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Main structures of the respiratory system.
Hello everyone! It's Megan from Kenhub here, and welcome to our tutorial about the respiratory system. In today's tutorial, we'll first define the terms upper and lower respiratory tracts. We'll then move on to look at the main structures of the respiratory system focusing specifically on their functions. We'll also take a look at the accessory structures of the respiratory system followed by some clinical notes. But before we dive in to all of these, I'd like to first give you a quick overview of the respiratory system in general.
The respiratory system is a vital system for life. Its job is to bring oxygen into the body, remove carbon dioxide from the body and to do this safely with minimal risk of infection, toxin absorption or blockage. We often divide the respiratory system into two sections – the upper respiratory tract and the lower respiratory tract. The upper respiratory tract ends and the lower respiratory tract begins where the throat divides into the trachea and the esophagus which is at roughly this level here. Unfortunately, the esophagus is obscured in this image as it's located posterior to the trachea.
The illustration you can see on the right will feature throughout the majority of this tutorial. It shows the body from the front with the respiratory system drawn over the external surface. The head is turned to the right and has been halved down the middle in the midsagittal plane to reveal the internal structures. All of the areas we'll be discussing today are highlighted in green in this image and when we talk about a specific part, we'll highlight that specific area. So let's get started.
We'll begin this tutorial by talking about the upper respiratory tract. The upper respiratory tract refers to all of the structures involved in respiration above the level of the beginning of the trachea which is this structure here. These structures include the nasal cavity, the oral cavity and the pharynx which is made up of three parts – the nasopharynx, the oropharynx and the laryngopharynx. Air is drawn into the body through the nasal cavity and the oral cavity. The nasal cavity is the air-filled space behind the nose. There are two sites separated by the nasal septum and each site corresponds to one nostril. The nasal cavity has several functions. One of its functions is to warm or cool air to body temperature. Another is to humidify incoming air. It also functions to trap dust, pathogens and other particles as well as providing our sense of smell.
The other entry point for air is the oral cavity. The oral cavity is the air-filled space within the mouth. In the image on the right, we can see an anterior view of the open mouth with the lining of the oral cavity highlighted in green. We can see the tongue inferiorly, the buccal mucosa laterally and the hard and soft palates superiorly. The oral cavity has a few respiratory functions. Similarly to the nasal cavity, it acts to warm or cool air to body temperature as well as functioning to humidify incoming air. It also provides our sense of taste. Special senses such as taste and smell are important in identifying the presence of potentially harmful air.
Inhaled air is brought from the nasal cavity and oral cavity into the pharynx – the area you might know as the throat. There are two pipes leaving the pharynx – the trachea and the esophagus. The pharynx guides air into the trachea and food into the esophagus. It also has a role in speech for producing vowel sounds. The pharynx has three parts – a part from the nasal cavity which is the nasopharynx seen here, a part from the oral cavity which is the oropharynx seen here and a common part where these two meet which is the laryngopharynx seen here. The laryngopharynx ends at the beginning of the trachea marking the end of the upper respiratory tract. Now we'll follow inhaled air down into the lower respiratory tract.
The lower respiratory tract is made up of several structures. The components in order are the trachea, the bronchi, the bronchioles, the alveoli, and more generally the lungs. We'll also mention the main and accessory muscles of respiration. So let's take a closer look at each of these structures.
The beginning of the trachea marks the top of the lower respiratory tract. As seen on the image, it travels down the neck into the thorax where it splits into two. Known more commonly as the windpipe, the trachea is responsible for bringing air down the neck and into the chest. It also produces mucus to trap pathogens and particles that managed to get past the defenses of the upper respiratory tract. The trachea is located towards the front of the neck in the center. It's surrounded by cartilage arranged in C-shaped bands making it firm but expandable to allow air in.
Next on our way down are the bronchi – a branching network of airways arranged in an inverted tree formation. Like the trachea, they have cartilaginous rings around them to hold them open. The amount of cartilage decreases as they go deeper into the lungs as more pliability of the airway is required. There are three levels of bronchi which are best shown in another diagram that displays just the airways with the lungs removed. First, we have the primary bronchi also known as the principle bronchi seen here then we have the secondary or lobar bronchi seen here, and finally we have the tertiary or segmental bronchi seen here.
The trachea splits into two primary bronchi – the right one here and the left one here. This split happens deep to the Angle of Louis which is the bump near the top of your breast bone. Each primary bronchus corresponds to one lung. Once they enter the respective lung, each primary bronchus splits into several smaller secondary bronchi which then split into many even smaller tertiary bronchi. In this way, they direct air to and from the bronchioles. So what exactly are the bronchioles?
