8

Chapters 8 through 11 focus on the region of the lung directly involved in gas exchange, often called the pulmonary parenchyma. This region includes the alveolar walls and spaces (with the alveolar-capillary interface) at the level of the alveolar sacs, ducts, and respiratory bronchioles. Although the broad group of disorders involving these structures traditionally has been described under the rubric of interstitial lung disease, the term diffuse parenchymal lung disease is increasingly used and more accurately reflects the breadth of the pathologic involvement.

This chapter provides a description of the normal anatomy of the gas-exchanging region of the lung and some aspects of its normal physiology. Chapter 9 provides an overview of the diffuse parenchymal lung diseases, emphasizing how disturbances in alveolar structure are closely linked with aberrations in function. Chapters 10 and 11 focus on specific disorders, generally subacute or chronic, the main pathologic features of which appear to reside within the alveolar wall. Pneumonia, acute lung injury (acute respiratory distress syndrome), and diseases of the pulmonary vasculature are deliberately excluded because of their different pathologic appearance and are considered separately in other parts of this text.

Although a wide variety of disorders affect the alveolar wall, many of the pathophysiologic features are common to a large number of individual diseases. Knowledge of these general pathophysiologic features and their effects on the normal function of the lung is useful for understanding the consequences of individual disease entities. For specific diseases with special characteristics, a consideration of these individual features is included.

Anatomy

For the lung to function efficiently as a gas-exchanging organ, a large surface area must be available where O₂ can be taken up and CO₂ released. At the alveolar wall, where gas exchange occurs, an extensive network of capillaries coursing through and coming into close contact with alveolar gas facilitates the exchange. In the normal lung, the capillaries are closely apposed to the alveolar lumen, and there is little tissue extraneous to the gas-exchanging process (Fig. 8-1).

Figure 8-1 Photomicrograph of alveolar walls shows a normal thin, lacy appearance. At top of photo is bronchial lumen, lined by bronchial epithelial cells (arrow). Peribronchial tissue lies between bronchial epithelium and alveolar walls. (Courtesy Dr. Earl Kasdon.)

The surface of the alveolar walls (the region bordering the alveolar lumen) is lined by a continuous layer of epithelial cells. Two different types of these lining epithelial cells, called type I and type II cells, can be identified. Type I cells are less numerous than type II cells but account for a much larger surface area. They have impressively long and delicate cytoplasmic extensions that line more than 95% of the alveolar surface (Fig. 8-2). Type I cells function as a barrier preventing free movement of material, such as fluid, from the alveolar wall into the alveolar lumen. Although they have few cytoplasmic organelles, increasing evidence indicates that type I cells play an important role in the regulation of ion and fluid balance in the lung, in part because they cover such a large part of the alveolar surface area.

Figure 8-2 Schematic diagram of normal alveolar structure. Type I and type II epithelial cells line alveolar wall. Type I cells are relatively flat and characterized by long cytoplasmic processes. Type II cells are cuboidal. Two capillaries are shown. A = Alveolar space; C = capillary endothelial cells; IS = interstitial space (relatively acellular region of the alveolar wall); L = type II cell cytoplasmic lamellar bodies (source of surfactant); RBC = erythrocytes in capillary lumen.

Type I alveolar epithelial cells have long cytoplasmic processes that line almost the entire alveolar surface.

The second lining epithelial cell is the type II cell. Type II cells have three well-defined functions: synthesis of surfactant, alveolar epithelial repair, and ion and fluid transport. In contrast to type I cells, type II epithelial cells have a cuboidal shape and often bulge into the alveolar lumen. Because they do not have long cytoplasmic extensions, type II cells cover less than 5% of the alveolar surface. The more numerous type II cells have many cytoplasmic organelles (mitochondria, rough endoplasmic reticulum, Golgi apparatus), which relate to their important synthetic role.

Type II cells produce surfactant, are important in the reparative process for type I cells, and are active in ion and fluid transport.

The primary product of the type II cells is surfactant. Specific inclusion bodies within the type II cells, termed lamellar inclusions

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. Tuberculosis and Nontuberculous Mycobacteria
2. Anatomic and Physiologic Aspects of Airways
3. Arterial Blood Gases: Guidelines for Interpretation and Sample Problems
4. Sample Problems Using Respiratory Equations

Stay updated, free articles. Join our Telegram channel

Join