Respiratory System Histology
The respiratory system consists of a system of tubes (the conducting zone) that allows the exchange of gases between the atmosphere and the lungs and membranes in the lungs (the respiratory zone) that promote the exchange of respiratory gases.
Conducting Zone Histology: The tubes in this zone are composed of three layers or tunics: an inner tunica intima lined with epithelium, an intermediate tunica media providing the structural support of the tube, and an outer tunica externa connecting the tube to surrounding tissues.
The tubes of the conducting zone warm, moisten, and filter inhaled air and provide an open pathway for air exchange. Warming, moistening and filtering inhaled air reduces heat and water loss with respiration and reduces the entry of particulate matter. Since movement of air into and out of the lungs is caused by changes in intrathoracic pressure, it is essential that the airway remain open (patent) at all times.
Respiratory Zone Histology: Respiratory gas exchange occurs by simple diffusion. The tissues in this zone increase the efficiency of this process. The smallest tubes (bronchioles and alveolar ducts) terminate in countless small air sacs (alveoli), where gas exchange occurs.
Slide 1 – Trachea – The trachea provides an open airway that also warms, moistens and filters incoming air. This low magnification view of a section through the trachea shows the Tunica Intima (A), Tunica Media (B) and a portion of the Tunica Externa (C).
In cross-section, you can view the three layers of the trachea. Closest to the lumen and labeled A in the image above, the tunica intima provides the lining to the trachea and is also called the mucosa and sub-mucosa. The lumen is lined with ciliated, pseudostratified columnar epithelium. Also visible in the tunica intima are an abundance of blood vessels (just deep to the epithelial lining) and close to the border with the next layer there are many adipocytes. The Tunica media, labeled B in the image, contains the readily identifiable hyaline cartilage. The tunica externa, labeled C above, is also sometimes called the adventitia. Note in this section the large amount of adipose in the tunica externa.
Slide 2 – Tunica interna
The second slide is a higher-magnification view of the tunica intima, showing that it can be subdivided into a mucosa (at the luminal surface, A) and a submucosa (the area below A).
At higher magnification you can see a clear definition between the mucosa and submucosa. The mucosal layer is defined by ciliated, pseudostratified columnar epithelium. While not seen in this image, there are frequently goblet cells that secrete mucous onto the luminal surface. Together, the mucous works to trap particles in inhaled air while the cilia move the mucous out of the trachea and into the pharynx. In this cross-section, there are two large blood vessels visible among the connective tissues. These blood vessels provide heat which warms the air causing its expansion before it arrives in the lungs.
Slide 3: Lung, low power
This slide shows a low-magnification view of the lung. Features shown in this view include smaller airways, alveolar sacs and blood vessels.
Lung tissue contains a great deal of open space. At low power you can see structures like airways (letter A), alveolar sacs and ducts (letter B), and blood vessels (letter C). The alveoli make up the large majority of the tissue in the lungs. They create a large surface area for gas exchange.
Slide 4: Bronchus
This slide shows a bronchus, one of the smaller branches of the pulmonary airway. Its lining is composed of non-ciliated columnar epithelium and areas of hyaline cartilage can be seen in its tunica media.
This image shows a small bronchus. There are small clusters of hyaline cartilage surrounding this airway. The cartilage helps to keep the lumen patent to allow for airflow.
Slide 5: Bronchiole
This slide shows two vessels, a bronchiole and venule, contained within the same connective tissue sheath. This provides an indication of the histological organization of the lung: specific regions are supplied by common airways and blood vessels.
By the time air arrives at a bronchiole, there is no more cartilage. What we can observe now is smooth muscle surrounding the bronchiole. This muscle can contract or relax to influence the movement of air into the smaller passageways in the lungs.
Slide 6: Lung, high power
The final slide tests your understanding of lung structure.
B and C represent Bronchi. As you traverse from B towards A in the image, you can see the changes in the mucosa that help to identify the changes from bronchus to bronchiole. Between B and C you can see the small light purple area that is cartilage, this indicates the bronchus level. By the time we arrive at A there is no sign of the cartilage, no you would be in a bronchiole.