria which prevents colds and sicknesses.

Pharynx:A muscular passageway for food and air, which is often referred to as the throat. • The air enters thenasopharynxfrom the nasal cavity and continues through theoropharynxand laryngopharynxto enter the esophagus below. The tonsils are located in the pharynx and are just clusters of lymphatic tissues • Larynx:Voice box which conducts air and food into proper pathways and plays an important role in speech. • It is made up of eight hyaline cartilages, thethyroid cartilagebeing the largest. It protrudes anteriorly and is called the Adams Apple. The epiglottis guides the air throughout the superior opening of the larynx.

-Bronchi: Two air tubes that branch off of the trachea enter the lungs and spread into treelike fashion into smaller tubes called bronchial tubes. -Alveoli: Tiny air sacs within the lungs where the exchange of oxygen and carbon dioxide takes place. -They are at the end of the bronchioles, running from the bronchi into the lobes of the lung.

Diaphragm: is a shelf of muscle extending across the bottom of the ribcage. • In order to draw air into the lungs, the diaphragm contracts, thus enlarging the thoracic cavity and reducing intra-thoracic pressure. When the diaphragm relaxes, air is exhaled.

Trachea:(Also called windpipe), is an important part of the vertebrate respiratory system. It is a bony tube that connects the nose and mouth to the lungs. • When an individual breathes, air is taken into the lungs for respiration. The air flows through the windpipe. Because of its primary function, any damage incurred to the trachea can be potentially life-threatening.

Lungs: main organs of the respiratory system. In the lungs oxygen is transported to the body and carbon dioxide is removed. • Structure: • Each lung is between 10 and 12 inches long. The left lung is divided into two sections, or lobes: superior and inferior. The right lung is somewhat larger than the left lung and is divided into three lobes: superior, middle, and inferior. The lungs are covered by a protective membrane called the pulmonary pleura. • The walls of the thoracic cavity are lined with parietal pleura. The pleural membranes function to produce pleural fluid, which is a slippery secretion which permits the lungs to glide easily over the thorax during breathing movements and allow the two pleural layers to cling to one and other. The pleurae have the ability to slide across each other, but resist being pulled apart. The position of tightly adhering pleural membranes is essential for normal breathing.

Lungs • Function: The main function of your lungs is respiration. • Each day, you take about 23,000 breaths, which bring almost 10,000 quarts of air into your lungs. The air that you breathe in contains several gases, including oxygen that your cells need to function. With each breath, your lungs add fresh oxygen to your blood, which then carries it to your cells.

How do you breathe? Breathing: • a mechanical process that depends on volume changes that occur in the thoracic cavity • breathing works by making the rib cage bigger, the pleural layers slide over each other and the pressure in the lung is decreased, so air is sucked in. • Breathing out does the reverse, the cage collapses and air is expelled. • The diaphragm is the main component. • When is contracts, it flattens and increases the space above it. • During relaxation, the abdominal contents push it up again. Voice Production: • Air pressure from the lungs travels up through the trachea, larynx, and pharynx. • The vocal folds in the larynx vibrate, which creates fluctuations in air pressure called sounds waves. • The vocal tract resonates and modifies the waves to match the shape of the jaw, lips, tongue, soft palate, and other vocal organs. This creates regions and differentiation in sound. • The openings of the mouth and nose release the sound to the external environment,

Oxygen and Carbon Dioxide transportation in the blood The Major function of Respiratory system is to supply the body with oxygen and to dispose of Carbon Dioxide.  • The walls of alveoli are a single, thin layer of squamous epithelial cells (a sheet of tissue paper is much thicker) • Alveolar Pores connect neighboring air sacs and provide alternate routes for air to reach alveoli. • External surfaces of alveoli are covered with pulmonary capillaries. • Together, alveolar and capillary walls and their fused basement membranes construct the respiratory membrane (air-blood barrier), which has gas (air) flowing past on one side and blood flowing past on the other side. ** • Gas exchanges occur by a simple diffusion through the respiratory membrane- oxygen passing from the alveolar air into the capillary blood and carbon dioxide leaving the blood to enter the gas-filled alveolus. • Total gas exchange surface provided by the alveolar walls estimated = 50-70 square meters. (40 times greater than surface area of skin.)

