9.4. Trachea and Bronchi
Features of the Trachea
These are incomplete rings of hyaline cartilage that are present throughout the length of the trachea. The cartilage is ‘C-shaped’, with the posterior aspect being deficient of cartilage. Instead of cartilage the trachealis muscle is present. The trachealis muscle facilitates the expulsion of air from the trachea by compressing its lumen, e.g. during coughing. This muscle also keeps the trachea open by preventing its collapse during inspiration (and the associated pressure changes), and prevents over-expansion of the trachea.
Bifurcation of the trachea
The trachea bifurcates, or divides into two main/primary bronchi, at an anatomical landmark of hyaline cartilage called the carina. The lies at the same level as the sternal symphysis joint (in fact, the trachea is said to bifurcate in the sternal plane).
Features of the Bronchi
Overall, there are three types of bronchi. These are:
1. Primary bronchus
Also called a principal bronchus or a main bronchus. There is one left primary bronchus and one right primary bronchus. The primary bronchus on each side retains the ‘C-shaped’ cartilage that is present in the trachea.
2. Secondary bronchus
Also called a lobar bronchus. These are formed from the dividing of the primary bronchi. Each secondary bronchus supplies one lobe of the lung. As such, on the left there are two secondary bronchi (one superior and one inferior) and on the right there are three secondary bronchi (superior, middle and inferior). Note on the diagram how the right primary bronchus first divides into the superior and middle secondary bronchi, followed by the inferior bronchus branching off the middle bronchus. Note also that the secondary (and tertiary) bronchi have smaller cartilage plates instead of the larger ‘C-shaped’ cartilage rings.
3. Tertiary bronchus
Also called a segmental bronchus. These branch off from the secondary bronchi. The tertiary bronchi each supply a ‘bronchopulmonary segment’ of each lobe. These are described in more detail in Section 9.6.