Trachea

Hi everyone! It's Nicole from Kenhub, and in this tutorial, we will be discussing the trachea and the bronchi. During this tutorial, we'll be looking at this image of the trachea also known as the windpipe. The trachea – a long cartilaginous tube – is a part of the lower respiratory tract connecting the inferior part of the larynx with the bronchi of the lungs. In this tutorial, we'll have a look at the main anatomical features of the trachea together with some of its surrounding structures such as the larynx and the bronchi.

So before we move on to talk about the structures associated with the trachea, I just wanted to have a brief chat about the lower respiratory system. And as you'll be able to see, the lower respiratory system – the organs of the body from the larynx downwards associated with the active respiration – is made up of three structures – the larynx, the trachea and the bronchi – and we'll talk about each as we come to it. But essentially the trachea begins at the lower border of the larynx around the level of the sixth cervical vertebra and ends at the level of the fifth to seventh thoracic vertebrae where the trachea bifurcates into the bronchi.

So as we can see at the top of our image of the lower respiratory tract, we have some of the cartilages of the larynx. And we're not going to be able to talk too much about the larynx, just the cartilages that we can see in the image. The larynx is a hollow muscular organ found anterior to the pharynx and it's made up of a collection of nine cartilages that allow for the passage of air as well as the production of the voice. And in this image as we've mentioned, we can see three cartilages and these are the thyroid cartilage, the arytenoid cartilage which is a paired cartilage, and the cricoid cartilage. The above cartilages all have a relationship to the trachea in some way and it's also worth noting that all three cartilages are made out of hyaline cartilage.

So the first cartilage of the larynx we're going to describe is this cartilage highlighted in green – the thyroid cartilage. And the thyroid cartilage is the largest of the laryngeal cartilages and is formed by a right and a left lamina that is separated quite widely posteriorly while converging anteriorly to create a forward projection named the laryngeal prominence also known as Adam's apple. The laryngeal prominence is more obvious in men than it is in women. And as you can see up top here, the laminae of the thyroid cartilage have two horns which are called the superior horns and two lower horns which are called the inferior horns. And as you can see in this image, the inferior horns attach to the cricoid cartilage which is in turn attached to the trachea.

The second cartilage I want to talk about is the arytenoid cartilages again highlighted in green. And as I mentioned before, these cartilages are paired cartilages, and as we can see in the image, the arytenoid cartilages are shaped like pyramids which give rise to their three processes – the apex, the vocal process, and the muscular process. And of course the cricoarytenoid joint located at the base of the arytenoid cartilages connects the arytenoids to the cricoid which is in turn attached to the trachea.

And of course our last relevant cartilage of the larynx and the last cartilage that we can see of our main image of the lower respiratory tract is the cricoid cartilage – the only cartilage that is completely circular. And although in this image only the narrow anterior aspect is shown, the posterior lamina is considerably broader. As we mentioned previously, the cricoid cartilage not only provides articulation sites for the two arytenoid cartilages and for the thyroid cartilage but it also connects to the trachea by the cricotracheal ligament. And the strength of the cricotracheal ligament is important as they connect the upper and lower respiratory tracts together.

So just one other structure I want to point out about the cartilages of the larynx is the cricothyroid ligament which again we can see on our image of the lower respiratory tract. And as you can see in the image, the cricothyroid ligament is a flat band of connective tissue connecting the anterior aspects of the cricoid and thyroid cartilages – the cricoid being here and the thyroid being here. And this is a particularly important ligament to note as in emergency situations when the airway is blocked, the median cricothyroid ligament is often perforated to establish another airway in an emergency procedure called a cricothyrotomy.

And now moving down the image of our lower respiratory tract, we're back at our main image with the trachea highlighted in green. And the trachea is around about 9-15 centimeters long and it's also made up of a series of cartilages which run down its length. We'll have a chat about these cartilages and their clinical significance over the next few slides.

So as we mentioned, the trachea is made up of incomplete tracheal cartilages which you can see highlighted in this slide from the anterior aspect and we sometimes refer to them as being C-shaped and, posteriorly, you can see the C-shaped rings opening out. The trachea is comprised of approximately 15-20 of these tracheal rings and each of these rings is made out of hyaline cartilage which of course provides structural rigidity to the trachea. But how are these small cartilages held together to provide rigidity for the trachea? The answer to this question is on these ligaments that you can see in green called annular ligaments of the trachea and these are circular horizontal fibrous bands and their role is to keep the tracheal rings together. Their secondary role is to provide further support to the tracheal wall.

So just coming back to our posterior view of the lower respiratory tract we can see the incomplete cartilaginous rings creating a straight membranous wall highlighted in green. And this membranous wall covers around 1/3 of the ring's diameter. This will consist of longitudinal elastic fibers that make the trachea a flexible tube and enable it to stretch and shift inferiorly during inspiration.