Respiratory Volume • There are many factors that influence lung capcity. • Size, age, sex, physical condition, ect. • Tidal Volume (TV) is the amount of quiet breathing into and out of the lungs with each breath (500mL normally) • Inspiratory Reserve Volume (IRV): The amount of air that can be taken in forcibly over the tidal volume. (2100-3200mL normally) • Expiratory Reserve Volume (ERV): The amount of air that can be forcibly exhaled after a tidal expiration. (1200mL normally) • Residual Volume: The amount of air still remaining in the lungs that cannot be voluntarily expelled (1200mL normally) It is important because it permits gas exchange to continue between breaths and keeps alveoli open. .

Lung Capacities • Respiratory Volumes are measured with a spirometer. • Vital Capacity (VC): The total amount of exchangeable air. It is the sum of the TV, IRV, and ERV. (4800 in healthy young males) • Dead Space Volume: The air that enters the respiratory tract but does not reach the alveoli. (Usually about 150mL, the other 350 reaches the alveoli)

External v Internal Respiration • External Respiration is the exchange of gases between the alveoli and the blood, while internal respiration is gas exchange between the systemic capillaries and he tissue cells.

Inspiration v Expiration • Inspiration, or inhalation, is the process by which air flows into the lungs. It involves the contraction of the diaphragm and external intercostals, which in turn increases the volume of the thoracic cavity. Consequently, the lungs and the gases within the lungs, expand to fill the new space. This effect produces a partial vacuum that sucks the air into the lungs until the intrapulmonary pressure is equal to atmospheric pressure. • Expiration, or exhalation, is the process that expels air out of the lungs. In healthy people, this process is very passive and relies mostly on the elasticity of the lungs rather than muscle contractions. After inspiration takes place, the muscles resume their original positions, forcing the ribcage to descend and the lungs to recoil. This causes the thoracic and intrapulmonary volumes decrease. As a result, the gases inside the lungs are forced more closely together and the intrapulmonary pressure rises to a point higher than atmospheric pressure, The gases then flow out of the lungs to stabilize pressures inside and outside of the lungs.

Nervous Control of Respiration • Voluntary • Phrenic and Intercostal Nerves • In the brain, breathing is controlled by neural centers called the ponsand medulla • Medulla- sets basic rhythm; contains a self-exciting inspiratory center as well as other respiratory centers • Pons- smoothes out basic rhythm of inspiration and expiration set by medulla • Impulses go back and forth b/w the two • Normal breathing rate = 12-15 respirations/min. (eupnea) • In case of overinflation of bronchioles and alveoli, impulses are sent from the stretch receptors to the medulla by the vagus nerves (inspiration ends, expiration occurs) • Hyperpnea-brain sends more impulses to resp. muscles to breathe (exercise, etc.) • Volition- conscious control • Emotional factors(fear, love, etc.) • Chemical factors- • Levels of co2 and o2 in blood (medulla)

Pressure Relationship of Breathing • Inspiration- taking in air (inhalation) • As the diaphragmand external intercostals contract, thoracic cavity gets bigger from top to bottom (diaphragm flattens out=bottom down, external intercostals contract= top up) • Sternum thrusted forward (increases front to back) • Lungs expand with thoracic cavity • Gas has more room to move; PRESSURE DECREASES • Body seeks to make intrapulmonary pressure equal to atmospheric pressure by inhaling more gas until lungs are full • Expiration- letting air out (exhalation) • Inspiratory muscles relax; thoracic and intrapulmonary volume decreases • Gases in lungs are forced more closely together; intrapulmonary pressure is greater than atmospheric pressure • Gases flow out to equalize pressure • Usually effortless unless resp. passages are narrowed (asthma, pneumonia, bronchitis, etc.); in forced expiration, internal intercostals muscles depress rib cage and abs contract to force air out by squeezing abdominal organs against diaphragm • The intrapleural pressure is always negative; prevents collapse of lungs • Protective Mechanisms • cilia and mucous to trap dust, bacteria, etc. • coughing (clears lower respiratory passageways), sneezing (clears upper respiratory passageways) • movement of larynx blocks food from entering trachea and directs it to esophagus

Respiratory Diseases • Infectionby viruses and bacteria: range from the common cold to the flu virus and Streptococcus(strep throat) • Many upper respiratory tract problems: • Epistaxis • Laryngitis • Pharyngitis • Rhinitis • Sinusitis • Sleep apnea • tonsilitis

