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Learning objectives

After completing this study unit you will be able to:

  1. Describe the structure of the lungs and the sections of the bronchial tree.
  2. Understand the histological differences of the different bronchi and bronchioles.

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The lungs consist of the pulmonary parenchyma, the branches of the bronchial tree and the pulmonary arteries and veins. It is a paired organ that is used for breathing, gas exchange and oxygen supply to the body.

The lungs are divided into different sections. The main bronchi, lobar bronchi and segmental bronchi have a similar structure to the trachea (windpipe). They consist of a mucosal layer with respiratory ciliated epithelium, goblet cells and bronchial glands, a fibromusculocartilagineous layer and an adventitial layer. The goblet cells produce protective mucus and the ciliated epithelium transports foreign substances and dirt particles out of the respiratory tract. The supporting fibromusculocartilagineous layer contains hyaline cartilage plates that decrease in size as the bronchi branch.

The bronchioles become smaller and smaller up to the terminal bronchioles and no longer have any pieces of cartilage, glands or goblet cells. The previous structures only serve to conduct air and gas exchange only begins in the respiratory bronchioli. The respiratory bronchioles have a gapped wall in which alveoli are located, optimal for gas exchange. The alveoli consist of a single-layer alveolar epithelium, formed by type I pneumocytes, and are in close proximity to capillary networks. Type II pneumocytes are also found sporadically in the alveolar epithelium, where they secrete surfactant, which reduces the surface tension of the alveoli and thus prevents the lung from collapsing.

Watch the following video to learn more about the histological appearance of the lung and its different components:

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