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FIGURE 4 The airway tree consists of two functional zones: The conducting zone includes the first 16 orders of branching and does not participate in gas exchange; the respiratory zone includes the last 7 orders of bronchial branching, which have alveoli that are responsible for gas exchange. (After Weibel, Morphometry of the human lung. Berlin: Springer, 1963.)

trachea divides into two slightly smaller main stem (or primary) bronchi to the right and left lungs. Figure 4 shows the types of airways in the 23 orders of bronchial branching that occur in a human lung. Note that all bronchi beyond the 16th generation contain some alveoli, so the number of alveoli (300 million) is even greater than the number predicted for one at the end of each branch in a tree with 23 bifurcations (223; 8.4 million). Although each airway generation is smaller, their number is increasing exponentially so the total cross-sectional area of the airways increases dramatically with each generation. The total cross-sectional area of the first 10 generations of airways is relatively constant at a few square centimeters. However, by the 17th generation, the total cross-sectional area of the airways is more than 200 cm2.

This dramatic increase in cross-sectional area has an important functional consequence for gas exchange. Once the total area is large enough, and the distances to the gas exchanging airways are short enough, the forward velocity of bulk flow decreases and diffusion becomes an effective mechanism for gas transport. Hence, diffusion is the primary mechanism of gas transport in airways after the terminal bronchioles. The airways and alveoli served by a terminal bronchiole

yConducting I zone y Respiratory I zone

FIGURE 4 The airway tree consists of two functional zones: The conducting zone includes the first 16 orders of branching and does not participate in gas exchange; the respiratory zone includes the last 7 orders of bronchial branching, which have alveoli that are responsible for gas exchange. (After Weibel, Morphometry of the human lung. Berlin: Springer, 1963.)

Alveolar sac

Alveolar sac are called an acinus (Fig. 3B). There are about 150,000 acini, with a path length for gas transport of about 5 mm in human lungs. The acinus is the functional unit of gas exchange because diffusion is so effective at mixing and equilibrating gas in it.

Figure 4 shows how the airways can be divided into a conducting zone and a respiratory zone. Airways in the respiratory zone are structurally stronger and function to distribute air by convection to peripheral airways. There is no significant uptake of O2 or elimination of CO2 across the walls of conducting airways. The respiratory zone consists of peripheral airways that function to equilibrate blood with lung gases, and this zone contains most of the lung volume. Airways and alveoli in the respiratory zone can be extremely delicate to facilitate diffusion from lung gas to blood because they are not subject to the larger stresses associated with ventilation and bulk flow.

Figure 5 shows how the structure and cellular biology of the airways change between the conducting and respiratory zones. These changes involve three principles of organization: (1) The airways form a barrier between gas and the body consisting of layers of epithelium, interstitium, and endothelium; (2) airway cells are differentiated according to their hierarchy in the tree of bronchial branching; and (3) a mosaic of airway cell types changes between the conducting and airway zones.

In the trachea and large bronchi, the airway walls are very thick and include cartilage and smooth muscle to provide structural support. The airway epithelial cells form a confluent (or continuous) sheet that is anchored by basement membrane and covers the entire internal surface of the airways, from the trachea to the alveoli. Tight junctions between epithelial cells limit and control molecular transport across this barrier. The mosaic of columnar epithelial cells in the bronchi includes ciliated cells and superficial Goblet cells that secrete mucous glands. Other mucous cells form invaginations in the airway that function as submucosal secretory glands. The surface cilia move secreted mucus toward the mouth to remove foreign objects from the airways. Neuroendocrine cells that secrete mediators into the bloodstream are relatively rare. Endothelial cells in the bronchi are part of the systemic circulation, which supplies the nutrient demands of large airways by the bronchial circulation.

In the transition zone, there is no cartilage, but helical bands of bronchial smooth muscle surround the airways. This smooth muscle controls bronchial caliber, airway resistance, and the local distribution of ventilatory gas flow. Ciliated epithelial cells are smaller and cuboidal in the transition zone. Goblet cells on the airway surface secrete mucus to be moved up and out of the airways by the ciliated epithelial cells. Clara cells are another type

gland

FIGURE 5 Cellular structure of airways showing how the layering and cellular forms at different levels from the trachea (left) to the alveoli (right). Both nonciliated epithelial cells (goblet cells) and submucosal glands secrete mucus in the bronchi. Secretory Clara cells occur in the bronchioles. The alveoli do not contain cilia but do contain type II epithelial cells that secrete surfactant. The epithelium (EP), basement membrane (BM), and interstitium (IN) are very thin in the alveoli to allow effective diffusion of O2 from alveolar gas through pulmonary capillary endothelium and into blood. (After Burri and Weibel, Rontgendiagnostik der lunge, Huber, 1973.)

gland

FIGURE 5 Cellular structure of airways showing how the layering and cellular forms at different levels from the trachea (left) to the alveoli (right). Both nonciliated epithelial cells (goblet cells) and submucosal glands secrete mucus in the bronchi. Secretory Clara cells occur in the bronchioles. The alveoli do not contain cilia but do contain type II epithelial cells that secrete surfactant. The epithelium (EP), basement membrane (BM), and interstitium (IN) are very thin in the alveoli to allow effective diffusion of O2 from alveolar gas through pulmonary capillary endothelium and into blood. (After Burri and Weibel, Rontgendiagnostik der lunge, Huber, 1973.)

of secretory epithelial cell in the small bronchi but their function is not known.

In the respiratory zone, there is no smooth muscle and little connective tissue. Type I alveolar epithelial cells are squamous (or flattened), with long and thin cyto-plasmic extensions and no cilia so the epithelial barrier to gas exchange is as thin as possible. The interstitial layer and pulmonary capillary endothelial cells are also very thin, reducing the barrier to gas exchange. Type II alveolar epithelial cells are specialized cells that synthesize and secrete surfactant, a substance that influences the mechanical properties of the lung, as described in Chapter 19. Type II cells are also precursors for type I cells and important for repair in lung injury.

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