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Bronchi: want to learn more about it?

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Bronchi

Bronchi are plural for bronchus and represent the passageways leading into the lungs. The first bronchi branch from trachea, and they are the right and left main bronchi. These bronchi are the widest and they enter the lung. After entering the lungs, the bronchi continue to branch further into the secondary bronchi, known as lobar bronchi, which then branch into tertiary (segmental) bronchi.

Segmental bronchi continue their branching until they reach the final, sixt generation of bronchi. Every generation, starting from primary, is supported by cartilage in its wall. After sixth generation, the passageways are very narrow to be supported by the cartillage, and thus are called bronchioles (small bronchi).

Key facts
Branching Trachea -> bifurcation -> main bronchi -> lobar bronchi -> segmental bronchi -> -> bronchi of sixth generation -> terminal bronchioles -> respiratory bronchioles -> alevolar duct -> alveolar sac -> alveolus
Respiratory units Respiratory bronchioles and alveoli
Bronchi histology Conducting portion (tracheal bifurcation ->-> terminal bronchiole):
- cartilage (decreases during branching)
- mucous membrane
- smooth muscle (increases during branching)
Respiratory portion (respiratory bronchiole ->-> alveoli)
- smooth muscle
- epithelium (simple cuboidal ->-> simple squamous)
Alveolar histology Surfactant
Capillary endothelium
Capillary lumen
Type I pneumocytes
Type II pneumocytes
Alveolar lumen
Erythrocytes
Elastric fibers 
Alveolar macrophages
Clinical relations Asthma, aspiration, bronchitis, infant respiratory distress syndrome (IRDS)

This article will discuss the anatomy of the bronchi.

Anatomy

A bronchus, which is also known as a main or primary bronchus, represents the airway in the respiratory tract that conducts air into the lungs. Bronchi will branch into smaller tubes that become bronchioles.

The trachea (windpipe) is found inferior to the thyroid cartilage and superior to division into the left and right main bronchus. The trachea divides into the left and right main bronchus, which is known as the tracheal bifurcation, at the level of the sternal angle and of the fifth thoracic vertebra (or up to two vertebrae higher or lower, depending on lung volume changes due to breathing).

Fresh lungs from a cadaver: Lung fissures divide the organ into segments. They also have important clinical applications during lung auscultation and for the localisation of pathological sounds. During lung dissection, observe them in-situ before removing them from the thoracic cavity. Identify each fissure in turn, then replace the anterior thoracic wall over the lungs and identify the surface landmarks representing each fissure. You can find the oblique fissure deep to the fifth rib and the sixth costal cartilage. The horizontal fissure is marked by the fourth rib and the corresponding fourth costal cartilage.

It is important to note that the right main bronchus is wider, shorter, and more vertical than the left main bronchus, and it enters the right lung at roughly the level of the fifth thoracic vertebra. The right main bronchus has 3 subdivisions, which become secondary bronchi also known as lobar bronchi, which deliver air to the 3 lobes of the right lung. Anatomically, the azygos vein arches over the right main bronchus from behind. The right pulmonary artery lies initially below the right bronchus and then later in front of it.

In contrast, the left main bronchus is smaller in size, but longer in length (~5 cm, as opposed to 2-3 cm long) than the right main bronchus. The left main bronchus enters the root of the left lung opposite to the sixth thoracic vertebra, passes underneath the aortic arch, and crosses in front of the esophagus, the thoracic duct, and the descending aorta. The left main bronchus subdivides into 2 secondary or lobar bronchi that deliver air to the 2 lobes of the left lung. The left pulmonary artery is found lying initially above the left main bronchus, then later in front of it. Secondary bronchi will further subdivide into the tertiary bronchi, which are also called the segmental bronchi, each of which supplies a bronchopulmonary segment.

Need a refresher on the basic anatomy of the respiratory system before diving into all things bronchi related? Revise with our respiratory system quizzes and labeled diagrams.

A bronchopulmonary segment is a division of the lung that is separated from the rest of the lung by a septum of connective tissue, which is an advantage during surgery since a bronchopulmonary segment can be removed without affects other nearby segments. There are 10 bronchopulmonary segments in the right lung (3 in the superior lobe, 2 in the middle lobe, 5 in the inferior lobe), and 8 segments in the left lung (4 in the upper lobe, 4 in the lower lobe). During development, there are initially 10 segments per lung, but since the left lung only has 2 lobes, 2 pairs of bronchopulmonary segments fuse to give 8 total segments, with 4 for each lobe. The segmental bronchi divide into many smaller bronchioles that divide into terminal bronchioles, and then into respiratory bronchioles, which divide into 2 to 11 alveolar ducts. Each alveolar duct has 5 or 6 associated alveolar sacs. The alveolus is the basic anatomic unit of gas exchange.

To summarize, once the trachea bifurcates into the main left and right bronchus, each bronchus segment is progressively smaller in diameter than the previous segment and subdivides from the segmental bronchus, into the large subsegmental bronchus, into the small subsegmental bronchus, and finally into the bronchioles. The bronchioles consist of first the terminal bronchioles, then the respiratory bronchioles, and finally the alveolar sacs (which allow for gas exchange).

For details about the anatomy of the lungs, take a look at the following:

Alveolar epithelial lining

Components that make up the alveolar epithelial lining are:

  • surfactant
  • capillary endothelial cells
  • the capillary lumen
  • type I pneumocytes
  • type II pneumocytes
  • the alveolar lumen
  • erythrocytes
  • elastic fibers in the interalveolar septum
  • alveolar macrophages

Function

No gas exchange takes place in the bronchi. The conducting portion of the bronchial tree thus extends from the tracheal bifurcation to the terminal bronchiole, inclusively. The respiratory portion includes the respiratory bronchiole, alveolar ducts, alveolar sacs, and alveoli. As the volume of the lung changes with the thoracic cavity during ventilation (respiration), the entire bronchial tree will move within the lung. Structural movements will be more pronounced in portions of the bronchial tree that are more distal to the pulmonary hilum.

Feeling ready for a recap test? Check out our quiz below:

Histology

The primary bronchi have cartilage and a mucous membrane that are similar to those found in the trachea. Additionally, hyaline cartilage forms an incomplete ring in the bronchi that gives them the characteristic "D"-shaped appearance in the larger bronchi, and as small “plates and islands” in smaller-sized bronchi.

As the branching continues throughout the bronchial tree, the amount of hyaline cartilage in the walls decrease until it reaches the bronchioles, which have a cartilage-free wall. The amount of smooth muscle increases as the amount of cartilage decreases, and smooth muscle is also present continuously around the bronchi. In addition, the mucous membrane will undergo a transition from ciliated pseudostratified columnar epithelium to simple cuboidal epithelium to simple squamous epithelium.

Are you curious to find out more about the histology of the lungs? Then take a look at the next video and quiz.

Bronchi: want to learn more about it?

Our engaging videos, interactive quizzes, in-depth articles and HD atlas are here to get you top results faster.

What do you prefer to learn with?

“I would honestly say that Kenhub cut my study time in half.” – Read more. Kim Bengochea Kim Bengochea, Regis University, Denver

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