Respiratory Diseases • Emphysema: usuallycaused by smoking • Can also be inherited (deficiency of alpha-I antitrypsin [AAT] protein) • Fourth leading cause of death in U.S. • How it works: • Damage to alveoli- Over-inflated alveoli fuse to become an enlarged alveoli. This results in less surface area for the normal gas exchange and blood oxygenation. Damage to the surfactant coating the alveoli causes a loss of elasticity. “Stale” air is never replaced by fresh air, and eventually alveoli can collapse, trapping the air. • Symptoms: breathlessness, barrel chest, weight loss, problems with breathing out

Respiratory Diseases • Emphysema(cont): • Treatment: • 1. surfactants to replace those lost • 2. oxygen therapy to oxygenate tissues • 3. treatment of symptoms • Pneumonia: an inflammation of the lungs caused by an infection or an injury in which the alveoli fill with fluid • Two to three million people affected in U.S. each year; out of these, about 40,000 to 70,000 people die • Proper gas exchange can not occur when fluid such as mucus or pus is present • Causes? • Bacteria, viruses, mycoplasmas, fungi, and various chemicals • Lead to a variety of types and strengths of pneumonia

Respiratory Diseases (cont) • Pneumonia (cont): • Symptoms: • severe shaking chill, high fever, cough, shortness of breath, rapid breathing, chest pains • Treatment: • antibiotics for the bacterial form; rest, drink fluids; anti-inflammatory meds • Preventative measures: • good hygiene, flu vaccination, don’t smoke tobacco Normal lungs Lungs with pneumonia

Respiratory Diseases (cont) • Tuberculosis: a bacterial infection of the lungs caused by the Myobacterium tuberculosis bacteria • Over-stimulates the inflammatory immune response, destroying lung tissue • Symptoms: a cough lasting 3 or more weeks, coughing up blood or mucus, fever and chills, night sweats; can be very deadly • Treatments: • Active: antibiotic treatment over a period of 6-12 months; if second occurrence (MDR-TB), requires special drugs that often have extreme side effects • Latent: isoniazid can prevent TB from becoming active • A vaccine for specifically Bacille Calmette-Guerin (BCG) strain

'Respiratory system," Hillendale Health. May 3, 2007. <http://hes.ucf.k12.pa.us/gclaypo/repiratorysys.html#Lungs> • "Anatomy of Respiratory System," Ohio State University Medical Center. May 3, 2007 <http://medicalcenter.osu.edu/patientcare/healthinformation/diseasesandconditions/respiratory/about/anatomy/> • "Oxygen Delivery System," The Franklin Institute. May 3, 2007. <http://www.fi.edu/biosci/systems/respiration.html> • "How the Lungs Work," National Heart Lung and Blood Institute. May 3, 2007. <http://www.nhlbi.nih.gov/health/dci/Diseases/Copd/Copd_HowLungsWork.html> • Ballard, Dr. Carol. The Lungs and Breathing. Farmington Hills, MI: Kidhaven Press, 2005. • Marieb, Elanie N. Essentials of Human Anatomy & Physiology. San Francisco, CA: Daryl Fox, 2003. • "Lung Toxicology Problem Set," The University of Arizona. 1997. May 7, 2007. http://www.biology.arizona.edu/chh/problem_sets/lung_toxicology/03t.html

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StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

StatPearls [Internet].

Upper respiratory tract infection.

Micah Thomas ; Paul A. Bomar .

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Last Update: June 26, 2023 .

• Continuing Education Activity

Upper respiratory tract infections can be defined as self-limited irritation and swelling of the upper airways with associated cough and no signs of pneumonia, in a patient with no other condition that would account for their symptoms, or with no history of chronic obstructive pulmonary disease, emphysema, or chronic bronchitis. Upper respiratory tract infections involve the nose, sinuses, pharynx, larynx, and large airways. This activity examines when an upper respiratory tract infections should be considered on differential diagnosis and how to properly evaluate it. This activity highlights the role of the interprofessional team in caring for patients with this condition.

• Describe the pathophysiology of upper respiratory tract infections.
• Review the history and physical of a patient with an upper respiratory tract infection.
• Outline the management options for upper respiratory tract infections.
• Explain interprofessional team strategies for improving care coordination and outcomes in patients with upper respiratory tract infections.
• Introduction

A variety of viruses and bacteria can cause upper respiratory tract infections. These cause a variety of patient diseases including acute bronchitis, the common cold, influenza, and respiratory distress syndromes. Defining most of these patient diseases is difficult because the presentations connected with upper respiratory tract infections (URIs) commonly overlap and their causes are similar. Upper respiratory tract infections can be defined as self-limited irritation and swelling of the upper airways with associated cough with no proof of pneumonia, lacking a separate condition to account for the patient symptoms, or with no history of COPD/emphysema/chronic bronchitis.  [1]   Upper respiratory tract infections involve the nose, sinuses, pharynx, larynx, and the large airways.

