Anatomy of the lungs and tracheobronchial tree

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Introduction0:00–1:11

The lungs are a pair of air-filled organs located in the thoracic cavity, on the left and right side, separated by a central mediastinum, which contains the heart, thoracic parts of the great vessels, thoracic parts of the trachea, esophagus, thymus, and other structures.
It’s also important to notice that the heart is turned and extends towards the left side of the thoracic cavity, imposing on the left lung markedly more than the right so the anatomy of the two lungs is not completely symmetrical.
Each lung is covered by a membrane called the pleura, which is subdivided into the visceral pleura that intimately adheres to the lung and the parietal pleura that lines the pulmonary cavity.
Between them, there’s the pleural cavity, which is normally filled with a thin film of fluid. Now, on the medial part of each lung, there’s the pulmonary hilum, where the root of the lung passes through.
The root of the lungs is made of structures like the main bronchus arteries, and veins. The lungs are light, soft and spongy, and each of them has an apex a base, 3 surfaces: costal, mediastinal and diaphragmatic, and 3 borders: anterior, inferior and posterior.

Lung structure1:11–2:39

The apex is the blunt superior end of the lung above the level of the first rib into the root of the neck, while the base is the concave inferior surface of the lung that rests on the diaphragm.
Now for the specifics. The right lung is larger and heavier than the left, but it’s shorter and wider, because the right dome of the diaphragm is higher and the heart and pericardium are more to the left.
The right lung is divided into three lobes, superior, middle and inferior, by the horizontal fissure and the oblique fissure, which can be seen on all the surfaces of the lung.
On the other hand, the left lung has a single left oblique fissure, which can also be seen on all surfaces and divides the left lung into two lobes: superior and inferior.
Also on the left lung, there’s a deep cardiac notch on the anterior border, caused by the deviation of the heart towards the left.
This leaves an impression on the antero-inferior aspect of the superior lobe and also shapes a tongue-like process called the lingula.
The lingula extends below the cardiac notch and goes in and out of the costomediastinal recess during breathing. Next, there are the lung surfaces.

Lung surfaces2:39–4:12

The costal surface of both lungs is large, smooth and convex and related to the costal pleura, which separates it from the ribs, costal cartilages and the intercostal muscles.
The posterior part of the costal surface is in relation to the bodies of the thoracic vertebrae and sometimes called the vertebral part of the costal surface.
The diaphragmatic surface of the lung, also concave, forms the base of the lung, which rests on the dome of the diaphragm.
The concavity is deeper in the right lung because the right dome is higher due to the liver sitting below. Finally, the mediastinal surface of the lung is concave because it’s related to the middle mediastinum, which contains the pericardium and heart.
This surface includes the hilum which receives the root of the lung. On an embalmed cadaver, several impressions can be seen on the mediastinal surface of the lung.
On the right lung, there’s a groove for the esophagus and arch of the azygos vein as it arches superior to the lung hilum to drain into the superior vena cava which may also cause a groove in the right lung.
Both lungs also have a cardiac impression for the heart,where on the left lung, there’s a much larger cardiac impression as the heart apex is directed towards the left.
The left lung will also have a continuous groove for the arch of the aorta, as well as the descending aorta and a smaller area for the esophagus.

Lung borders4:12–4:52

And just a few words on borders. The anterior border is where the costal and mediastinal surfaces meet anteriorly.
On the right lung, it’s almost straight, while on the left lung it’s deviated by the cardiac notch. The inferior border is thin and sharp and bounds the diaphragmatic surface of the lung and separates it from the costal and mediastinal surfaces.
It also projects into the costodiaphragmatic recess of the pleura. The posterior border is where the costal and mediastinal surfaces meet posteriorly, and it’s broad and rounded, and lies at the side of the thoracic region of the vertebral column.
The lungs are attached to the mediastinum by the roots of the lungs, made up by the bronchi and associated bronchial vessels, the pulmonary arteries, the superior and inferior pulmonary veins, as well as the pulmonary plexuses of nerves and lymphatic vessels.

