Anatomic and Physiologic Aspects of the Pulmonary Parenchyma
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.
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).
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.