Common cold continues to be a large burden on society, economically and socially. The most common virus is rhinovirus. Other viruses include the influenza virus, adenovirus, enterovirus, and respiratory syncytial virus. Bacteria may cause roughly 15% of sudden onset pharyngitis presentations. The most common is S. pyogenes, a Group A streptococcus.

Risk factors for a URTI

• Close contact with children: both daycares and schools increase the risk fo URI
• Medical disorder: People with asthma and allergic rhinitis are more likely to develop URI
• Smoking is a common risk factor for URI
• Immunocompromised individuals including those with cystic fibrosis, HIV, use of corticosteroids, transplantation, and post-splenectomy are at high risk for URI
• Anatomical anomalies including facial dysmorphic changes or nasal polyposis also increase the risk of URI
• Epidemiology

Across the country, URIs are one of the top three diagnoses in the outpatient setting.   Estimated annual costs for viral URI, not related to influenza, exceeds $22 billion. [2] Upper respiratory tract infections account for an estimated 10 million outpatient appointments a year. Relief of symptoms is the main reason for outpatient visits amongst adults during the initial couple weeks of sickness, and a majority of these appointments result with physicians needless writing of antibiotic prescriptions. Adults obtain a common cold around two to three times yearly whereas pediatrics can have up to eight cases yearly. [3] , [4] , [5]   Fall months see a peak in incidence of common cold caused by the rhinovirus. Upper respiratory tract infections are accountable for greater than 20 million missed days of school and greater than 20 million days of work lost, thus generating a large economic burden. [6]

• Pathophysiology

A URTI usually involves direct invasion of the upper airway mucosa by the organism. The organism is usually acquired by inhalation of infected droplets. Barriers that prevent the organism from attaching to the mucosa include 1) the hair lining that traps pathogens, 2) the mucus which also traps organisms 3) the angle between the pharynx and nose which prevents particles from falling into the airways and 4) ciliated cells in the lower airways that transport the pathogens back to the pharynx.

The adenoids and tonsils also contain immunological cells that attack the pathogens.

The incubation period for influenza is 1 to 4 days, and the time interval between symptom onset is estimated to be 3 to 4 days. Viral shedding can occur 1 day before the onset of symptoms. It is believed that influenza can be transferred among humans by direct contact, indirect contact, droplets, or aerosolization. Short distances (<1 meter) are generally required for contact and droplet transmission to occur between the source person and the susceptible individual. Airborne transmission may occur over longer distances (>1 m). Most evidence-based data suggest that direct contact and droplet transfer are the predominant modes of transmission for influenza. [7]

• Common Cold

The pathogens are responsible for causing the common cold include rhinovirus, adenovirus, parainfluenza virus, respiratory syncytial virus, enterovirus, and coronavirus. The rhinovirus, a species of the Enterovirus genus of the Picornaviridae family, is the most common cause of the common cold and causes up to 80% of all respiratory infections during peak seasons. [8] Dozens of rhinovirus serotypes and frequent antigenic changes among them make identification, characterization, and eradication complex. After deposition in the anterior nasal mucosa, rhinovirus replication and infection are thought to begin upon mucociliary transport to the posterior nasopharynx and adenoids. As soon as 10 to 12 hours after inoculation, symptoms may begin. The mean duration of symptoms is 7 to 10 days, but symptoms can persist for as long as 3 weeks. Nasal mucosal infection and the host's subsequent inflammatory response cause vasodilation and increased vascular permeability. These events result in nasal obstruction and rhinorrhea whereas cholinergic stimulation prompts mucus production and sneezing.

• History and Physical

Acute upper respiratory tract infections include rhinitis, pharyngitis, tonsillitis, and laryngitis. Symptoms of URTIs commonly include:

• Sore throat
• Nasal congestion
• Low-grade fever
• Facial pressure

The onset of symptoms usually begins one to three days after exposure and lasts 7–10 days, and can persist up to 3 weeks.

