space of the alveolus (Fig. 19.20). In air spaces, they scavenge the surface to remove inhaled particulate matter (e.g., dust and pollen), thus giving them one of their alternative names, dust cells. Alveolar macrophages are derived from blood monocytes and belong to the mononuclear phagocytotic system (see page 185). They phagocytize red blood cells that may enter the alveoli in heart failure (see Fig. 19.20). Some engorged macrophages pass up the bronchial tree in the mucus and are disposed of by swallowing or expectoration when they reach the pharynx. Other macrophages return to or remain in the septal connective tissue, where, filled with accumulated phagocytized material, they may remain for much of an individual’s life. Thus, at autopsy, the lungs of urban dwellers and smokers usually show many alveolar and septal macrophages filled with carbon particles, anthracotic pigment, and birefringent needle-like particles of silica. Alveolar macrophages also phagocytose infectious organisms such as Mycobacterium tuberculosis, which can be recognized in the cells in appropriately stained specimens. These bacilli are not digested by macrophages, however, other infections or conditions that damage alveolar macrophages can cause release of the bacteria and recurrent tuberculosis.
Collateral air circulation through alveolar pores allows air to pass between alveoli.
Scanning electron microscopic studies of alveolar structure show openings in the interalveolar septa that allow circulation of air from one alveolus to another. These alveolar pores (of Kohn) can be of great significance in some pathologic conditions in which obstructive lung disease blocks the normal pathway of air to the alveoli. The alveoli distal to the blockage may continue to be aerated, via the pores, from an adjacent lobule or acinus.
A basic summary of information related to the respiratory system is included in Figure 19.21.
surfactant discharged into lumen of alveolus
FIGURE 19.17 • Diagram of a type II alveolar cell and electron micrograph of lamellar bodies. a. Surfactant is an oily mixture of proteins, phospholipids, and neutral lipids that are synthesized in the rER from precursors in the blood. These precursors are glucose, fatty acids, choline, and amino acids. The protein constituents of surfactant are produced in the rER and stored in the cytoplasm within lamellar bodies, which are discharged into the lumen of the alveolus. With the aid of surfactant protein, surfactant is distributed, on the surface of epithelial cells lining the alveolus, as a thin film that reduces the surface tension. b. Higher-magnification electron micrograph showing the typical lamellar pattern of the secretory vesicles of type II alveolar cells. These vesicles contain the pulmonary surfactant precursor proteins. 38,000. (Courtesy of Dr. A. Mercuri.)
FIGURE 19.18 • Diagram of the interalveolar septum. This diagram shows the thick and thin portions of the interalveolar septum. The thin portion forms the air–blood barrier and is responsible for most of the gas exchange that occurs in the lung. Arrows indicate the direction of CO2 and O2 exchange between the alveolar air space and the blood. The thick portion of the interalveolar septum plays an important role in fluid distribution and its dynamics. It contains connective tissue cells. Note the macrophage in the thick portion that extends its processes into the lumen of the alveolus.