Respiratory Airways - Anatomy and Histology

Overview
The airways of the lung provide a pathway for bringing external air to the gas exchange surfaces of the lung. They are organized in a tree-like configuration of rapidly branching tubes with progressively smaller diameters. Because these airways do not contain any gas exchange surfaces they are known as “conducting airways”. The term “conducting” is also used because the movement of gas through these areas occurs by bulk flow, similar to air being blown through a straw.
Anatomy
The conducting airways of the lower respiratory tract begin with the trachea which divides into the two main bronchi that serve the right and left lungs, respectively. These in turn split into the lobar bronchi, each of which supplies an entire lobe of the lung. The lobar bronchi then progressively split into shorter and narrower airways until the final, smallest conducting airway known as the terminal bronchiole is reached. On average there are roughly 17 branch points between the trachea and any particular terminal bronchiole.
Histology
  • Overview
    • The histological architecture of all the conducting airways is roughly the same, and is organized as a series of concentric layers. All conducting airways are lined by the respiratory epithelium composed of a layer of respiratory epithelial cells which begin as a ciliated pseudostratified columnar epithelium in the trachea and slowly transition to that of a non-ciliated simple cuboidal epithelium in the terminal bronchioles. Mucin-secreting Goblet Cells are scattered throughout the respiratory epithelium with the greatest density in the trachea. Their density decreases as the airways progressively branch, completely disappearing by the terminal bronchioles.
  • Respiratory Lamina Propria
    • The respiratory epithelium is surrounded by a band of vascular fibro-elastic tissue known as the respiratory lamina propria which also contains clusters of immune cells important for pulmonary immunity.
  • Respiratory Smooth Muscle
    • Deep to the lamina propria lies a band of smooth muscle forming the bronchial smooth muscle. While absent from the trachea, the smooth muscle layer gradually increases in thickness as airways branch and is used regulate the diameter and in turn the resistance of the airways to airflow.
  • Respiratory Submucosa
The submucosa is a loose layer that largely contains serous and mucous glands. This layer is most prominent in the trachea and largely disappears by a few branches after the lobar bronchi.
  • Cartilage
    • A layer of cartilage may invest the submucosa and helps maintain patency of the airways during the breathing cycle. The cartilage layer is most prominent in the trachea and largely disappears a few branches after the lobar bronchi.
Gas Exchange Areas
  • The gas exchange areas are those areas of the lung where external air and blood are brought into such close contact that gas molecules can directly diffuse between them. This occurs in structures termed alveoli that appear as extremely thin, air-containing sacks.
  • Alveoli directly branch off of all airways distal to the terminal bronchiole. As a result, all such airways are termed “respiratory airways” because they can participate in gas exchange. The first such alveoli-containing airways distal to the terminal bronchiole are known as respiratory bronchioles in whose walls can be found isolated alveoli. Typically, however, respiratory bronchioles branch into alveolar ducts that are in turn surrounded by tight, grape-like clusters of multiple alveoli.
  • The surface area of the lung required for gas exchange is enormous and the surface area provided the nearly 500 million alveoli in the average lung is roughly 80 m2, or about that of one side of a tennis court. Because the surface area of the lung expands so rapidly as the alveoli are reached, bulk flow of air comes to a near standstill at this point. In fact, at the threshold of the alveoli, air moves into and out of alveoli solely by random diffusion as described further in Alveolar Air Composition section.
Alveolar Histology
  • Alveoli are the ultimate unit of respiratory gas exchange and are invested with a highly specialized thin wall ideal for this purpose. The alveolar wall consists of a specialized simple epithelium made of highly flat cells known as Type I Pneumocytes that are surrounded by a rich network of pulmonary capillaries, all of which is embedded in a delicate meshwork of elastin and collagen fibers. In many areas, the basement membrane investing the Type I Pneumocytes and capillary endothelium is fused, thus reducing the diffusion barrier for gas exchange. Although Type I Pneumocytes numerically make up slightly less than half of the alveolar cells, they account for over 95% of the entire pulmonary surface area due to their extremely flat and wide geometry.
  • Type II Pneumocytes are cuboidal cells found scattered within the alveolar walls which are responsible for secreting surfactants that assist in reducing lung compliance, our discussion of Lung Compliance.  Additionally, Type II Pneumocytes are thought divide and replenish Type I Pneumocytes in the event of respiratory injury.