Lung roots4:52–7:12

On a section of the right and left lung root, we can identify the position of the main bronchus, and the pulmonary artery and vein in relationship to one another.
But keep in mind there may be slight variations from person to person. First, the main bronchus is typically against the posterior middle boundary in both lung roots.
Depending on the precise location of the section, you may see the main bronchus dividing into the superior lobar bronchus and intermediate bronchus on the right, and into the superior lobar bronchus and inferior lobar bronchus on the left.
On anatomical dissection, the bronchus is easiest to identify due to its firm cartilaginous structure when feeling it. Second, the pulmonary artery is the superior most structure on both the left and right roots.
However, on the right side, you may see the first two branches of the pulmonary artery. Third, the superior and inferior pulmonary veins are typically the anterior most and inferior most structures on both lung roots.
These roots enter and exit the lungs through a doorway that is called the hilum of the lung, similar to how the roots of a plant enter into the ground.
Medial to the hilum, towards the central mediastinum, the lung root is enclosed within the area of continuity between the parietal and visceral layers of the pleura.
Inferior to the root of the lung, this continuity between parietal and visceral pleura forms the pulmonary ligament, which extends between the lung and mediastinum, directly anterior to the esophagus..
The pulmonary ligament consists of a double layer of pleura separated by a small amount of connective tissue. To visualize this, think of wearing a large lab coat.
Your forearm represents the root of the lung, and the sleeve is the pleura, which if it is too large will droop down in a double layer.
Now, let’s play a game and see if you can tell which lung is which based on their external appearance! Alright, now let's look at the tracheobronchial tree which starts with the trachea, right below the larynx.

Quiz7:12–7:24

Tracheobronchial tree7:24–11:28

Below the larynx, the walls of the trachea are surrounded by C-shaped rings of hyaline cartilage. The trachea bifurcates into two main bronchi or primary bronchi at the level of the transverse thoracic plane or sternal angle.
They pass inferolaterally to enter the lungs at each hilum. The right main bronchus is wider, shorter and has a more vertical trajectory than the left one and passes directly into the right hilum.
The left main bronchus passes inferolaterally, inferior to the aortic arch and anterior to the esophagus and thoracic aorta to reach the left hilum.
Now, within each lung, the main bronchi divide to form the branches of the bronchial tree, all of which actually belong to the root of the lung.
Each main bronchus divides into secondary lobar bronchi, each of which supply a lobe of the lung - so on the right, there are three secondary lobar bronchi: right superior lobar bronchus, right middle lobar bronchus and right lower lobar bronchus.
The portion of the right main bronchus that continues inferiorly and gives rise to the middle and lower lobar bronchi is often referred to as the intermediate bronchus.
On the left, there are only two lobar bronchi: the left superior lobar bronchus and left inferior lobar bronchus. Each secondary lobar bronchus then divides into several tertiary segmental bronchi that supply the bronchopulmonary segments Each bronchopulmonary segment is independently supplied by a tertiary segmental bronchus, in addition to a branch of the pulmonary artery - so there’s 10 bronchopulmonary segments for the right lung, and 8 to 10 for the left lung as some of the segments may be combined.
These bronchopulmonary segments are functionally distinct segments which is important for surgeons who operate on the lungs.
In both lungs, the superior lobe has an apical, posterior, and anterior segment where the apical and posterior segment may be combined in the left.
Furthermore, the left superior lobe also has a superior lingular and inferior lingular segment. Because the right lung has a middle lobe, the middle lobe has a lateral and medial segment, which are the counterparts to the superior lingular and inferior lingular sesgments.
Finally, both of the lung’s lower lobes have a superior segment, along with an anterior, posterior, medial and lateral basal segment, where in the left lung the anterior and medial segments may be combined.
Beyond the tertiary segmental bronchi, the tracheobronchial tree branches 20 to 25 more times, resulting in about 20 to 25 generations of branching conducting bronchioles.
Finally, conducting bronchioles end as terminal bronchioles, which are the smallest conducting bronchioles and don’t have any cartilage in their walls.
Each terminal bronchiole gives rise to several generations of respiratory bronchioles, which have scattered alveoli, or thin-walled outpouchings, extending from their lumens.
The pulmonary alveolus is the basic structural unit of gas exchange. Due to these tiny structures, the respiratory bronchioles can transport air and exchange gas.
Each respiratory bronchiole gives rise to 2 to 11 alveolar ducts, each of which gives rise to 5 to 6 alveolar sacs. Alveolar ducts are elongated airways lined with alveoli, leading to the alveolar sacs where clusters of alveoli open.

