Bronchi are plural for bronchus and represent the passageways leading into the lungs. A bronchus, which is also known as a main or primary bronchus, represents the airway in the respiratory tract that conducts air into the lungs. Bronchi will branch into smaller tubes that become bronchioles.
The trachea (windpipe) is found inferior to the thyroid cartilage and superior to division into the left and right main bronchus. The trachea divides into the left and right main bronchus, which is known as the tracheal bifurcation, at the level of the sternal angle and of the fifth thoracic vertebra (or up to two vertebrae higher or lower, depending on lung volume changes due to breathing).
It is important to note that the right main bronchus is wider, shorter, and more vertical than the left main bronchus, and it enters the right lung at roughly the level of the fifth thoracic vertebra. The right main bronchus has 3 subdivisions, which become secondary bronchi also known as lobar bronchi, which deliver air to the 3 lobes of the right lung. Anatomically, the azygos vein arches over the right main bronchus from behind. The right pulmonary artery lies initially below the right bronchus and then later in front of it.
In contrast, the left main bronchus is smaller in size, but longer in length (~5 cm, as opposed to 2-3 cm long) than the right main bronchus. The left main bronchus enters the root of the left lung opposite to the sixth thoracic vertebra, passes underneath the aortic arch, and crosses in front of the esophagus, the thoracic duct, and the descending aorta. The left main bronchus subdivides into 2 secondary or lobar bronchi that deliver air to the 2 lobes of the left lung. The left pulmonary artery is found lying initially above the left main bronchus, then later in front of it. Secondary bronchi will further subdivide into the tertiary bronchi, which are also called the segmental bronchi, each of which supplies a bronchopulmonary segment.
A bronchopulmonary segment is a division of the lung that is separated from the rest of the lung by a septum of connective tissue, which is an advantage during surgery since a bronchopulmonary segment can be removed without affects other nearby segments. There are 10 bronchopulmonary segments in the right lung (3 in the superior lobe, 2 in the middle lobe, 5 in the inferior lobe), and 8 segments in the left lung (4 in the upper lobe, 4 in the lower lobe). During development, there are initially 10 segments per lung, but since the left lung only has 2 lobes, 2 pairs of bronchopulmonary segments fuse to give 8 total segments, with 4 for each lobe. The segmental bronchi divide into many smaller bronchioles that divide into terminal bronchioles, and then into respiratory bronchioles, which divide into 2 to 11 alveolar ducts. Each alveolar duct has 5 or 6 associated alveolar sacs. The alveolus is the basic anatomic unit of gas exchange.
To summarize, once the trachea bifurcates into the main left and right bronchus, each bronchus segment is progressively smaller in diameter than the previous segment and subdivides from the segmental bronchus, into the large subsegmental bronchus, into the small subsegmental bronchus, and finally into the bronchioles. The bronchioles consist of first the terminal bronchioles, then the respiratory bronchioles, and finally the alveolar sacs (which allow for gas exchange).
Alveolar epithelial lining
Components that make up the alveolar epithelial lining are:
- capillary endothelial cells
- the capillary lumen
- type I pneumocytes
- type II pneumocytes
- the alveolar lumen
- elastic fibers in the interalveolar septum
- alveolar macrophages
No gas exchange takes place in the bronchi. The conducting portion of the bronchial tree thus extends from the tracheal bifurcation to the terminal bronchiole, inclusively. The respiratory portion includes the respiratory bronchiole, alveolar ducts, alveolar sacs, and alveoli. As the volume of the lung changes with the thoracic cavity during ventilation (respiration), the entire bronchial tree will move within the lung. Structural movements will be more pronounced in portions of the bronchial tree that are more distal to the pulmonary hilum.
The primary bronchi have cartilage and a mucous membrane that are similar to those found in the trachea. Additionally, hyaline cartilage forms an incomplete ring in the bronchi that gives them the characteristic "D"-shaped appearance in the larger bronchi, and as small “plates and islands” in smaller-sized bronchi.
As the branching continues throughout the bronchial tree, the amount of hyaline cartilage in the walls decrease until it reaches the bronchioles, which have a cartilage-free wall. The amount of smooth muscle increases as the amount of cartilage decreases, and smooth muscle is also present continuously around the bronchi. In addition, the mucous membrane will undergo a transition from ciliated pseudostratified columnar epithelium to simple cuboidal epithelium to simple squamous epithelium.
The most common cause of respiratory compromise at the bronchial level is asthma, which is the hyperreactivity of the bronchi to an inflammatory component, such as from allergens. Bronchi will constrict in response to inflammation and result in shortness of breath, difficulty breathing, and ultimately resulting in decreased amounts of oxygen available for cellular processes.
Some patients are often described as “pink puffers” due to their efforts required to breathe in enough air and devices known as “asthma puffers” (or more commonly, inhalers) can help alleviate asthma. These puffers contain a bronchodilator that will help soothe the constricted bronchi to allow them to expand the airways and allow more airflow again. Albuterol is a commonly used short acting β2-adrenergic receptor agonist that quickly allow for bronchodilation.
- Standing/sitting position - Since the right main bronchus is wider, shorter, and more vertical, aspiration in adults whilst standing has higher chance of resulting in obstruction of right main bronchus instead of the left (which departs from the trachea at a more acute angle). Patients who aspirate while standing can have bilateral lower lung lobe infiltrates. However, in children, this may not always be the case. In children, depending on age, the left mainstem bronchus may be closer in size to the right mainstem bronchus, and the left mainstem bronchus also will not branch at the same acute angle as in adults, so there may not be as much of an increased chance of aspiration into the right bronchus. In general, two-thirds of aspirated objects lodge in main stem bronchi rather than in the distal bronchi, and will aspirate into the right lower lobe area.
- Supine position - When lying flat on the back, aspiration will often result in obstruction of the superior segment of the right lower lobe.
- Lying on the right side - This position will result in obstruction of the right middle lobe or posterior segment of the right upper lobe
- Endotracheal tube - If the endotracheal tube is inserted too far down the trachea, then it will lodge in the right main stem bronchus, which will allow ventilation of the right lung but not the left lung.
Bronchitis is defined as inflammation of the bronchi, which can be an acute or chronic condition. Acute bronchitis is often caused by viral or bacterial infections. Patients with chronic bronchitis often also suffer from chronic obstructive pulmonary disease (COPD), with common associations with smoking or long-term exposure to irritants. In diseases such as emphysema that occurs in COPD, the alveoli are damaged or destroyed, which reduces the surface area available for effective gas exchange.
Surfactant is a phospholipoprotein produced and distributed by type II alveolar cells, and absorbed by type I alveolar epithelial cells, with the main lipid component of the surfactant being dipalmitoylphosphatidylcholine (DPPC), which decreases surface tension of alveoli therefore making it easier for the lungs to expand.
Immature lungs of a preterm infant often fail to produce sufficient surfactant, which leads to respiratory problems. Commonly used is the lecithin-sphingomyelin ratio ("L/S ratio"), where a L/S ratio of less than 2:1 means that the fetal lungs may be surfactant deficient, which can result in infant respiratory distress syndrome (IRDS) and ultimately neonatal death. Another recent ratio used is the surfactant/albumin (S/A) ratio, where a S/A ratio of less than 35 indicates immature lungs, between 35-55 is indeterminate, and more than 55 indicates mature surfactant production (which correlates with a L/S ratio of 2.2 or greater).