Bronchial Anatomy

The tracheobronchial tree is the anatomical and functional segment of the respiratory system that conducts air from the upper airways to the lung parenchyma. It is composed of the trachea and the intrapulmonary airways, including the bronchi, bronchioles and respiratory bronchioles. Different histological characteristics are seen at each level and serve specific purposes. ^[1]

The trachea and bronchi (from the Greek bronkhos, meaning "windpipe") have cartilaginous walls. Bronchi undergo multiple divisions and eventually give rise to the terminal bronchioles, which by definition, lack cartilage. The most distal respiratory bronchioles and alveoli are in charge of gas exchange. ^[2]

The trachea is the segment that connects the upper airways to the bronchi. It has 16-22 cartilaginous rings in the anterior and lateral walls (cartilaginous portion) and a thin band of smooth muscle in the posterior wall (membranous portion). This configuration supports the tracheal anatomy during inspiration and expiration. ^[3]

The trachea extends distally 10-12 cm and divides into the right and left mainstem bronchi (primary) at the level of the T5 vertebra (see image below).  

Bronchi.

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The right mainstem bronchus originates higher than the left mainstem bronchus; it is also shorter, wider, and more vertical. It measures about 2.5 cm and appears as a direct continuation of the trachea. The left mainstem bronchus is about 5 cm in length.

Mainstem bronchi divide into the lobar bronchi (secondary) and subsequently into the segmental (tertiary) bronchi (see the image below). Arteries, veins, and lymphatics also enter the lungs at the hilum along with the bronchi. A bronchopulmonary segment is a portion of lung that is supplied by a segmental bronchus and its adjacent blood vessels.

Bronchi, anterior view.

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The right mainstem bronchus divides into the right upper lobe bronchus and the bronchus intermedius (BI). The former then divides into 3 segments: apical (RB1), posterior (RB2), and anterior (RB3). The bronchus intermedius divides into the right middle lobe and right lower lobe bronchi. The right middle lobe bronchus has two segments: lateral (RB4) and middle (RB5). The right lower lobe bronchus has 5 segments: superior (RB6), medial basal (RB7), anterior basal (RB8), lateral basal (RB9), and posterior basal (RB10).

The left mainstem bronchus divides into the left upper lobe and lower lobe bronchi. The left upper lobe bronchus subsequently divides into the left upper division bronchus and the lingular division. The former gives rise to 3 segments: apical (LB1), posterior (LB2), and anterior (LB3). The apical and posterior segments are usually fused in a single apicoposterior (LB1/2) segment. The lingular bronchus has 2 segments: superior lingular (LB4) and inferior lingular (LB5). The left lower lobe bronchus branches into the superior (LB6), anteromedial basal (LB7/8), lateral basal (LB9), and posterior basal (LB10) segments.

Bronchi undergo multiple divisions (on average 23) along the bronchial tree. The initial 16-17 generations of bronchi make up the conducting zone of the airways and do not participate in gas exchange. The surface of the airways that does not contribute to gas exchange is referred to as “dead space.”

As bronchi divide into smaller airways, the respiratory epithelium undergoes histological changes and gives rise to terminal bronchioles. The 17^th to 19^th generations of bronchioles constitute the transitional zone. These bronchioles enter pyramid-shaped pulmonary lobules separated from one another by a thin septum, with the apex directed toward the hilum, comprising 5-7 terminal bronchioles. The last 2-3 generations of bronchioles have some alveoli in their walls and make up the respiratory zone.

The area of the lung that is distal to a terminal bronchiole is termed the acinus. ^{[2, 4]}  The final division is called the respiratory bronchiole, which further branches into multiple alveolar ducts. Alveoli, the functional units of the respiratory system, start appearing at the level of the respiratory bronchioles.

The bronchial wall is made up of mucosa, lamina propria, smooth muscle, and submucosa with interspersed cartilage. The initial generations of the bronchi are similar to each other in their histologic structure, except for the amount of hyaline cartilage. In the trachea, the cartilage encircles the lumen, but in subsequent divisions of bronchi, it is replaced by diminishing quantities of cartilage plates.

