Special Histology and Embryology: Practical Course - V. K. Naphanyuk 2001

Respiratory System
Airways
Lungs

The Lungs (pulmonum) occupy the greater part of the thoracic cavity and constantly change their shape depending on the phase of respiration.

The surface of the lung is covered by a serous membrane known as the visceral Pleura.

The lung consists of a system of air-conducting pathways—the Bronchi (bronchial tree)—and a system of pulmonary sacs (alveoli), which function as the proper respiratory compartments of the Respiratory system.

The bronchial tree (arbor bronchialis) has the following Structure:

— main bronchi (right, left);

— extrapulmonary lobar bronchi (large 1st-order bronchi);

— zonal extrapulmonary bronchi (four in each lung), large 2nd-order bronchi;

— intrapulmonary segmental bronchi (ten in each lung), large;

— subsegmental bronchi (3rd–5th-order), medium bronchi;

— small bronchi;

— terminal bronchioles.

Although The structure of the bronchi varies throughout the bronchial tree, they share common features. The bronchial wall consists of the following layers:

1) a mucosa lined with ciliated epithelium; its thickness gradually decreases due to cellular modification from high columnar to low cuboidal Cells. In addition to ciliated, goblet, endocrine, and basal cells, secretory, brush (bordered), and non-ciliated cells are found in the distal regions;

2) a lamina propria of the bronchial mucosa, which is rich in longitudinally oriented elastic fibers;

3) a submucosal Connective Tissue layer containing the terminal portions of mixed protein-mucous glands;

4) a fibrocartilaginous layer characterized by a gradual transition (as caliber decreases) from open Cartilage rings to cartilage plates and islands of cartilage tissue. It is absent in small-caliber bronchi;

5) an adventitial layer formed by Fibrous connective tissue.

Large-caliber bronchi (from 15 to 5 mm) possess all four layers and form folds. The mucosa is lined with pseudostratified ciliated epithelium. The muscularis mucosae is well developed. Large-caliber bronchi are characterized by the presence of glands and hyaline cartilage plates in the fibrocartilaginous layer.

Medium-caliber bronchi (from 5 to 2 mm) are distinguished by a reduced height of the epithelial layer and thickness of the mucosa, the presence of glands, and smaller sizes of cartilage islands.

Small-caliber bronchi (from 2 to 1 mm) are characterized by marked folding of the mucosa. The epithelium is double-layered and transitions to single-layered further down; cartilage and glands are absent, and the muscularis mucosae is maximally developed relative to the total wall thickness. The absence of cartilage combined with well-developed, circularly arranged muscular tissue serves as the morphological basis for complete luminal closure occurring in various spastic conditions.

Terminal bronchioles (diameter about 0.5 mm): the mucosa is lined with a simple cuboidal ciliated epithelium containing bordered, secretory, and non-bordered cells. The lamina propria of these bronchioles contains longitudinal elastic fibers interspersed with isolated bundles of smooth Muscle cells. Consequently, the bronchioles easily stretch during inhalation and return to their initial position during exhalation.