The presence of classical features for rhinovirus infection, coupled with the absence of signs of bacterial infection or serious respiratory illness, is sufficient to make the diagnosis of the common cold. The common cold is a clinical diagnosis, and diagnostic testing is not necessary. When testing for influenza, obtain specimens as close to symptom onset as possible. Nasal aspirates and swabs are the best specimens to obtain when testing infants and young children. For older children and adults, swabs and aspirates from the nasopharynx are preferred. Rapid strep swabs can be used to rule out bacterial pharyngitis, which could help decrease number of antibiotics being prescribed for these infections.

• Treatment / Management

The goal of treatment for the common cold is symptom relief. Decongestants and combination antihistamine/decongestant medications can limit cough, congestion, and other symptoms in adults. [9] Avoid cough preparations in children. [10] H1-receptor antagonists may offer a modest reduction of rhinorrhea and sneezing during the first 2 days of a cold in adults. [3] First-generation antihistamines are sedating, so advise the patient about caution during their use. Topical and oral nasal decongestants (i.e., topical oxymetazoline, oral pseudoephedrine) have moderate benefit in adults and adolescents in reducing nasal airway resistance. [10] , [3] Evidence-based data does not support the use of antibiotics in the treatment of the common cold because they do not improve symptoms or shorten the course of illness. [10] , [3] There is also a lack of convincing evidence supporting the use of dextromethorphan for acute cough.

According to a Cochrane Review, [11] vitamin C used as daily prophylaxis at doses of =0.2 grams or more had a "modest but consistent effect" on the duration and severity of common cold symptoms (8% and 13% decreases in duration for adults and children, respectively). When taken therapeutically after the onset of symptoms, however, high-dose vitamin C has not shown clear benefit in trials. [11]

Early antiviral treatment for influenza infection shortens the duration of influenza symptoms, decreases the length of hospital stays, and reduces the risk of complications.[ Recommendations for the treatment of influenza are updated frequently by the Centers for Disease Control and Prevention based on epidemiologic data and antiviral resistance patterns. Give antiviral therapy for influenza within 48 hours of symptom onset (or earlier), and do not delay treatment for laboratory confirmation if a rapid test is not available. Antiviral treatment can provide benefit even after 48 hours in pregnant and other high-risk patients. [12]

Vaccination is the most effective method of preventing influenza illness. Antiviral chemoprophylaxis is also helpful in preventing influenza (70% to 90% effective) and should be considered as an adjunct to vaccination in certain scenarios or when vaccination is unavailable or not possible. Generally, antiviral chemoprophylaxis is used during periods of influenza activity for (1) high-risk persons who cannot receive vaccination (due to contraindications) or in whom recent vaccination does not, or is not expected to, afford a sufficient immune response; (2) controlling outbreaks among high-risk persons in institutional settings; and (3) high-risk persons with influenza exposures. [13]

• Differential Diagnosis
• Allergic rhinitis
• Tracheobronchitis
• Atypical Pneumonia
• Epiglottitis
• Streptococcal Pharyngitis/Tonsillitis
• Infectious Mononucleosis

URI are common during the winter season and for the most part, are benign, but they can seriously affect the quality of life for a few weeks. A few individuals may develop pneumonia, meningitis, sepsis, and bronchitis. Each year, there are isolated cases of death reported from a URI. Time off work and school is very common. In addition, patients spend billions of dollars on worthless remedies. There is little evidence that any treatment actually shortens the duration of a viral URI. Even the vaccine only works in 40-60% of individuals, at best.

• Complications

Complications of upper respiratory tract infections are relatively rare, except with influenza. Complications of influenza infection include primary influenza viral pneumonia; secondary bacterial pneumonia; sinusitis; otitis media; coinfection with bacterial agents; and exacerbation of preexisting medical conditions, particularly asthma and chronic obstructive pulmonary disease. Pneumonia is one of the most common complications of influenza illness in children and contributes significantly to morbidity and mortality.

• Enhancing Healthcare Team Outcomes

Upper respiratory tract infections are one of the most common illnesses that healthcare workers will encounter in an outpatient setting. The infection may vary from the common cold to a life-threatening illness like acute epiglottitis. Because of the diverse causes and presentation, upper respiratory tract infections are best managed by an interprofessional team.

The key is to avoid over-prescribing of antibiotics but at the same time not missing a life-threatening infection. Nurse practitioners who see these patients should freely communicate with an infectious disease expert if there is any doubt about the severity of the infection. The pharmacist should educate the patient on URI and to refrain from overusing unproven products.

