Bronchi | Research & Encyclopedia Articles

Bronchi

Bronchi are the respiratory passageways between the trachea and the lobes of the lung. After the left and right primary bronchi (also termed principal bronchi) arise from the trachea, they decrease in diameter as they ramify (create multiple extensions) and enter the lobes of the lung as lobar and segmental bronchi. The multiple branching of the bronchi is referred to as a bronchial tree.

Corresponding to the lobes of the lung they serve, the right principal bronchus divides into a right superior lobe broncus, a middle lobe bronchus, and right inferior lobe bronchus. The left principal bronchus divides into the left superior principal bronchus and the left inferior lobe bronchus.

In the embryo, the bronchi develop from the caudal end of the tracheal tube as primordial main stem bronchi. As ultimately reflected in adult anatomy, the left main bronchus assumes a more transverse course than the right main bronchus. Splanchnopleuric mesoderm differentiates into the fibro-elastic and muscular structure of the developing bronchial tree. Blood vessels serving both bronchial and lung tissue also develop from this mesodermal layer. Vagal nerves invest the developing lobes of the lung surrounding the bronchi serving each lung lobe bud. Pulmonary arteries and veins also develop within the bronchial tree to form a bronchopulmonary segment--a functional unit of lung--wherein a single segmental bronchus and accompanying pulmonary artery serve a defined region of the lung. Blood returns to the heart via pulmonary veins that lie in the connective tissue between the bronchopulmonary segments.

Bronchopulmonary segments, surrounded by connective tissue, form the functionally independent respiratory units of the lung. This redundancy is important, because, if one segment or lobe of the lung is punctured, or otherwise becomes non-functional, other branches of the bronchopulmonary tree are not compromised.

In the adult, there are six main bronchopulmonary segments within the right superior lobe of the lung (one apical, two posterior, three anterior). Within the right middle lobe, there are nine bronchopulmonary segments (four lateral, five medial). The inferior lobe of the right lung usually contains far more bronchopulmonary segments than the other right lobes combined. The inferior lobe contains 40 bronchopulmonary segments investing all regions of lobe.

On the left side of the lung, the left superior lobe contains 15 bronchopulmonary segments, the same number as the superior and middle lobes of the right lung. As with the right side the left inferior lobe contains 40 bronchopulmonary segments.

The lining of the bronchial tree consists of a layer of pseudostratified ciliated epithelium interspersed with goblet cells. The cilia beat so that they move debris and mucous upwards. This broncho-tracheal elevator is an important component in the removal of debris and the resistance to infections (e.g., anthrax infection) that can become severe if sufficient bacteria, debris, or spores are able to invade the terminal portions of the bronchopulmonary tree.

It possible to view the interior structure and mucosa of individual bronchi (a broncus) via fiberoptic bronchoscopic examination. A bronchoscope is introduced through the naso-oral cavity and trachea. It is also possible to remove bronchial tissue samples for pathological examination. Bronchoscopy also allows for the visualization and removal of foreign objects that become lodged in the bronchial tree. Inflammation of one or more bronchi results in bronchitis. Acute onset bronchitis, with varying degrees of severity, is often associated with upper respiratory infections.

Chronic inflammation of the bronchi may result from a number of pathological conditions, including low-level exposure to allergens (allergic reaction inducing substances), smoking, and a variety of chemical pollutants and toxins. Whether stimulated by immune response or chemical irritation, inflammation in the bronchial tree results in increased mucosal production that blocks the airway and results in bronchospasm, coughing, and the expectoration (spitting) of mucosal fluids.

A congenital abnormality of the alveoli may result in an increased diameter (dilation) in the terminal portions of the bronchial tree. Infections such as pneumonia and flu (influenza) can also produce chronic dilation of the terminal portions of the bronchial tree (brochiectasis).