And while we're here, let's have a quick look at these numerous glands located principally in the submucosa of the trachea – the tracheal glands. And as you can see, these glands which are exocrine glands has greater combination of water and mucus through narrow ducts both of which have important purposes. And the watery secretions from these glands humidify the inspired air while the mucus traps particles from the air which are in turn transported upwards towards the pharynx by the cilia on the epithelium keeping the lungs free of particles and bacteria.

And finally at the base of the trachea is the carina of the trachea – a ridge that separates the openings of the right and left main bronchi. The carina is of clinical significance as its mucous membrane is one of the sites in addition to the larynx and the bifurcation of the main bronchi that contributes to the cough mechanism. And as we can see in this image at the level of the carina, the trachea bifurcates into the two main bronchi. Anatomically, this bifurcation occurs at the level of the sternal angle around the level of the fourth or fifth vertebral body and we can see this in this image highlighted in green. And as you can also see although perhaps in this image it's not as visible, the trachea is displaced slightly to the right by the arch of the aorta on its left. And of course after bifurcation, the two main bronchi each enter the root of each lung passing through the hilum.

And of course, we're back at our image of the lower respiratory tract and highlighted in green on this image are our bronchi which are the organs through which inhaled and exhaled air is passed in and out of the lungs. And as you can see, the bronchi can be broken down into three levels beginning with the right and left main bronchi also sometimes called the primary bronchi. The next level of bronchi are called the lobar bronchi or the secondary bronchi followed by the segmental bronchi or the tertiary bronchi. And as they run exterior to the lung itself, the right and left main bronchi are considered extrapulmonary bronchi whereas the lobar and the segmental bronchi are considered intrapulmonary as they enter the lung.

So let's begin with the right main bronchus which we can see highlighted in green in the image. And the right main bronchus is clearly wider than the left, shorter and takes a more vertical course through the root and the hilum. It enters the right lung at approximately the fifth thoracic vertebra and as you can see, it can be further divided within the lung into three lobar bronchi – the right superior, the right middle and the right inferior lobar bronchus. Each of these bronchi can be further subdivided into smaller segmental bronchi supplying the pulmonary lobes and segments with air but these are not visible in this image.

The left main bronchus is smaller in caliber but longer than the right being almost 5 centimeters long and having a more horizontal course than the right main bronchus. It enters the root of the left lung opposite the sixth thoracic vertebra and the left main bronchus dives into two lobar bronchi – the left superior and the left inferior.

And I just wanted to bravely talk about the segmental bronchi and, as we mentioned, the segmental bronchi are the third level of bronchi and each segmental bronchus supplies a bronchopulmonary segment which is the division of the lung separated from the rest of the lung by connective tissue. And there are ten bronchopulmonary segments and therefore ten segmental bronchi in the right lung and eight bronchopulmonary segments and therefore eight segmental bronchi on the left. The segmental bronchi divide into smaller branches called bronchioles and then finally into alveoli but as they're not pictured in this image, we'll save them for another tutorial.

And finally I just wanted to talk a little bit about the bronchial wall. So, as you can see in this image, the mucous membrane of the trachea is continuous with that of the bronchi and, like the trachea, the bronchi also has layers of smooth muscle although the rings of the cartilage decrease with the increasing divisions of the bronchi. And the walls of the bronchi like the walls of the trachea also have ciliated epithelium.

So now that we've reached the end of the tutorial, I just wanted to have a little chat on some clinical notes starting with some notes on the trachea. So in contrast to the cricothyrotomy which we mentioned earlier which is an emergency procedure in which the median cricothyroid ligament is pierced to provide an emergency airway, a tracheostomy is a procedure during which a hole is made in the wall of the trachea usually around the second and third tracheal rings and a plastic tube is inserted in order to enable ventilation.

Now, this process is typically performed when there is an obstruction of the larynx because of inhalation of a foreign body or because of severe edema due to anaphylactic reaction or severe trauma. A small transverse incision in the lower third of the neck anteriorly and after the trachea is made visible and then an incision is made into the trachea and a small tracheostomy tube is inserted. After the tracheostomy is left in its place for the required length of time, it is then removed and the hole should just usually closed without any other intervention. It's also important to note that patients with long term tracheostomies are unable to vocalize because there is no air passing through the vocal cords.

Regarding the bronchi, the right main bronchus is wider than the left, shorter and takes a more vertical course through the root and the hilum than the left one as we mentioned before, therefore, the right bronchus appears to be a more direct continuation of the trachea than the left so that any inhaled foreign body would naturally be directed towards the right bronchus.

Patients that have a lesion within a bronchus can undergo a procedure named bronchoscopy which includes the evaluation of the trachea and its main branches. A bronchoscope is used for this process which is passed through the nose into the oropharynx and then directed into the trachea. During this process, there is an optical evaluation of the trachea and the bronchi but in most cases small biopsies are usually taken. Bronchoscopy is extremely useful for the diagnosis and the histological classification of specific types of lung cancer.

And that's all for our tutorial on the trachea and the bronchi. Thanks for watching.

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Now, good luck everyone, and I will see you next time.