Similarly, the emergency department physician should not readily discharge patients home with antibiotics for the common cold. Overall, upper respiratory tract infections lead to very high disability for short periods. Absenteeism from work and schools is common; in addition, the symptoms can be annoying and extreme fatigue is the norm. Patients should be encouraged to drink ample fluids, rest, discontinue smoking and remain compliant with the prescribed medications. [14]

Nursing can monitor the patient's condition and symptoms, counsel on medication compliance, and report any concerns to the clinicians managing the case. INterprofessinoal cooperation is key to good outcomes. [Level 5]

Finally, clinicians should urge patients to get vaccinated before the flu season. While the vaccine may not decrease the duration of the infection, the symptoms are much less severe.

The outcomes in most patients are good, particularly with the interprofessional team approach. [Level 5]

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Disclosure: Micah Thomas declares no relevant financial relationships with ineligible companies.

Disclosure: Paul Bomar declares no relevant financial relationships with ineligible companies.

This book is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ), which permits others to distribute the work, provided that the article is not altered or used commercially. You are not required to obtain permission to distribute this article, provided that you credit the author and journal.

• Cite this Page Thomas M, Bomar PA. Upper Respiratory Tract Infection. [Updated 2023 Jun 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

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COVID-19: Who's at higher risk of serious symptoms?

Advanced age and some health conditions can raise the risk of serious COVID-19 (coronavirus disease 2019) illness.

Many people with COVID-19, also called coronavirus disease 2019, recover at home. But for some, COVID-19 can be a serious illness. Some people may need care in the hospital, treatment in the intensive care unit and the need for breathing help. In some people, severe COVID-19 illness can lead to death.

What raises the risk of severe or critical COVID-19 illness?

The risk for serious COVID-19 illness depends on your health status, age and activities. Your risk also depends on other factors. This includes where you live, work or learn, how easy it is for you to get medical care, and your economic stability.

If you have more than one risk factor, your risk goes up with each one.

Age raises the risk of serious COVID-19

People age 65 and older and babies younger than 6 months have a higher than average risk of serious COVID-19 illness. Those age groups have the highest risk of needing hospital care for COVID-19.

Babies younger than 6 months aren't eligible for the COVID-19 vaccine, which adds to their risk. For older people, the challenge is that the immune system is less able to clear out germs as people age. Also, as people age, medical conditions that raise the risk of severe COVID-19 are more likely. In the U.S. as of March 2024, about 76% of all deaths from COVID-19 have been among people age 65 and older.

Aging plus disease raises the risk of serious COVID-19

Severe COVID-19 disease is more likely for people who have other health issues.

Some common diseases linked to aging are:

• Heart disease. Examples are heart failure or coronary artery disease.
• Diabetes mellitus. The risk is higher for both type 1 and type 2.
• Chronic lung diseases. This includes airway disease and conditions that damage lung tissue.
• Obesity. The risk goes up as body mass index (BMI) increases, with the highest risk for a BMI of 40 or greater.
• Chronic kidney disease. Especially if you are on dialysis.

These diseases become more common as people age. But they can affect people of any age. The risk of serious COVID-19 illness is linked to having one or more underlying medical condition.

Asthma, COPD, other lung diseases raise risk of severe COVID-19

Your risk of having more severe COVID-19 illness is higher if you have lung disease. Having moderate to severe asthma raises some risks of serious COVID-19 illness. It raises the risk of needing care in the hospital, including intensive care, and needing mechanical help breathing.

The risk of serious COVID-19 illness also is higher for people who have conditions that damage lung tissue over time. Examples are tuberculosis, cystic fibrosis, interstitial lung disease, bronchiectasis or COPD, which stands for chronic obstructive pulmonary disease. These diseases raise the risk of needing care in the hospital for COVID-19. Depending on the condition, the risk of needing intensive care and the risk of death from COVID-19 also may go up.

Other lung conditions, such as a history of pulmonary hypertension or pulmonary embolism affect a person's risk of serious illness after COVID-19. The risk of death may be higher after these conditions.

Cancer raises the risk of severe COVID-19

In general, people with cancer have a greater risk of getting serious COVID-19. People who have or had blood cancer may have a higher risk of being sick for longer, or getting sicker, with COVID-19 than people with solid tumors.

Having cancer raises the risk of needing care in the hospital, intensive care and the use of breathing support. Having blood cancer and getting COVID-19 raises the risk of death from the illness.

