Purpose of preparation: To visualize the blood-air barrier in the alveoli.
Introduction
Lungs from a rat and from a pig were used for this preparation. The photo below[1] clearly shows the location of the lung in the mouse. Photo 'A' shows the mouse with skin removed. Photo 'B' has part of the chest (rib cage) removed and the right lung lobe is clearly visible. Mice are widely used in histology (doctrine of healthy tissues) and pathology (doctrine of diseased tissues) because they are easy to obtain and their tissue is almost identical to that of humans.
The photo below[1] shows a dissected lung. 'A' is a normal lung. At the bottom, the heart is visible. 'B' is the same lung but inflated with air. Here the spongy structure is already clearly recognizable.
Anatomy of the lungs[2]
In the lungs (pulmones), located in the pleural cavity, gas exchange takes place between inhaled air and blood. The inhaled air enters the lungs through the bronchi (airways). The lungs consist of several structures connected by connective tissue and overlaid by the pleura (lung membrane). The bronchi branch out at the top of the lungs to the left and right of the trachea. In the lung, the bronchus branches into secondary and tertiary bronchioles and these in turn branch into bronchioli. The bronchioli branch further into alveoli.
Physiology of the lungs[2]
The lungs belong to the respiratory system. The main function of respiration is gas exchange, where oxygen (O2) from the outside air is taken into the blood and carbon dioxide (CO2) from the venous blood is released to the outside. This process is called external respiration. The transport of O2 from the arterial blood into the cell and the release of CO2 from the cell into the blood is called internal respiration. Humans need their own transport system for gas exchange, the circulatory system. Through the circulatory system, internal and external respiration are linked. The oxygen, inhaled through the trachea, bronchi and bronchioli, enters the alveoli and diffuses from there into the blood. The wall thickness of the alveoli is approximately 1-2 µm. The oxygenated blood is carried to the pulmonary vein and then to the left half of the heart after which it is pumped into the aorta. Through the arteries, the blood travels through the body, to the capillaries. There O2 is delivered to the tissue cells and the CO2 is taken away again. This now venous blood enters the right half of the heart via the veins and then into the lungs, where there are also capillaries. These are in close contact with the alveoli. Here, too, gas exchange takes place by diffusion. That is, CO2 enters the alveoli and O2 is absorbed from them into the blood. This in turn makes arterial (oxygenated) blood available, which is pumped back into the aorta via the left heart half.
Preparation Description
To obtain a good overview of the lung tissue, the largest possible section is desirable. A paraffin-embedded block of tissue is preferred.
However, in order to be able to see the details of this very fragile lung tissue, it is necessary to cut the section even thinner. Therefore, a plastic block (Technovit 7100) was also made, with which a section thickness of 1µm can be easily achieved.
Cells of the alveoli[3]
So it is in the alveoli (lung alveoli) that the actual gas exchange takes place. The red blood cells (erythrocytes) flow through very fine capillaries (capillaries) located in the alveoli. Where the capillaries make direct contact with the air in the alveoli, the exchange takes place.
Red blood cells consist mainly of the protein hemoglobin. Hemoglobin is an iron-containing protein molecule that is red in color and can bind oxygen and carbon dioxide molecules.
The further cellular components of the alveoli are type I and type II pneumocytes, fibrocytes, and macrophages. The cytoplasm of the pneumocytes forms the walls of the alveoli. In the parts where no capillaries run, there are fine collagen and elastic fibers. These fibers provide some strength and prevent the delicate alveoli from collapsing. In the inhaled air there are also all kinds of contaminants such as dust and soot particles, plant pollen and, for example, bacteria. The coarse particles are already caught in the nose and bronchi, but the very fine particles also end up in the alveoli and can form inflammations there. In order to clean up and digest these fine particles, macrophages are found in the alveoli. Macrophages differentiate from monocytes that are in the bloodstream. Monocytes enter the alveoli via the pulmonary arteries (artery) and crawl through the wall of the alveoli into the air space. There they move along the wall of the alveoli and through the air space into the alveoli. Thus, they are the clearing cells of our body. In a histological section they are clearly visible and easily distinguishable from other cells. Macrophages that have done their job are removed from the lungs by coughing. We usually swallow the coughing sputum and it is further digested in the digestive system.
Source reference:
[1] 'A color atlas of sectional anatomy of the mouse’, pag. 26, ISBN: 4-900659-58-4 .
[2] Document: ‘Help, een longembolie! Wat nu? Het zorgtraject voor de patiënt met een longembolie’ van de hbo-kennisbank. https://hbo-kennisbank.nl/
[3] Prof. Dr. med. Ulrich Welsch (2006, auflage 2), Lehrbuch Histologie, München. Uitgeverij Elsevier GmbH, Urban & Fisher. pag. 323, 8.2 Alveolarraum, ISBN: 978-3-437-44430-2