Corpus: Bronchial system

The term bronchial system or bronchial tree (bronchial arbor) refers to the airways in the lungs below the trachea.

The trachea divides into two main bronchi (bronchi principales) at the level of the 4th to 5th thoracic vertebrae. A distinction is therefore made between the right and left bronchial tree. The trunk is formed by the right and left main bronchus (bronchus principalis).

Bronchi[Bearbeiten]

The right main bronchus continues approximately in the direction of the trachea, the left branches off at an angle of about 50-100 degrees. As the heart, which lies behind the sternum, protrudes into the left half of the thorax, the left lung is smaller than the right and has only 2 lobes, whereas the right lung has three.

The left main bronchus is divided into 2 lobar bronchi, the right into three, each with a diameter of 8 to 12 mm. The lobular bronchi are in turn divided into 9 or 10 segmental bronchi - corresponding to the structure of the lung in segments - which are numbered consecutively on both sides to facilitate communication between the doctors. The right lung also has a cardiac lower lobe segment and therefore has 10 segmental bronchi.

Each segmental bronchus is divided into two so-called rami subsegmentales. Further divisions take place up to a diameter of about 1 mm and only up to this point does the bronchial wall contain cartilage to ensure that the bronchi remain open and thus ventilation of the entire lung. As the division progresses, the glandular cells and ciliated epithelium decrease and a ring-shaped muscle system forms under the mucous membrane.

Right bronchial tree[Bearbeiten]

The right upper lobe bronchus arises from the right main bronchus approx. 3 cm distal to the carina: it divides after a few millimetres:

• B1: apical segmental bronchus of the upper lobe
• B2: posterior segmental bronchus of the upper lobe
• B3: anterior segmental bronchus of the upper lobe

The segment following the bifurcation of the right upper lobe bronchus is called the bronchus intermedius. After approx. 3 cm it divides into the middle lobe and lower lobe bronchus. The middle lobe bronchus branches into:

• B4: lateral segmental bronchus of the middle lobe
• B5: medial segmental bronchus of the middle lobe

The right lower lobe bronchus divides into:

• B6: apical segmental bronchus of the lower lobe
• B7: cardiac segmental bronchus of the lower lobe
• B8: anterobasal segmental bronchus of the lower lobe
• B9: laterobasal segmental bronchus of the lower lobe
• B10: posterobasal segmental bronchus of the lower lobe

Left bronchial tree[Bearbeiten]

The left main bronchus branches into the left upper lobe and lower lobe bronchus. The upper lobe bronchus is divided into the following segmental bronchi:

• B1/B2: apicoposterior segmental bronchus of the upper lobe
• B3: anterior segmental bronchus of the upper lobe
• B4: superior segmental bronchus of the lingula (of the upper lobe)
• B5: inferior segmental bronchus of the lingula (of the upper lobe)

The left lower lobe bronchus is divided into the following segmental bronchi:

• B6: apical segmental bronchus of the lower lobe
• B8: anterobasal segmental bronchus of the lower lobe
• B9: laterobasal segmental bronchus of the lower lobe
• B10: posterobasal segmental bronchus of the lower lobe

Bronchioli[Bearbeiten]

With the loss of the mural cartilage, the corresponding tubes are now called bronchioli. The multi-row ciliated epithelium becomes a single-layered ciliated epithelium. The bronchioli terminales still have a single-layered ciliated epithelium, but the goblet cells have disappeared. In the bronchioli respiratorii, the kinocilia are also missing and only a single-layered isoprismatic epithelium is present.

The opening of the lumen is now only ensured by the tension of elastic fibres. The first section is formed by the bronchioli lobulares (lobular bronchioles). These bronchioles divide into 4-5 bronchioli terminales (terminal bronchioles) and these again into bronchioli respiratorii, which are approx. 1-3.5 mm long and 0.4 mm wide. Several alveolar ducts emerge from each terminal branch of a bronchiolus respiratorius, from which the alveolar sacs arise. The sac-shaped epithelial protrusions in which gas exchange takes place are known as alveoli. Increasing distally, they are found in the bronchioli respiratorii, the alveolar ducts and the alveolar sacs.

Bronchioli terminales, bronchioli respiratorii and alveoli essentially form the basis for the organisation of the lungs into lobules. The primary lobule comprises all structures dependent on a bronchiolus respiratorius (ductus and sacculi alveolares, approx. 10-20 alveoli). It is the smallest functional unit of the lung.

All structures distal to a bronchiolus terminalis (including vessels, nerves and interstitial connective tissue) are referred to as acinus. It contains approx. 400 alveoli. 3-5 acini form a secondary lobule. It contains all structures dependent on a bronchiolus lobularis (approx. 30-50 primary lobules) and is the smallest unit surrounded by connective tissue septa (interlobular septa).

Only individual larger, mainly orthograde bronchi are recognisable in the chest X-ray. In computed tomography (CT), the bronchial walls are visible as tubular structures tapering towards the periphery. Larger bronchioles can only be identified on HR CT. In the case of pathological changes, smaller bronchioles up to the bronchiolus terminalis may also be visible on HR CT.

Inflammation of the bronchial mucosa is known as bronchitis.

• Image source podcast: © Tj Holowaychuk / Unsplash