The bronchial submucosa contains mixed compound tubuloacinar glands, composed largely of mucin and serous-secreting cells. These cells secrete water and electrolytes into the bronchial lumen.

The bronchial mucosa is made of pseudostratified ciliated columnar epithelium with goblet cells and basal cells. Goblet cells are devoid of apical cilia and are responsible for secretion of mucin. The density of goblet cells progressively decreases from the periphery and disappears at the level of terminal bronchioles. Basal cells are located close to the basal lamina away from the lumen of the bronchus. ^[4]

Mucin is a complex glycoprotein that is responsible for trapping particulate material (ie, cells and debris) in the bronchi. The presence of mucin, water, and electrolytes contributes to the solubility of bronchial secretions.

The cilia present in the luminal aspect of epithelial cells are in charge of the rhythmic upward movement of bronchial secretions from within the lung to the pharynx. The respiratory epithelial cells are composed of basal bodies, also called terminal bars, which form a dark band just beneath the cilia and are modified centrioles.

The airway epithelial cells have apical junctions between them, comprising the zonula occludens, the zonula adherens, and the macula adherens (or desmosome).

The terminal bronchioles contain ciliated cuboidal epithelium, thin discontinuous smooth muscle, and submucosal connective tissue. Respiratory bronchioles are only partially lined by cuboidal epithelium; the remainder of their wall is lined by squamous epithelium. In contrast, alveolar walls are only composed squamous respiratory epithelium. ^[1]

Minor anatomical tracheobronchial variations may be found incidentally during bronchoscopy and/or CT scanning in 1-12 % of patients. ^[5]  However, these are rarely clinically relevant and usually remain asymptomatic. Variations may include different branching patterns and accessory lobar or segmental bronchi.

Many airway diseases involve the bronchial tree in addition to lung parenchyma.

Asthma is a heterogeneous disease, usually characterized by chronic airway inflammation that leads to variable expiratory airflow limitation (bronchoconstriction). It presents with respiratory symptoms such as wheeze, shortness of breath, chest tightness, and cough that vary over time. Disease severity varies. ^[6]  It has a complex pathophysiology. Airway hyperresponsiveness, eosinophilic inflammation, noneosinophilic inflammation, and airway remodeling that leads to chronic obstruction have all been implicated in its development. The mainstays of therapy are inhaled corticosteroids; inhaled bronchodilators are also used. ^[7]  Systemic steroids may be used for severe exacerbations.

Cystic fibrosis is an autosomal recessive disease caused by a mutation in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene on chromosome 7. This mutation leads to abnormal development or function of the sodium channel in the luminal aspect of epithelial cells, thus affecting the solubility of secretions. In the lungs particularly, it causes decreased mucociliary clearance and subsequent recurrent lung infections. The disease may also affect other organs. ^[8]

Emphysema refers to permanent dilatation of the most distal airways and damage of alveolar walls. Parenchymal architectural destruction leads to airway obstruction through the loss of the elastic properties of the lung. Alveolar damage and replacement with large air sacs (bullae) leads to an impaired capacity for gas exchange.

Chronic bronchiectasis (a type of chronic obstructive lung disease) is characterized by persistent abnormal dilatation and destruction of bronchi and bronchioles. This results in impaired drainage of secretions, airway obstruction, and subsequent recurrent lower respiratory tract infections. Clinically, it is characterized by chronic cough with sputum production and frequent acute exacerbations. ^[9]

Bronchoscopy is the process of direct visualization of the tracheobronchial tree. It may be performed with either a rigid bronchoscope or a flexible one. Flexible bronchoscopy is a minimally invasive procedure that can be performed with minimal preparation and sedation in an outpatient setting (see the video below). Conscious sedation is given before the procedure.

Bronchoscopy enables visualization of the bronchial tree up to the level of the segmental bronchi and allows the clinician to obtain tissue and fluid samples from more distal airways and lung parenchyma by means of bronchoalveolar lavage, brushings, and transbronchial biopsies. Microbiologic and histopathologic analyses of these samples facilitate accurate diagnosis of various pulmonary diseases.