Treatment for blood cancer may raise the risk of severe COVID-19 but the research is still unclear. Cancer treatment may also affect your COVID-19 vaccine. Talk to your healthcare professional about additional shots and getting vaccinated after treatments that affect some immune cells.

Other conditions that raise the risk of severe COVID-19

If an organ or body system is already weakened by disease, infection with the COVID-19 virus can cause further damage. In other cases, medicine for the original condition can lower the immune system's response to the virus that causes COVID-19.

Many different diseases can raise the risk of severe COVID-19 illness.

• Brain and nervous system diseases, such as strokes.
• Chronic liver disease, specifically cirrhosis, nonalcoholic fatty liver disease, alcoholic liver disease and autoimmune hepatitis.
• HIV not well managed with medicine.
• Heart disease, including congenital heart disease and cardiomyopathies.
• Mood disorders or schizophrenia.
• Having received an organ or stem cell transplant.
• Sickle cell anemia and thalassemia blood disorders.

Other risk factors for severe COVID-19 are:

• Not getting enough physical activity.
• Pregnancy or having recently given birth.
• Use of medicines that lower the immune system's ability to respond to germs.

Also, as a general group, disability is linked to an increased risk of severe COVID-19. The risks are different depending on the disability.

• Down syndrome is linked to a higher risk of needing care in the hospital. The risk of death from severe COVID-19 also is higher than typical for people with Down syndrome.
• Attention deficit/hyperactivity disorder is linked to an increased risk of needing care in the hospital from severe COVID-19.
• Cerebral palsy is linked to an increased risk of needing care in the hospital from severe COVID-19.

These are not the only conditions that increase the risk of severe COVID-19. Talk to your healthcare professional if you have questions about your health and risk for getting a serious COVID-19 illness.

A COVID-19 vaccine can lower your risk of serious illness

The COVID-19 vaccine can lower the risk of death or serious illness caused by COVID-19. Your healthcare team may suggest added doses of COVID-19 vaccine if you have a moderately or seriously weakened immune system.

How else can you lower the risk of severe COVID-19?

Everyone can lower the risk of serious COVID-19 illness by working to prevent infection with the virus that causes COVID-19.

• Avoid close contact with anyone who is sick or has symptoms, if possible.
• Use fans, open windows or doors, and use filters to move the air and keep any germs from lingering.
• Wash your hands well and often with soap and water for at least 20 seconds. Or use an alcohol-based hand sanitizer with at least 60% alcohol.
• Cough or sneeze into a tissue or your elbow. Then wash your hands.
• Clean and disinfect high-touch surfaces. For example, clean doorknobs, light switches, electronics and counters regularly.
• Spread out in crowded public areas, especially in places with poor airflow. This is important if you have a higher risk of serious illness.
• The U.S. Centers for Disease Control and Prevention recommends that people wear a mask in indoor public spaces if COVID-19 is spreading. This means if you're in an area with a high number of people with COVID-19 in the hospital. They suggest wearing the most protective mask possible that you'll wear regularly, that fits well and is comfortable.

These basic actions are even more important for people who have weakened immune systems, and their caregivers.

The FDA also has authorized the monoclonal antibody pemivibart (Pemgarda) to prevent COVID-19 in some people with weakened immune systems.

People can take other actions based on their risk factors.

• If you're at a higher risk of serious illness, talk to your healthcare professional about how best to protect yourself. Know what to do if you get sick so you can quickly start treatment.
• Lower your risk of COVID-19 complications by making sure that any health issues are well managed. This includes staying on track with managing medical conditions, going to all healthcare appointments and planning ahead to avoid running out of medicine. Keep taking medicines as suggested by your healthcare professional.
• Stay up to date on vaccines. This includes vaccines for flu, pneumonia and RSV. These vaccines won't prevent COVID-19. But becoming ill with a respiratory illness may worsen your outcome if you also catch COVID-19.

You may consider making a care plan. In the care plan, write your medical conditions, the medicine you take, and any special food or diet needs you have. The care plan also includes who you see for care and your emergency contacts.

