The respiratory system includes the lungs and a branching system of tubes that link the sites of gas exchange with the external environment. Air is moved through the lungs by a ventilating mechanism, consisting of the thoracic cage, intercostal muscles, diaphragm, and elastic components of the lung tissue. The respiratory system is divided anatomically into structures of the upper and lower respiratory tracts (Figure 17–1). Functionally, these structures make up the system's conducting portion, which consists of the nasal cavities, nasopharynx, larynx, trachea, bronchi (Gr. bronchos, windpipe), bronchioles, and terminal bronchioles; and a respiratory portion (where gas exchange takes place), consisting of respiratory bronchioles, alveolar ducts, and alveoli. Alveoli are saclike structures that make up the greater part of the lungs. They are the main sites for the principal function of the lungs—the exchange of O2 and CO2 between inspired air and blood.
Anatomy of the respiratory system.
Anatomically, the respiratory tract has upper and lower parts. Histologically and functionally, the respiratory system has a conducting portion, which consists of all the components that condition air and bring it into the lungs, and a respiratory portion, where gas exchange actually occurs, consisting of respiratory bronchioles, alveolar ducts, and alveoli in the lungs. Portions of two sets of paranasal sinuses are shown here.
The conducting portion serves two main functions: to provide a conduit through which air moves to and from the lungs and to condition the inspired air. To ensure an uninterrupted supply of air, a combination of cartilage, elastic and collagen fibers, and smooth muscle provides the conducting portion with rigid structural support and the necessary flexibility and extensibility.
Most of the conducting portion is lined with ciliated pseudostratified columnar epithelium known as respiratory epithelium (Figure 17–2). This epithelium has at least five cell types, all of which touch the thick basement membrane:
- Ciliated columnar cells (described in Chapter 2) are the most abundant, each with about 300 cilia on its apical surface (Figure 17–2).
Immotile cilia syndrome, a disorder that causes infertility in men and chronic respiratory tract infections in both sexes, is caused by immobility of cilia and flagella induced, in some cases, by deficiency of dynein, a protein normally present in the cilia. Dynein participates in the ciliary movement (see Chapter 2).
- Goblet cells are also abundant in some areas of the respiratory epithelium (Figure 17–2), filled in their apical portions with granules of mucin glycoproteins.
- Brush cells are a much more sparsely scattered and less easily found, columnar cell type, which has a small apical surface bearing a tuft of many short, blunt microvilli (Figure 17–2c). Brush cells express some signal transduction components like ...
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