Respiratory Department Clinical Handbook

Author: Chief Editor: Yu Jianmin
Publisher:
Publish Date: 1999-09-01
Features:
Segment: 2. Bronchi: The trachea divides into the left and right main bronchi (also known as principal bronchi) at the bifurcation. The angle between the left and right bronchi (i.e., the angle of tracheal bifurcation) is generally 65–80°, with an average of 70°. The size of this angle is significant; an excessively large angle suggests enlargement of the subcarinal lymph nodes, while an excessively small angle may indicate compression and displacement of one bronchus. The structure of the bronchial wall is similar to that of the trachea, but the cartilaginous rings are relatively smaller, while the membranous walls are relatively larger. The number of cartilaginous rings is unequal on the left and right sides. The right bronchus typically has 3–4 rings, while the left bronchus generally has 7–8 rings.
(1) Right Bronchus: It is wider, shorter, and more steeply inclined than the left bronchus. The average length is 2.1 cm in males and 1.9 cm in females. The angle between the bronchus and the prolongation of the tracheal midline is 25–30°, corresponding to the level of the 5th thoracic vertebra, passing through the right pulmonary hilum into the right lung. Foreign bodies are more likely to enter the right bronchus, and inhalation diseases such as lung abscesses are also more common on the right side, particularly in the right lower lobe. Additionally, bronchoscopy or bronchial catheterization is often easier on the right side.
(2) Left Bronchus: It is narrower, longer, and more horizontal than the right bronchus. The average length is 4.8 cm in males and 4.5 cm in females. The angle between the bronchus and the prolongation of the tracheal midline is 40–50°, corresponding to the level of the 5th thoracic vertebra, passing through the left pulmonary hilum into the left lung. The length of the left bronchus is approximately 2.5 times that of the right bronchus. The cartilage of the bronchial wall gradually becomes irregular spiral or fragmented into incomplete pieces from the lobar, segmental, and subsegmental bronchi. By the time it reaches the 7th-order bronchioles, the diameter decreases from 3.5 mm to 1–2 mm, and the cartilage pieces rapidly decrease and disappear. From the small bronchioles to the terminal bronchioles, these are the last five membranous air-conducting tubes, continuous with the cuboidal epithelial cells of the surface. The submucosal tissue gradually degenerates and thins, while the muscle fibers increase, intertwining to form a double helical structure on both sides of the wall. When the muscle fibers contract, the mucosa of the terminal bronchioles may exhibit longitudinal folds. The small bronchioles and their branches lack cartilaginous support, making the lumen more susceptible to the effects of intrathoracic pressure fluctuations compared to cartilaginous airways. The small bronchioles average 20 branches with a diameter of about 0.5 mm, and each terminal bronchiole gives rise to approximately 50 respiratory bronchioles of similar diameter, which serve as the gas-exchange airways.
(3) Bronchial Branching in the Lungs: The left and right bronchi at the pulmonary hilum divide into lobar bronchi (secondary bronchi), which further divide into segmental bronchi (tertiary bronchi). Each lung is generally divided into 10 segmental bronchi, with each segmental bronchus supplying the corresponding pulmonary tissue (lung segment). The segmental bronchi then branch into smaller bronchioles and terminal bronchioles. Respiratory bronchioles branch further downward, with alveoli protruding from their walls. When the bronchi within the lungs are stripped away or observed via bronchography in vivo, all bronchi are seen to branch repeatedly, resembling the branching of a tree, hence they are often referred to as the "bronchial tree."
Branching of the Right Bronchus in the Lungs: After the right bronchus gives rise to the upper lobar bronchus at 1–2.5 cm, it descends to become the intermediate bronchus, from which the middle lobar bronchus arises. The continuation of the main bronchus becomes the lower lobar bronchus. The upper lobe gives rise to the apical branch (1), posterior branch (2), and anterior branch (3); the middle lobe gives rise to the lateral branch (4) and medial branch (5); the lower lobe gives rise to the posterior branch (6), medial basal branch (7), anterior basal branch (8), lateral basal branch (9), and posterior basal branch (10), among other segmental bronchi.
Branching of the Left Bronchus in the Lungs: The left bronchus enters the lung at a distance of 3 cm from the tracheal bifurcation. The upper lobar bronchus divides into two branches: the upper branch gives rise to the apicoposterior branch (1+2) and the anterior branch (3), while the lower branch is the lingular bronchus (equivalent to the middle lobe of the right lung), which divides into the upper lingular branch (4) and the lower lingular branch (5). The lower lobe is formed by the continuation of the left bronchus downward. After giving rise to the posterior branch (6), it further gives rise to the medial basal branch, which merges with the anterior basal branch (7+8), the lateral basal branch (9), and the posterior basal branch (10). Since the apicoposterior branch of the upper lobe and the medial and anterior basal branches of the lower lobe are merged, there are actually only 8 segmental bronchi in the two lobes of the left lung.