(Fig. 19.13). They have a well-developed basal rER, a lateral or supranuclear Golgi apparatus, secretory granules that stain for protein, and numerous cisternae of sER in the apical cytoplasm. Clara cells secrete a surface-active agent, a lipoprotein that prevents luminal adhesion should the wall of the airway collapse on itself, particularly during expiration. In addition, Clara cells produce a 16-kilodalton protein known as Clara cell secretory protein (CC16), which is an abundant component of the airway secretion.
Chronic lung diseases such as chronic obstructive pulmonary disease (COPD) and asthma are associated with changes in the abundance of CC16 in airway ﬂuid and serum. CC16 is used as a measurable pulmonary marker in bronchoalveolar lavage ﬂuid and serum. Secretion of CC16 into the bronchial tree decreases during lung injury (because of damage to the Clara cells), whereas serum levels of CC16 may increase because of leakage across the air–blood barrier.
FIGURE 19.13 • Diagram of a Clara cell between bronchiolar ciliated epithelial cells. The nucleus is in a basal location. Rough-surfaced endoplasmic reticulum (rER), a Golgi apparatus, and mitochondria are chiefly in basal and paranuclear locations of the cell. Smooth-surfaced endoplasmic reticulum (sER) and secretory vesicles are chiefly in the apical cytoplasm. One of the secretory vesicles is shown discharging its contents onto the surface of the cell.
Respiratory bronchioles are the first part of the bronchial tree that allows gas exchange.
Respiratory bronchioles constitute a transitional zone in the respiratory system; they are involved in both air conduction and gas exchange. They have a narrow diameter and are lined by cuboidal epithelium. The epithelium of the initial segments of the respiratory bronchioles contains both ciliated cells and Clara cells (see Fig. 19.12). Distally, Clara cells predominate. Occasional brush cells and dense-core granule cells are also present along the length of the respiratory bronchiole. Scattered, thin-walled outpocketings, alveoli, extend from the lumen of the respiratory bronchioles (see Fig. 19.11). Alveoli are the sites at which air leaves and enters the bron chiole to allow gas exchange.
Alveoli are the site of gas exchange.
The surface area available for gas exchange is increased by the lung alveoli. Alveoli are the terminal air spaces of the respiratory system and are the actual sites of gas exchange between the air and the blood. Each alveolus is surrounded by a network of capillaries that brings blood into close proximity to inhaled air inside the alveolus. About 150 to 250 million alveoli are found in each adult lung; their combined internal surface area is approximately 75 m2, roughly the size of a tennis court. Each alveolus is a thin-walled polyhedral
FIGURE 19.14 • Photomicrograph showing an alveolar sac with adjacent alveoli. This photomicrograph shows the terminal components of the respiratory system, namely, the alveolar sac (AS) and the surrounding alveoli (A). The alveoli are surrounded and separated from one another by a thin connective tissue layer, the interalveolar septa, containing blood capillaries. On the right is the lung surface, which is covered by visceral pleura containing simple squamous epithelium and an underlying layer of connective tissue. 360.
chamber approximately 0.2 mm in diameter that is confluent with an alveolar sac (Fig. 19.14).  Alveolar ducts are elongated airways that have almost no walls, only alveoli, as their peripheral boundary. Rings of smooth muscle are present in the knoblike interalveolar septa (see the next paragraph). Alveolar sacs are spaces surrounded by clusters of alveoli.
The surrounding alveoli open into these spaces.