Blood supply11:28–11:57

When thinking of the blood supply to the lungs, you need to consider two different circuits. First, they have a functional circulation, where low-oxygen blood comes from the heart and reaches the lungs in order to exchange carbon dioxide with oxygen from all the tissues throughout the body.
Then there’s a nutritive circulation, which also comes from the heart, but this time, oxygenated blood nourishes the lungs themselves.

Pulmonary circulation11:57–14:05

Regarding functional, or pulmonary circulation, briefly speaking, each lung has a pulmonary artery supplying low-oxygen blood to it and two pulmonary veins draining oxygenated blood from it.
The right and left pulmonary artery arise from the pulmonary trunk at the level of the sternal angle and they carry low-oxygen blood to the lungs.
Although they’re called arteries, they’re colored blue, just like veins, in almost all anatomical illustrations, because they transport low-oxygen blood.
Each pulmonary artery becomes part of the root and then divides into secondary lobar arteries. At this point, let’s keep in mind that the arteries pair together with the bronchi and usually sit anterior to them.
The right and left superior lobar arteries to the superior lobes arise first, even before entering the hilum. Continuing into the lung, the artery descends posteriorly and laterally to the main bronchus.
In the right lung, it continues as the intermediate artery that will divide into middle and inferior lobar arteries, and, in the left lung, it continues as the inferior lobar artery.
Lobar arteries further divide into tertiary segmental arteries, which are what supply the bronchopulmonary segments. Two pulmonary veins, a superior and an inferior pulmonary vein on each side, carry oxygen-rich blood from the lungs to the left atrium of the heart.
Because they carry oxygen rich blood, they’re colored red in medical illustration. Unlike the pulmonary arteries, the pulmonary veins don’t buddy up with respective bronchi and arteries, they just course within the lung receiving oxygenated-blood from adjacent bronchopulmonary segments.
The veins from the visceral pleura and the bronchial venous circulation drain into the pulmonary veins, with the exception of the central, perihilar region of the lung; this means that a small volume of low-oxygen blood enters the large volume of oxygenated blood.
Regarding the nutritive, or bronchial circulation, there are two left bronchial arteries and a single right bronchial artery that supply the root of the lungs, tissues of the lungs, and the visceral pleura.

Bronchial circulation14:05–15:14

The parietal pleura is typically supplied by the arteries supplying the thoracic wall. Now, the two left bronchial arteries usually arise from the thoracic aorta.
The right bronchial artery usually arises either from the proximal part of one of the upper posterior intercostal arteries, usually the 3rd posterior intercostal artery, or from a common trunk with the left superior bronchial artery.
Sometimes, the single right bronchial artery may arise directly from the aorta. The bronchial arteries also provide branches to the upper esophagus.
The bronchial arteries pass on the posterior aspect of the main bronchi and supply all the branches as far as the respiratory bronchioles.
The most distal branches of the bronchial arteries anastomose with branches of the pulmonary arteries in the walls of the bronchioles and also in the visceral pleura.

Venous drainage15:14–15:52

The right and left bronchial veins only drain the part of the blood that’s distributed near the more proximal part of the root and they also drain some blood from esophageal veins.
The right bronchial vein drains into the azygos vein, while the left bronchial vein drains into the accessory hemi-azygos vein or into the left superior intercostal vein.
The blood returning from the visceral pleura, as well as from the more peripheral regions of the lung and the distal components of the root is drained by the pulmonary veins.
While we’re at it, let’s not forget about the lymphatics of the lungs. First, there are two lymphatic plexuses: a superficial and a deep one, which communicate freely.

