Update March 7, 2018
The conducting airway begins at the trachea and branches dichotomously. The first 16 generations of passages form the conducting zone of the airways. These branches are made up of bronchi, bronchioles, and terminal bronchioles.
The trachea receives some support by the tracheal rings.
The mucosal epithelium is attached to a thin basement membrane, and beneath this, the lamina propria. Smooth muscle cells are found beneath the epithelium and an enveloping connective tissue is likewise interspersed with cartilage that is more predominant in the portions of the conducting airway of greater caliber. The epithelium is organized as a pseudostratified epithelium and contains several cell types, including ciliated and secretory cells (eg, goblet cells and glandular acini) that provide key components for airway innate immunity, and basal cells that can serve as progenitor cells during injury.
As the conducting airway transitions to terminal and transitional bronchioles, secretory glands are absent from the epithelium of the bronchioles and terminal bronchioles, smooth muscle plays a more prominent role and cartilage is largely absent from the underlying tissue. Club cells (formerly termed “Clara cells”), nonciliated cuboidal epithelial cells that secrete important defense markers and serve as progenitor cells after injury, make up a large portion of the epithelial lining in the latter portions of the conducting airway.
The bronchioles are supplied by the broncial arteries.[https://en.wikipedia.org/wiki/Bronchial_artery}
Once the bronchial arteries reach the level of capillaries, three anastomoses occur: the larger bronchial arteries can merge directly with the alveolar microvasculature; the smaller bronchial arteries can merge with the veins of the pleural and pulmonary drainage system; and bronchial capillaries can merge directly with pulmonary capillaries.7 These connections produce a physiological right-to-left shunt comprising 5% of the total cardiac output.
The mucosa of the conducting zone of the respiratory system is composed primarily of ciliated pseudostratified columnar cells. These ciliated cells play a critical role in sweeping mucus secretions and debris out of the lungs and toward the mouth. The density of these cells gradually decreases as the airway progresses distally from the conducting to respiratory zones, and the respiratory bronchioles are the last segment of the airway in which these ciliated cells can be found.
Goblet cells are also abundant within the conducting zone epithelium and produce mucus secretions that trap particulate matter and help moisten the air. These cells (like the ciliated pseudostratified columnar epithelial cells) also progressively decrease in number as the airway progresses distally. They are no longer found beyond the terminal bronchioles. Thus, they are exclusive to the conducting zone and are not found in the respiratory zone. This is logical , because if goblet cells were present distal to the ciliated pseudostratified columnar cells, mucus would be produced that could not be cleared.
Kulchitsky cells (enterochromaffin cells) are neuroendocrine cells found throughout the conducting zone of the tracheobronchial tree. These cells secrete peptide hormones that regulate airway and vascular tone. They are important for the USMLE because they are the cells of origin for small cell carcinoma of the lung, a particularly aggressive form of lung cancer that will be discussed later in more detail.
The mucosa of the respiratory bronchioles is composed primarily of cuboidal cells and some Clara cells.
Clara cells are nonciliated, dome-shaped cells found within the airways of the conducting zone and respiratory bronchioles. These cells secrete a surfactant like material. These secretions coat the luminal surface of the bronchioles to prevent luminal adhesion so that should they collapse (as they often do), these bronchioles are able to reexpand.
After passing through the conducting airways, the inspired air enters the alveoli, where it comes into contact with the mixed venous blood in the pulmonary capillaries.
The trachea is lined with pseudostratified ciliated columnar epithelium cellswith goblet cells that produce mucus. This mucus lines the cells of the trachea to trap inhaled foreign particles that the cilia then waft upward toward the larynx and then the pharynx where it can be either swallowed into the stomach or expelled as phlegm.
Patients with primary ciliary dyskinesia ( Kartagener’s syndrome when accompanied by situs invertus) do not have functioning cilia. These patients are predisposed to recurrent respiratory infections.