Histology of the respiratory system
The respiratory system consists of several organs working together to facilitate the exchange of oxygen and carbon dioxide between the air and the bloodstream, supporting cellular respiration. The respiratory tract is divided into two sections; the upper and lower respiratory tract.
The upper respiratory tract includes the paranasal sinuses, nasal cavity, pharynx and the segment of the larynx superior to the vocal cords. The lower respiratory tract includes the segment of the larynx inferior to the vocal cords, the trachea, lungs, bronchi and bronchioles.
Here, we will describe the microscopic appearance of the trachea and lungs.
The trachea (windpipe) connects the larynx with the two main bronchi (principal bronchi) and represents the beginning of the lower respiratory tract.
The tracheal wall consists of several layers. The inner mucosa is lined with respiratory ciliated epithelium and goblet cells. The middle fibromusculocartilagineous layer is composed of three components. Many elastic fibers enable the trachea to adjust its size depending on breathing. A total of 16 to 20 horseshoe-shaped hyaline cartilage rings support the trachea and keep it open. These cartilage rings are open towards the back, where the trachealis muscle connects the rings. The trachea is embedded in its surroundings via the loose connective tissue of the outer adventitia.
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The lungs consist of the pulmonary parenchyma, the branches of the bronchial tree and the pulmonary arteries and veins. The main bronchi, lobar bronchi and segmental bronchi have a similar structure to the trachea. They consist of a mucosal layer with respiratory ciliated epithelium, goblet cells and bronchial glands, a fibromusculocartilagineous layer and an adventitial layer.
The bronchioles become smaller and smaller up to the terminal bronchioles and no longer have any pieces of cartilage, glands or goblet cells. The respiratory bronchioles have a gapped wall in which alveoli are located, optimal for gas exchange.
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