Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 13

The trachea has cartilaginous horseshoe‐shaped ‘rings’ supporting its anterior and lateral walls. The posterior wall is flaccid and bulges forward during coughing, for example. This results in narrowing of the lumen, which increases the shearing force from the moving air on the mucus lying on the tracheal walls.

The trachea divides into the right and left main bronchi at the level of the sternal angle (angle of Louis). The left main bronchus is longer than the right and leaves the trachea at a more abrupt angle. The right main bronchus is more directly in line with the trachea, so that inhaled material tends to enter the right lung more readily than the left.

The main bronchi divide into lobar bronchi (upper, middle and lower on the right; upper and lower on the left) and then segmental bronchi, as shown in Fig. 1.1. The position of the lungs in relation to external landmarks is shown in Fig. 1.2. Bronchi are airways with cartilage in their walls, and there are about 10 divisions of bronchi beyond the tracheal bifurcation. Smaller airways without cartilage in their walls are referred to as bronchioles. Respiratory bronchioles are peripheral bronchioles with alveoli in their walls. Bronchioles immediately proximal to alveoli are known as terminal bronchioles. In the bronchi, smooth muscle is arranged in a spiral fashion internal to the cartilaginous plates. The muscle coat becomes more complete distally as the cartilaginous plates become more fragmentary.

The epithelial lining is ciliated and includes goblet cells. The cilia beat with a whip‐like action, and waves of contraction pass in an organised fashion from cell to cell so that material trapped in the sticky mucus layer above the cilia is moved upwards and out of the lung. This ‘mucociliary escalator’ is an important part of the lung’s defences. Larger bronchi also have acinar mucus‐secreting glands in the submucosa; these are hypertrophied in chronic bronchitis.

Alveoli are about 0.1– 0.2 mm in diameter and are lined by a thin layer of cells, of which there are two types: type I pneumocytes have flattened processes that extend to cover most of the internal surface of the alveoli; type II pneumocytes are less numerous and contain lamellated structures, which are concerned with the production of surfactant (Fig. 1.3). There is a potential space between the alveolar cells and the capillary basement membrane, which is only apparent in disease states, when it may contain fluid, fibrous tissue or a cellular infiltrate.