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• Goldman L, et al., eds. COVID-19: Epidemiology, clinical manifestations, diagnosis, community prevention, and prognosis. In: Goldman-Cecil Medicine. 27th ed. Elsevier; 2024. https://www.clinicalkey.com. Accessed April 5, 2024.
• Regan JJ, et al. Use of Updated COVID-19 Vaccines 2023-2024 Formula for Persons Aged ≥6 Months: Recommendations of the Advisory Committee on Immunization Practices—United States, September 2023. MMWR. Morbidity and Mortality Weekly Report 2023; doi:10.15585/mmwr.mm7242e1.
• Underlying medical conditions associated with higher risk for severe COVID-19: Information for healthcare providers. Centers for Disease Control and Prevention. https://www.cdc.gov/coronavirus/2019-ncov/hcp/clinical-care/underlyingconditions.html. Accessed April 2, 2024.
• Stay up to date with COVID-19 vaccines. Centers for Disease Control and Prevention. www.cdc.gov/coronavirus/2019-ncov/vaccines/stay-up-to-date.html. Accessed April 2, 2024.
• COVID data tracker. Centers for Disease Control and Prevention. https://covid.cdc.gov/covid-data-tracker/#demographics. Accessed April 2, 2024.
• Najafabadi BT, et al. Obesity as an independent risk factor for COVID‐19 severity and mortality. Cochrane Database of Systematic Reviews. 2023; doi:10.1002/14651858.CD015201.
• People with certain medical conditions. Centers for Disease Control and Prevention. https://www.cdc.gov/coronavirus/2019-ncov/need-extra-precautions/people-with-medical-conditions.html. Accessed April 2, 2024.
• AskMayoExpert. COVID-19: Outpatient management (adult). Mayo Clinic; 2023.
• Emergency use authorizations for drugs and non-vaccine biological products. U.S. Food and Drug Association. https://www.fda.gov/drugs/emergency-preparedness-drugs/emergency-use-authorizations-drugs-and-non-vaccine-biological-products. Accessed April 2, 2024.
• How to protect yourself and others. Centers for Disease Control and Prevention. https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/prevention.html. Accessed April 2, 2024.
• COVID-19: What people with cancer should know. National Cancer Institute. https://www.cancer.gov/about-cancer/coronavirus/coronavirus-cancer-patient-information. Accessed April 2, 2024.
• Hygiene and respiratory viruses prevention. Centers for Disease Control and Prevention. https://www.cdc.gov/respiratory-viruses/prevention/hygiene.html. Accessed April 2, 2024.
• Preventing respiratory viruses. Centers for Disease Control and Prevention. https://www.cdc.gov/respiratory-viruses/prevention/index.html. Accessed April 2, 2024.
• Maintaining a care plan. Centers for Disease Control and Prevention. https://www.cdc.gov/aging/publications/features/caregivers-month.html. Accessed April 2, 2024.
• COVID-19: What People with Cancer Should Know. National Cancer Institute. https://www.cancer.gov/about-cancer/coronavirus/coronavirus-cancer-patient-information. Accessed April 11, 2024.

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• Coronavirus infection by race
• COVID-19 travel advice
• COVID-19 vaccine: Should I reschedule my mammogram?
• COVID-19 vaccines for kids: What you need to know
• COVID-19 vaccines
• COVID-19 variant
• COVID-19 vs. flu: Similarities and differences
• Debunking coronavirus myths
• Different COVID-19 vaccines
• Extracorporeal membrane oxygenation (ECMO)
• Fever: First aid
• Fever treatment: Quick guide to treating a fever
• Fight coronavirus (COVID-19) transmission at home
• Honey: An effective cough remedy?
• How do COVID-19 antibody tests differ from diagnostic tests?
• How to measure your respiratory rate
• How to take your pulse
• How to take your temperature
• How well do face masks protect against COVID-19?
• Is hydroxychloroquine a treatment for COVID-19?
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• Mayo Clinic Minute: You're washing your hands all wrong
• Mayo Clinic Minute: How dirty are common surfaces?
• Multisystem inflammatory syndrome in children (MIS-C)
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• Pregnancy and COVID-19
• Safe outdoor activities during the COVID-19 pandemic
• Safety tips for attending school during COVID-19
• Sex and COVID-19
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• Thermometers: Understand the options
• Treating COVID-19 at home
• Unusual symptoms of coronavirus
• Vaccine guidance from Mayo Clinic
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Related information

• Coronavirus disease 2019 (COVID-19) - Related information Coronavirus disease 2019 (COVID-19)
• COVID-19 vaccines: Get the facts - Related information COVID-19 vaccines: Get the facts
• COVID-19 Whos at higher risk of serious symptoms

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