Atlas of plant and animal histology

The trachea branches into the primary bronchi, which carry air to each lung. Within each lung, the primary bronchi divide into secondary bronchi. In humans, the number of secondary bronchi corresponds to the number of lobes in each lung: two on the left and three on the right. The histological structure of the bronchi is similar to that of the trachea. However, as they branch out, the cartilage gradually disappears: the semilunar bodies become scattered plates and then disappear. In addition, smooth muscle becomes more prominent and forms a more visible layer separating the mucosa from the submucosa. The epithelium, which is pseudostratified ciliated columnar in the larger bronchi, as in the trachea, transforms into pseudostratified cuboidal epithelium, with progressively decreasing thickness. It contains three main types of cells: ciliated, mucous, and basal. As the cartilage, mucous glands, and goblet cells disappear, and the smooth muscle becomes less dense yet remains visible, the resulting ducts have a diameter of approximately 1 mm. These ducts are called bronchioles.

Bronchioles lack both cartilage and glands. While the larger brochioles have a ciliated pseudostratified columnar epithelium, the epithelium changes to simple columnar and subsequently to simple cuboidal as their diameter decreases. The cell types include ciliated secretory club cells, neuroendocrine cells, and mucous cells, which are the most abundant. Each bronchiole has a clearly visible layer of smooth muscle. The larger bronchioles are called terminal bronchioles, and the smaller ones are respiratory bronchioles, whose walls have very thin expansions that form cavities called pulmonary alveoli (Figures 1 and 2).

Alveoli are the sites where gas exchange between the blood and the air takes place. They are cavities lined with a simple squamous epithelium that greatly increases the surface area in contact with the air. The alveolar epithelium contains squamous type I pneumocytes and cuboidal type II pneumocytes. Within the alveoli, a subpopulation of type II pneumocytes is responsible for regenerating both type I and type II pneumocytes. Type II pneumocytes in the pulmonary alveoli release a surfactant that help to prevents the collapse of the pulmonary alveoli. This substance is primarily composed of lipids along with some proteins. Each alveolus is surrounded by a network of capillaries so that the distance between red blood cells and the air is very small, facilitating gas exchange. In humans, it is estimated that there are between 150 and 250 million alveoli. Sometimes, several alveoli form alveolar ducts, which are expansions of the respiratory bronchioles consisting of numerous associated alveoli. At the ends of the respiratory bronchioles, alveolar sacs often form open spaces into which numerous alveoli open.

The lower respiratory tract (trachea, bronchi, bronchioles, and terminal air spaces) is lined with mucus and a variety of microorganisms, collectively referred to as the microbiota. The microbiota of the respiratory tract is sparser and more variable than that of other mucosal surfaces, such as the digestive tract, because the airways have mechanisms to clear and renew this mucus layer, thanks to the cilia in the epithelial layers, coughing, and a more active innate immune system. A correlation has been found between specific microbiota and certain respiratory diseases. In some cases, a correlation has also been observed between disease progression and an altered microbiota (dysbiosis). However, due to the limited number of studies, it has not been determined whether dysbiosis is a cause or a consequence of these conditions.