Tracheobronchial Tree

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Top Contributors - Stella Constantinides and Kim Jackson  

Introduction

Figure 1: The lower respiratory tract 1:Trachea, 2: Mainstem bronchus 3: Lobar bronchus,4: Segmental bronchus, 5: Bronchiole,6: Alveolar duct, 7:Alveolus

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

• the trachea
• the bronchi
• the bronchioles.

Its main function is the transfer of air from the environment to the lungs for gas exchange.

The tracheobronchial tree has a branching structure of approximately 23 branches or generations extending from the trachea (generation 0) to the last order of terminal bronchioles (generation 23).

Trachea

Figure 2: Anatomy of the trachea

The main functions of the trachea is to allow the passage of air into the lungs and provide a vital role in the mucociliary clearance of the respiratory system.

Characteristics of the trachea

1. The trachea descends from the larynx into the lung.
2. The trachea is tube 12 cm.
3. Is situated anterior to the oesophagus.
4. .Extends from cricoid cartilage (c6 level) by cricothyroid membrane  to carina (level of sternal angle).
5. At the level of sternal angle, the trachea bifurcates into the right and left main bronchi.

The trachea is a C-shaped structure, flexible tube that is composed of hyaline cartilage on the anterior and lateral walls. The posterior wall of the trachea is formed by the trachealis smooth muscle .Also the trachea is composed of several primary structural annular ligaments. (anatomy book )

Hyaline cartilage

1. Series of 16 to 20 hyaline cartilage rings 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.

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. By decreasing the diameter of the airway, it allows for increased expiratory force during coughing and thus allows greater velocity to be achieved for the particle to be expelled. This action is helpful, especially when eating food, which requires the expansion of the esophagus.
• 

  Figure 3: Cross-section of a trachea

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.

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 1 .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 each 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 (2.5 cm) -       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

Table 1

Bronchioles

Bronchioles arise from segmental bronchi and represent the smaller branches of the bronchial tree. They differ histologically from bronchi as they lack C shaped cartilages in their walls.

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

Conducting bronchioles (part of conducting zone)

• Segmental bronchi give rise to 20 to 25 generations of conducting bronchioles that further divide to terminal bronchioles.
• Function - as part of the conducting system they allow air to travel from the trachea to alveoli where gas exchange occurs.

Terminal bronchioles (part of conducting zone)

• Represent the last part of the conducting part of the bronchial tree and give rise to several generations of respiratory bronchioles
• Function - as part of the conducting system they allow air to travel from the trachea to alveoli where gas exchange occurs.

Respiratory bronchioles ( part of respiratory zone)

• Distinguishable to other types of bronchioles by the presence of alveoli extending from their lumens
• Function - represent the first part of the respiratory airways where they facilitate gas exchange

Gas exchange occurs in the respiratory bronchioles, alveolar ducts and alveoli, which collectively form the respiratory zones deep in the lungs.

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