Bronchial Tree

The bronchial tree is a system of airways within the lungs that conducts air from the trachea to the alveoli where gas exchange occurs. It begins at the bifurcation of the trachea and branches progressively into smaller tubes, ultimately terminating in the respiratory bronchioles and alveolar sacs. The bronchial tree forms the conducting portion of the lower respiratory tract and is essential for air distribution throughout the lungs.

Location

The bronchial tree lies within the thoracic cavity, extending from the inferior end of the trachea at the level of the sternal angle (T4–T5 vertebral level) into both lungs. It is asymmetrical due to differences in the right and left lungs. The airways are supported by cartilaginous rings and smooth muscle, and are embedded within the lung parenchyma.

Main Components

The bronchial tree is classically divided into several key structural levels:

1. Primary (Main) Bronchi

• Two main bronchi arise from the trachea: the right main bronchus and the left main bronchus.
• Right main bronchus: Wider, shorter, and more vertical. Enters the right lung at the hilum and divides into three lobar bronchi.
• Left main bronchus: Longer, narrower, and more horizontal. Enters the left lung and divides into two lobar bronchi.

2. Secondary (Lobar) Bronchi

• Each main bronchus divides into lobar bronchi, one for each lobe of the lung.
• Right lung: 3 lobar bronchi – superior, middle, and inferior
• Left lung: 2 lobar bronchi – superior and inferior

3. Tertiary (Segmental) Bronchi

• Each lobar bronchus divides into segmental bronchi that supply individual bronchopulmonary segments.
• Right lung: 10 segmental bronchi
• Left lung: 8–10 segmental bronchi (some segments may fuse)

4. Smaller Bronchi and Bronchioles

• Segmental bronchi divide into smaller bronchi and then into bronchioles (less than 1 mm in diameter).
• Bronchioles lack cartilage and glands in their walls, unlike larger bronchi.
• They further branch into terminal and respiratory bronchioles.

5. Terminal and Respiratory Bronchioles

• Terminal bronchioles: Last part of the conducting system; lead to respiratory bronchioles.
• Respiratory bronchioles: Mark the beginning of the respiratory portion where gas exchange begins; they give rise to alveolar ducts and sacs.

Bronchopulmonary Segments

Each tertiary bronchus supplies a bronchopulmonary segment—a functionally independent unit of the lung, each with its own vascular supply and airway. These segments are separated by connective tissue septa and can be surgically resected without affecting adjacent segments.

Right Lung (10 segments)

1. Apical
2. Posterior
3. Anterior
4. Lateral
5. Medial
6. Superior (of lower lobe)
7. Medial basal
8. Anterior basal
9. Lateral basal
10. Posterior basal

Left Lung (8–10 segments)

1. Apicoposterior
2. Anterior
3. Superior lingular
4. Inferior lingular
5. Superior (of lower lobe)
6. Anteromedial basal
7. Lateral basal
8. Posterior basal

Histological Changes Along the Bronchial Tree

As the bronchial tree branches and narrows, several histological changes occur:

• Cartilage becomes less prominent and disappears in bronchioles
• Pseudostratified columnar epithelium transitions to cuboidal
• Smooth muscle becomes more prominent in bronchioles
• Goblet cells decrease in number

Structural Features

• Cartilage: Present in bronchi as plates; absent in bronchioles
• Epithelium: Ciliated pseudostratified columnar in bronchi, cuboidal in terminal bronchioles
• Muscle: Smooth muscle present throughout; important in bronchioles for airway regulation

Blood Supply

• Bronchial arteries: Branches from the thoracic aorta that supply oxygenated blood to the bronchial tree and lung tissues (except alveoli)
• Bronchial veins: Drain into azygos and hemiazygos veins

Innervation

The bronchial tree is innervated by the pulmonary plexuses:

• Parasympathetic (via vagus nerve): Bronchoconstriction, vasodilation, secretion from glands
• Sympathetic (from sympathetic trunk): Bronchodilation, vasoconstriction, reduced secretion
• Sensory fibers: Detect stretch, pain, and irritants; contribute to reflexes like coughing

Function

• Conduct air from the trachea to the alveoli
• Warm, humidify, and filter inspired air through mucosal lining
• Regulate airflow via smooth muscle in bronchioles
• Provide anatomical compartmentalization (bronchopulmonary segments)

Clinical Relevance

• Bronchial obstruction: Commonly occurs in the right main bronchus due to its vertical alignment
• Bronchiectasis: Permanent dilation of bronchi from chronic infection or obstruction
• Asthma: Bronchial hyperreactivity and smooth muscle constriction affecting bronchioles