Porth's Essentials of Pathophysiology, 4e

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Respiratory Function

U N I T 6

The alveoli, which consist of alveolar ducts and sacs, are the terminal air spaces of the respiratory tract and the actual sites of gas exchange between the air and the blood. The alveolar ducts are elongated airways that have almost no walls at their peripheral boundary. The alveolar sacs are cup-shaped, thin-walled structures that are separated from each other by thin alveolar septa. A single network of capillaries occupies most of the septa, so blood is exposed to air on both sides. There are approximately 300 million alveoli in the adult lung, with a total surface area of approximately 50 to 100 m 2 . Unlike the bronchioles, which are tubes with their own separate walls, the alveoli are intercon- necting spaces that have no separate walls. As a result of this arrangement, there is a continual mixing of air in the alveolar structures. Small holes in the walls of adjacent alveoli, the minute pores of Kohn , contribute to the mixing of air. The alveolar epithelium is composed of two types of cells: type I and type II alveolar cells (Fig. 21-8). Type I alveolar cells are extremely thin squamous cells with a thin cytoplasm and flattened nucleus that occupy about 95% of the surface area of the alveoli. Type I alveolar cells are not capable of regeneration. Type II alveolar cells, which are found interspersed between the type I alveolar cells, are secretory cells that produce the sur- face-active agent called surfactant. In addition to secret- ing surfactant, type II alveolar cells are the progenitor cells for type I cells. Following lung injury, they prolifer- ate and restore both type I and type II alveolar cells. The alveoli also contain brush cells and macrophages. The brush cells, which are few in number, are thought to act as receptors that monitor the air quality of the lungs. The surfactant molecules produced by the type II alveolar cells reduce the surface tension at the air-epithe- lium interface, and they modulate the immune functions of the lung. Recent research has identified four types of surfactant, each with a different molecular structure: surfactant proteins A, B, C, and D. Surfactants B and C serve to reduce the surface tension at the air-epithelium interface and increase lung compliance and ease of lung inflation. Surfactant B is particularly important to the generation of the surface-reducing film that makes lung expansion possible (to be discussed). Surfactants A and D do not reduce surface tension, but contribute to innate immune defenses that protect against pathogens that have entered the lung. They bind pathogens, damage microbial membranes, regulate microbial phagocytosis, and activate or deactivate the inflammatory response (see Chapter 16). The alveolar macrophages , which are present in both the connective tissue of the septum and in the air spaces of the alveolus, are responsible for the removal of offending substances from the alveoli (see Fig. 21-8). In the air spaces, they scavenge the surface to remove inhaled particulate matter, such as dust and pollen. Some macrophages move up the bronchial tree in the mucus and are disposed of by swallowing or coughing when they reach the pharynx. Others enter the septal connective tissue where, filled with phagocytosed mate- rials, they remain for life. Thus, at autopsy, the lungs

convert angiotensin I to angiotensin II, and serve as a reservoir for blood storage. Heparin-producing cells are particularly abundant in the capillaries of the lung, where small clots may be trapped. Pleura The lungs are encased in a thin double-layered closed sac, called the pleura (see Fig. 21-1). The outer pari- etal layer of the pleural sac lines the pulmonary cavities and adheres to the thoracic wall, the mediastinum, and the diaphragm. The inner visceral pleura closely covers the lung and is adherent to all its surfaces. It is con- tinuous with the parietal pleura at the hilus of the lung, where the major bronchus and pulmonary vessels enter and leave the lung. A thin film of serous fluid separates the two layers, allowing them to glide over each other, yet hold together so there is no separation between the lungs and the chest wall. The pleural cavity, or space between the two pleural layers, is also a potential space in which serous fluid or inflammatory exudate can accu- mulate. The term pleural effusion is used to describe an abnormal collection of fluid in the pleural cavity. Respiratory Lobules The gas exchange function of the lung takes place in the respiratory lobules of the lungs. Each lobule, which is the smallest functional unit of the lung, is supplied by a terminal bronchiole, alveoli, and pulmonary blood vessels (Fig. 21-7). Blood enters the lobules through a pulmonary artery and exits through a pulmonary vein. Lymphatic structures surround the lobule and aid in the removal of plasma proteins and other particles from the interstitial spaces.

Air

Lymphatics

Bronchiole

Pulmonary artery

Smooth muscle

Pulmonary vein

Pulmonary capillaries

Pores of Kohn

Alveoli

FIGURE 21-7. Lobule of the lung, showing the bronchial smooth muscle fibers, pulmonary blood vessels, and lymphatics.