The bronchioles are the smallest airways in the lung even smaller than the tertiary bronchi. In fact, they're so small that we can't see them in this diagram. They are distributed throughout the lungs and do not have cartilaginous rings. Their primary function is to carry air in and out of the alveoli, however, they can also secrete mucus to trap any particles or pathogens which may be inhaled that far into the respiratory tract.
Continuous with the bronchioles are the alveoli which are tiny air sacs and the terminal points of the airways. The alveoli are not shown in this illustration as again they are too small but they exist as clumps at the end of each bronchiole so are also distributed throughout the lungs. Oxygen from inhaled air is absorbed through the walls of the alveoli into the blood stream. Similarly, carbon dioxide and certain other waste products diffuse through the wall from the blood and are subsequently exhaled. This process is called gas exchange and is their role in respiration.
While we're in the lower respiratory tract, let's discuss the lungs which we can see here highlighted in green. The lungs are the organs responsible for gas exchange. The secondary and tertiary bronchi – the bronchioles and the alveoli – all branch within the lungs. The lungs also consist of the pulmonary blood vessels and the tissues lining and supporting the airways. They are located within the thoracic cavity and inflate and deflate with the chest wall movement. Each lung is divided into lobes. There are three lobes on the right and two lobes on the left. There are only two lobes on the left-hand side to allow space for the heart.
Now let's quickly talk about the special blood supply within the lower respiratory tract.
The bronchi, bronchioles and lungs benefit from a dual blood supply and drainage. The first supply is through the bronchial arteries which fuel the cells of the tract. They bring oxygenated blood from the thoracic aorta and drain into the bronchial veins and the pulmonary veins. The second supply comes from the pulmonary arteries which bring deoxygenated blood from the right ventricle of the heart to the alveoli of the lungs for gas exchange. They then drain into the pulmonary veins to take the now oxygenated blood to the left atrium of the heart, ready to be pumped into the systemic circulation to supply the tissues of the body. Through this pulmonary circulation, the lungs enact their function of removing waste from the blood and supplying the body with oxygen.
In order to move air and out of the lungs, the chest wall must expand and compress. It achieves this through a series of muscles. At rest, two groups of muscles are involved. The first of which is the diaphragm which is a large concave muscle that separates the thoracic and abdominal cavities. When it contracts, it flattens which compresses the abdominal cavity and expands the thoracic cavity creating a negative pressure in the lungs which subsequently draws air in. When the diaphragm relaxes, it springs back into its original position to increase the pressure in the thoracic cavity and force air out of the lungs. The lungs in the chest wall have an innate elasticity which assists with this passive exhalation. The external intercostal muscles are a set of muscles found between the ribs. They use the stability of the spine to pull against bringing the ribs up and out like a bucket handle. This expands the thoracic cavity decreasing the intrathoracic pressure and subsequently draws air into the lungs.
Now that we've talked about the main muscles of respiration, it's time to focus on the accessory muscles of respiration. In periods of increased metabolic demand, in other words, when more oxygen or waste removal is required, accessory muscles are activated to increase the depth and rate of breathing. We'll use diagram showing just the skeleton and muscles to display where they are. Let's run through some of the main ones starting with those that assist inspiration.
The sternocleidomastoid muscle and the scalene muscles attach to structures above the ribs. When they contract, they pull the ribs up and out assisting the external intercostal muscles. Here we can see the sternocleidomastoid which originates from the sternum and clavicle and then inserts on the mastoid process of the skull. Next, we can see the posterior scalene muscle which originates from the cervical vertebrae and inserts on the second rib. Then we have the middle scalene muscle which also originates from the cervical vertebrae but inserts on the first rib. Finally we have the anterior scalene muscle which also originates from the cervical vertebrae and inserts on the first rib.
Muscle of the anterior chest wall such as the pectoralis major muscles attach between the ribs and the shoulder joint. When the shoulder joints are fixed in position, the muscles pull the ribcage up and out towards each shoulder enlarging the intrathoracic cavity. It's possible to fix the shoulder joint in place using the back muscles or by pushing your arms against something. Muscles of the posterior chest wall such as latissimus dorsi which we can see here and the trapezius muscle which we can see here facilitate greater inspiration by fixing the shoulders in position to allow the anterior chest wall muscles to assist as just mentioned. Others use the spine's stability to pull against raising the ribs in a similar fashion to the external intercostal muscles.
Muscles around the spine such as the iliocostalis muscle shown here help out by stabilizing the spine and lower ribs. This gives other muscles fixed structures to pull against and by extending the spine the intrathoracic volume is increased.
