M01.05.022 Tracheobronchial Tree

The tracheobronchial tree is a network of airways that facilitates the movement of air into the lungs, where gas exchange occurs. It consists of the trachea, bronchi, and bronchioles and is located in the neck and thoracic cavity. In this article, we will explore the anatomical position, structure, neurovascular supply, and clinical relevance of these airways.

Anatomical Position

The trachea begins at the lower border of the cricoid cartilage and continues as a direct extension of the larynx. It travels inferiorly into the superior mediastinum and bifurcates at the level of the sternal angle into the right and left main bronchi.

Key Points:

• The trachea is anterior to the esophagus and inclines slightly to the right as it descends.
• Bifurcation occurs at the level of the sternal angle, marking the division into the primary bronchi.

Structure of the Airways

The trachea, bronchi, and bronchioles are lined with ciliated pseudostratified columnar epithelium, interspersed with goblet cells that secrete mucus. Together, these form the mucociliary escalator, which traps particles and pathogens, moving them out of the airways.

Trachea and Bronchi Structure:

• Trachea: C-shaped cartilage rings support its structure. The free ends of these rings are connected by the trachealis muscle.
• Carina: A ridge of cartilage at the bifurcation of the trachea that is sensitive for triggering the cough reflex.

Bronchi Structure:

• Right Main Bronchus: Wider, shorter, and more vertical; higher risk of foreign body inhalation.
• Left Main Bronchus: Passes inferiorly to the arch of the aorta and anteriorly to the thoracic aorta and oesophagus.

Neurovascular Supply

Trachea:

• Sensory Innervation: Recurrent laryngeal nerve.
• Arterial Supply: Tracheal branches from the inferior thyroid artery.
• Venous Drainage: Brachiocephalic, Azygos, and accessory hemiazygos veins.

Bronchi:

• Innervation: Pulmonary branches of the vagus nerve (CN X).
• Blood Supply: Bronchial arteries.
• Venous Drainage: Bronchial veins.

Bronchioles

Bronchioles are smaller airways that arise from the Segmental bronchi. They lack cartilage and mucus-secreting cells, but instead have club cells that produce a surfactant lipoprotein, preventing airway collapse during expiration.

Bronchioles Structure:

• Conducting Bronchioles: Transport air but do not participate in gas exchange.
• Terminal Bronchioles: Lead to respiratory bronchioles, which contain alveoli, the primary sites of gas exchange.

Alveoli:

• Tiny, air-filled sacs with simple squamous epithelium.
• Around 300 million alveoli in the lungs provide a large surface area for gas exchange.

Clinical Relevance: Asthma

Pathophysiology:

• Asthma is a chronic inflammatory disorder of the airways, characterized by hypersensitivity, bronchospasm, and reversible airflow obstruction.
• During an asthma attack, a trigger (e.g., allergens, exercise) leads to inflammation and contraction of smooth muscle around the bronchioles, narrowing the airways, resulting in wheezing and difficulty breathing.

Asthma in Bronchioles:

• Airway Remodeling: Increased smooth muscle thickness, damaged epithelium, and thickened basement membrane occur in the bronchioles.

Points to Remember

1. The trachea, bronchi, and bronchioles form the tracheobronchial tree, essential for air passage into the lungs.
2. The trachea bifurcates at the sternal angle into the right and left main bronchi.
3. Carina at the bifurcation is sensitive for the cough reflex.
4. Asthma causes inflammation and bronchospasm, leading to narrowed airways and difficulty breathing.