Tracheobronchial Tree

Original Editor - User Name

Top Contributors - Stella Constantinides and Kim Jackson  

Introduction

The tracheobronchial tree is a branching tree of airways composed of:

• the trachea
• the bronchi
• the bronchioles.

There are approximately 23 generations (divisions) of airway in the human lung extending from trachea - generation 0 to the last order of terminal bronchioles -generation 23.

• 0 generation - Trachea
• 1 generation - Primary bronchi
• 2 generation - Lobar brochi.
• 3 generation - Segmental bronchi
• 4 generations - Subsegmental bronchus
• 17 -22 generations of divisions = Bronchioles
• 23 - Alveolar duct https://www.nature.com/articles/pr199230.pdf?origin=ppub

Trachea

The main functions of the trachea comprise air flow into the lungs, mucociliary clearance.

The trachea is a C-shaped structure, flexible tube that is composed of hyaline cartilage on the anterior and lateral walls. Also contains of the trachealis smooth muscle forming the posterior wall of the trachea .Also the trachea is composed of several primary structural annular ligament.https://www.sciencedirect.com/topics/medicine-and-dentistry/trachealis-muscle , https://www.ncbi.nlm.nih.gov/books/NBK448070/. Posterior surface of the trachea is flat where its cartilaginous rings are incomplete (anatomy book ).

1. 
   The trachea descends from the larynx into the lung.https://www.ncbi.nlm.nih.gov/books/NBK556044/#:~:text=The%20tracheobronchial%20tree%20is%20composed,the%20lungs%20for%20gas%20exchange.
2. The trachea is tube 10-12 cm.
3. Is situated anterior to the oesophagus. (biblio)
4. .Extends from cricoid cartilage (c6 level) by cricothyroid membrane  to carina (level of sternal angle ). At the level of carina the trachea bifurcates into the right and left main bronchi.https://www.ncbi.nlm.nih.gov/books/NBK556044/#:~:text=The%20tracheobronchial%20tree%20is%20composed,the%20lungs%20for%20gas%20exchange.https://www.ncbi.nlm.nih.gov/books/NBK448070/

Inner mucosal layer made of ciliated pseudostratified columnar epithelium with goblet cells. Goblet cells produce mucus that traps airborne particles and microorganisms, and the cilia propel the mucus upward, where it is either swallowed or expelled.

Hyaline cartilage

1. Series of 16 to 20 hyaline cartilage rings one top the other each is individually connected by an annular ligament.
2. The tracheal cartilaginous rings, are responsible for keeping the trachea lumen open in spite of the changes in intrathoracic pressure that occur during respiration, and hence prevent air flow limitation.

Trachealis muscle

• Type of muscle = Smooth muscle
• Connects the ends of the C-shaped tracheal cartilages.
• Is in contact with the anterior esophagus
  
  The trachealis muscle functions to constrict the airway by pulling the cartilages together, decrease the diameter , allowing for increased expiratory force during coughing with the smaller aperture allowing greater velocity to be achieved and the particle to be expelled. http://education.med.nyu.edu/Histology/courseware/modules/respiratory-sy/respiratory.07.html. This action is helpful, especially when eating food, which requires the expansion of the esophagus.

Bronchi

At the level of the fifth thoracic vertebra, the trachea divides into the left and right main bronchi. The right main bronchus has a length of 2.5 cm, and the left main bronchus is 5 cm long. https://www.sciencedirect.com/topics/medicine-and-dentistry/trachealis-

The trachea divides at the carina (at the level of the sternal angle) to give rise to the two primary bronchi – the right and left bronchus (figure X). Differences between right and left bronchus are illustrated below in table X .The right and left bronchus conduct air from the trachea into the right and left lung respectively.

The primary bronchi then branch into secondary bronchi (lobar bronchi) – three on the right and two on the left. Each secondary bronchus supplies a lobe of the lung.

Each secondary bronchi (lobar bronchi) subsequently divide into narrower tertiary bronchi (segmental bronchi) that supply bronchopulmonary segments (largest subdivisions of a lobe).

Segmental bronchi undergo further branching to form numerous smaller airways, called the bronchioles, .

Right Bronchus	Left Bronchus
-       Wider -       More vertical -       Shorter (2-3cm) -       Supported by C shaped cartilages -       20 to 30 degree angle	-       Narrower -       More angular -       Longer (5cm) -       Supported by C shaped cartilages -       40 to 60 degree angle

Bronchioles

Bronchioles arise from segmental bronchi and represent the smaller branches of the bronchial tree.

Bronchioles differ histologically from bronchi as they lack C shaped cartilages in their walls. ????

Bronchioles initially divide into many generations of conducting bronchioles that eventually divide further into terminal bronchioles which further divide into respiratory bronchioles.

• Conducting bronchioles – segmental bronchi give rise to 20 to 25 generations of conducting bronchioles that further divide to terminal bronchioles.
  • Terminal bronchioles – represent the last part of the conducting part of the bronchial tree and give rise to several generations of respiratory bronchiole.
    • Respiratory bronchioles- distinguishable to other types of bronchioles by the presence thin walled outpocketing (alveoli) extending from their lumens. Therefore, represent the first part of the respiratory airways where they facilitate gas exchange.

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