Lymphatic drainage15:52–17:50

The superficial, or subpleural plexus, lies deep to the visceral pleura and drains the visceral pleura and lung tissue. Lymphatic vessels from this plexus drain into the bronchopulmonary or hilar lymph nodes located in the region of the lung hilum.
The deep lymphatic plexus is located in the submucosa of the bronchi and in the peribronchial connective tissue, and it drains the structures that make up the root of the lung.
Lymphatic vessels here continue to follow the bronchi and pulmonary vessels to the hilum, where they also drain into the bronchopulmonary lymph nodes.
At this point, lymph from both the superficial and deep plexus drains to the superior and inferior tracheobronchial lymph nodes, which are located superior and inferior to the bifurcation of the trachea.
The right lung drains primarily through the consecutive nodes on the right side, and the superior left lobe drains primarily through corresponding nodes on the left.
However, a lot of the lymphatics from the lower lobe of the left lung drain to the right superior tracheobronchial nodes.
Now, lymph from the tracheobronchial lymph nodes passes to the right and left bronchomediastinal lymph trunks, which are the major conduits that are draining the thoracic viscera.
They usually end on each side at the venous angles, which are the junctions of the subclavian and internal jugular veins.
Finally, lymph from the parietal pleura drains into the lymph nodes of the thoracic wall and a few lymphatics from the cervical parietal pleura drain into the axillary lymph nodes.
After having fun with the lymphatics, let’s now tackle the nerves of the lungs and pleura. The nerves of the lungs and visceral pleura come from the pulmonary plexuses, located anterior and posterior to the roots of the lungs and contain parasympathetic, sympathetic, and visceral afferent fibers.

Innervation17:50–20:02

The parasympathetic fibers conveyed to the pulmonary plexus are presynaptic fibers from the vagus nerve. They synapse with parasympathetic ganglion cells in the pulmonary plexuses and along the branches of the bronchial tree.
The parasympathetic fibers are active during rest mode, so they cause bronchoconstriction of the smooth muscle in the bronchial tree, vasodilation of the pulmonary vessels through inhibition, and secretion of glands in the bronchial tree.
The sympathetic fibers of the pulmonary plexuses are postsynaptic fibers. Their cell bodies are in the paravertebral sympathetic ganglia of the sympathetic trunks.
The sympathetic fibers are active during the fight or flight mode, so they cause bronchodilation, vasoconstriction in the pulmonary vessels and inhibit the secretion in the alveolar glands of the bronchial tree.
The visceral afferent fibers of the pulmonary plexuses are either reflexive, meaning they coordinate an automatic response to a stimulus or nociceptive, meaning they respond to painful stimuli.
Reflexive fibers travel with parasympathetic fibers, and carry impulses responsible for reflexive control such as coughing.
Nociceptive fibers from the visceral pleura and bronchi accompany the sympathetic fibers, and respond to painful stimuli..
Finally, the nerves of the parietal pleura derive from the intercostal and phrenic nerves. The costal pleura and the peripheral part of the diaphragmatic pleura are supplied by intercostal nerves, which mediate both the sensation of touch and pain.
The central part of the diaphragmatic pleura and the mediastinal pleura are supplied by the phrenic nerves. All right, as a quick recap.

Review20:02–22:01

The lungs are located on each side of the thorax and are covered by the pleura. They have an apex, a base, 3 surfaces: costal, mediastinal and diaphragmatic and 3 borders: anterior, inferior and posterior.
The right lung is divided by the horizontal fissure and oblique fissure into three lobes, while the left lung is divided by the left oblique fissure into two lobes and due to the cardiac notch, on the antero-inferior aspect of the superior lobe there’s the lingula.
The tracheobronchial tree begins below the larynx with the trachea which splits into the main bronchi, which further split into secondary lobar bronchi, tertiary segmental bronchi.
At the tertiary level, the right lung can be divided into 10 bronchopulmonary segments and the left lung into 8-10 bronchopulmonary segments.
The tertiary segmental bronchi then split into conducting bronchioles, terminal bronchioles and respiratory bronchioles which contain the alveoli.
The functional circulation of the lungs consists of the right and left pulmonary arteries which supply oxygen-poor blood to the lungs, and the 4 pulmonary veins which carry oxygen-rich blood back to the heart.
The nutritive circulation is assured by the two left bronchial arteries and the single right bronchial artery. The right and left bronchial veins are responsible with venous drainage, along with the pulmonary veins in some regions.
Lymphatics of the lungs and pleura include the superficial and deep lymphatic plexuses. Finally, nerves come from the pulmonary plexuses and contain parasympathetic, sympathetic and afferent fibers.