When you're breathing hard you need to force air out of your lungs. Normally, this wouldn’t need much, if any, muscle contraction due to the elasticity of the lungs, thoracic wall and diaphragm but when stressed, the body enlists the help of some muscles to assist with expiration. The internal intercostal muscles are a set of muscles found between the ribs situated deep to the external intercostal muscles. They pull the ribs down and together to increase the intrathoracic pressure and push air out. The abdominal wall muscles also help and there are four muscles in this group. These muscles include the rectus abdominis muscle seen here, the internal oblique muscle seen here, the external oblique muscle seen here and, finally, the transversus abdominis muscle seen here. When these muscles contract, they increase the pressure inside the abdominal cavity. This forces the diaphragm upwards which increases the pressure in the thoracic cavity and helps push air from the lungs.
Now that we covered the main structures of the respiratory system, let's go on to discuss the accessory structures. The accessory structures which we can now see highlighted in green generally have two main functions. One is to help optimize gas exchange and the other is to minimize risks to the airway. There are quite a few structures but we'll work our way through them in a similar way to before following the path of inhaled air.
At the beginning of this tutorial, we talked about the nasal cavity and its functions. To help maximize its ability to carry out these functions, the nasal cavity has some accessory structures within it. These are the nasal conchae and nasal meatus. The nasal conchae are curled shelves of bone protruding from the skull. In the image on the right, we can see the superior, middle and inferior nasal conchae highlighted in green. Between the nasal conchae are the nasal meatus which are these air-filled areas of the nasal cavity we can now see highlighted in our image. Together, the nasal conchae and meatus function to increase the surface area of the nasal cavity. This optimizes its ability to control the temperature and humidity of inhaled air and provides more opportunity to catch particles and pathogens. The conchae also cause turbulence of inhaled air which enhances smell detection.
Another important structure in the nasal cavity is the cribriform plate which we can see here highlighted in green. It's part of the ethmoid bone of the skull and forms the superior aspect of the nasal cavity. What makes it unique is that it's pierced by lots of tiny holes through which branches of the olfactory nerve protrude. The olfactory nerve is responsible for the sense of smell so it means to have nerve endings in the nasal cavity thus the cribriform plate facilitates the sense of smell. Smell is important in respiration as it allows the detection of potentially toxic or risky environments.
Moving backwards in the head we come to the Eustachian tubes which are air-filled canals that connect the middle ear to the nasopharynx. This tiny hole here is where the Eustachian tube communicates with the nasopharynx. There's one tube on either side of the head each connecting with an ear. We can see this clearer in the next diagram where this tube – the Eustachian tube – connects the middle ear and the nasopharynx. The Eustachian tubes have two functions. They're responsible for pressure equalization of the eardrum to prevent strain on the eardrum with environmental changes and they drain the middle ear of mucus secretions and debris to minimize infection risk.
Looking into the mouth, a few accessory structures can be seen. The first one which we're looking at now is the tongue. The tongue is a group of muscles in the mouth which can be controlled unconsciously and consciously. Because it is a group of muscles, the tongue is highly controllable and has many functions. The tongue's functions in respiration are to humidify and heat or cool inhaled air to body temperature and to detect the temperature of anything in the mouth for example food to allow you to react and avoid damage to the throat. It's also responsible for swallowing where it guides food away from the trachea, and for taste to detect potentially toxic environments. Finally, it also facilitates speech.
Moving up to the top of the mouth, we have the hard and soft palates which form the roof of the mouth. This can be seen in this anterior view of the open mouth. First, we have the hard palate and posterior to the hard palate, we have the soft palate. They separate the nasal cavity and nasopharynx from the oral cavity and oropharynx. The hard palate is made up of bones of the skull whereas the soft palate is a group of muscles that are coordinated with the tongue. Collectively, the palates have three functions – closing the nasal airway during swallowing and vomiting, providing sensation for the gag reflex and in modifying speech.
Attached to the edge of the soft palate is this structure here which is called the uvula. This is a small dangling grape-like structure which moves when you say ahhh. It provides sensation for the gag reflex and aids with speech. Sometimes confused with the uvula are the tonsils. There are three sets of paired tonsils – two pharyngeal tonsils on the back wall of the nasopharynx – two palatine tonsils one each side of the soft palate, and two lingual tonsils found at the base of the tongue. The palatine tonsils often called the adenoids when infected are the tonsils removed in a tonsillectomy. We can see the right and left palatine tonsils in this anterior view of the open mouth. As a collective unit, the three sets of tonsils' job is to trap and identify particles and pathogens entering the body. They form an almost complete ring around the throat called Waldeyer's ring. This allows the tonsils to catch everything going into the body.
