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The main function of the respiratory system is to supply the body with O2 from the air and to remove CO2 from the bloodstream via a system of tubes or airways. Functionally, the respiratory system can be divided into the following two divisions, which differ significantly in structure and function:

  • Conducting division. Consists of a series of airways located both outside and inside the lung. Inspired air is warmed, moistened, and cleaned in the conducting airways before entering the respiratory airways. Conducting airways do not directly participate in gas exchange.

  • Respiratory division. Consists of airways located entirely within the lungs. Respiratory airways are specialized for gas exchange and are distinguished by the presence of alveoli, saclike structures where the exchange of gasses occurs between inhaled air and blood in capillaries.

Airways in the lung branch repeatedly, producing progressively smaller conduits, described collectively as a bronchial tree.

The lungs receive blood from two sources: the pulmonary arteries deliver deoxygenated blood to capillaries in the alveoli for oxygenation, and the bronchial arteries supply nutrients and oxygenated blood to the lung tissue itself.

O2 from the air is exchanged for CO2 from the blood across the blood-air barrier, a specialized, trilayered region of the alveolus composed of the alveolar and capillary endothelia and their fused basal laminae.



Air from the external environment travels through a series of conducting airways (nasal cavity, nasopharynx, larynx, trachea, and extrapulmonary primary bronchi) before entering the lungs. Within the lungs, air continues through still more conducting airways, which branch repeatedly into progressively smaller airways (intrapulmonary bronchi, bronchioles, and terminal bronchioles) (Figure 10-1A). As air leaves the terminal bronchioles, it enters the respiratory airways, where O2 is exchanged for CO2. No gas exchange occurs as air is transported through conducting airways, but the inspired air is warmed, moistened, and cleaned (i.e., conditioned) along this journey, which helps to prevent damage to delicate alveoli. This chapter will focus on the functional histology of the trachea and lung.

Figure 10-1:

Organization of conducting airways and structure of the trachea. A. Airways in the conducting division: the nasal cavity, nasopharynx, larynx, trachea, bronchi, and bronchioles. The walls of the trachea and large bronchi are supported by rings or plates of cartilage, respectively (blue). Terminal bronchioles, which branch to produce respiratory bronchioles, are the most distal conducting airways. B. Cells in respiratory epithelium: mucous goblet, ciliated columnar, basal (short), brush, and small granule cells. Note that brush cells have afferent nerve endings on their basal surface. C. Section through the respiratory epithelium (pseudostratified ciliated columnar epithelium) lining the trachea. Respiratory epithelium is composed of the five types of cells illustrated schematically ...

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