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: surfactant, capillary endothelial cells, the capillary lumen, type I pneumocytes, type II pneumocytes, the alveolar lumen, erythrocytes, elastic fibers in the interalveolar septum, and 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.