Going down into the neck, we have the epiglottis. This is a flap of connective tissue attached to the entrance of the larynx. When swallowing, the epiglottis closes over the larynx to prevent swallowed substances from entering the lower respiratory tract. Below the epiglottis, we can see this structure here which is called the larynx. The larynx is a collection of cartilage structures and it houses and protects the vocal cords. Adam's apple is a useful surface marker of the front of the larynx. The vocal cords' functions are modulating pitch and voice production and to act as another barrier in preventing aspiration of larger objects.
Before we conclude this tutorial, let's go over some clinical notes relating to the respiratory system. Respiratory diseases can be separated into two categories. Obstructive diseases affect the airways you can see highlighted and examples include asthma or chronic obstructive pulmonary disease also known as COPD. Restrictive diseases on the other hand cause problems with lung expansion. Examples of which would be pulmonary fibrosis and obesity hypoventilation syndrome.
So obstructive diseases involve narrowing or blocking of airways. In asthma, there's a reversible antibody-mediated hypersensitivity reaction in the airways in response to a trigger, for example, cold air or pollen. This causes mucus secretion and constriction of the muscles around the smaller bronchi and bronchioles which together obstruct the airflow. It improves rapidly with bronchodilator therapy and this is what's meant by the term reversible and is a crucial part of an asthma diagnosis. In COPD, persistent inflammation of all the airways results in narrowing due to swelling, increased mucus secretion and reduced lung elasticity. As we mentioned when talking about the muscles of respiration, the lung's elasticity is the main way air is expelled. So if the elasticity is decreased, it's more difficult to expel air and it remains trapped in the lungs. This takes up space and prevents nice new oxygenated air from coming in and reduces the ability of the lungs to transfer oxygen to the blood. Unlike asthma, it's only minimally reversible, if at all, with bronchodilator therapy.
Restrictive defects are down to the lungs not being able to expand as much as normal. Pulmonary fibrosis as a term means scarring of the lungs and it refers to a group of conditions with various causes, however, a lot of the time a cause isn’t found. In other cases, medical treatments such as radiotherapy are the precipitating factor. The scarring results in less compliance of the lungs causing an inability to expand and allow as much air in. Obesity hypoventilation syndrome occurs in obese individuals where the weight of fat on the chest wall is greater than the respiratory muscles can cope with. This means the thorax cannot expand properly restricting airflow into the lungs. It's worse when lying down and on exertion; however, it's important to emphasize that exercise is one of, of note, the best treatment of this condition.
So now we know the path of air in and out of the body. To summarize, first we covered the upper respiratory tract which consists of the nasal cavity which is located behind the nose, the oral cavity which contains accessory structures such as the tongue and uvula, and then we had the three components of the pharynx. We had the nasopharynx which we can see here then the oropharynx which we can see here and finally we had the laryngopharynx which we can see here. Then we dived into the lower respiratory tract which is made up of the trachea which bifurcates into the two principle bronchi. The principle bronchi further divide into the secondary and tertiary bronchi until they give off the bronchioles which are continuous with the alveoli. We also mentioned the lungs as a respiratory unit that contains the secondary and tertiary bronchi, the bronchioles and the alveoli.
We also talked about the main muscles of respiration namely the diaphragm which we can see here and the external intercostal muscles which we can see here. Once we'd gone over the main muscles of respiration, we looked at the accessory muscles of respiration including the sternocleidomastoid muscle; the posterior scalene, middle scalene and anterior scalene muscles; the anterior chest wall muscles such as the pectoralis major muscles, the posterior chest wall muscles for example the latissimus dorsi and trapezius muscles, as well as the spinal muscles like the iliocostalis muscle. We also looked at the internal intercostal muscles which are found between the ribs and, finally, the abdominal wall muscles.
Lastly, we looked at the accessory structures of the respiratory tract which are the nasal conchae and nasal meatus. Here we can see the superior, middle and inferior nasal meatus highlighted in green. Then we had the cribriform plate which is located at the roof of the nasal cavity followed by the Eustachian tubes which are represented by this tiny hole here. Then we had the tongue which is located at the floor of the mouth and the hard and soft palates which form the roof of the mouth. Next, we had the uvula which is continuous with the soft palate and the tonsils of which there are three types. In this image, we can see the palatine tonsils. Next, we had the epiglottis which prevents food from entering the lower respiratory tract and finally we had the larynx – this structure here which houses the vocal cords.
So that brings us to the end of our tutorial on the respiratory system. I hope you enjoyed it and thank you for listening!
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