Figure 1820

Photomicrographs of emphysema and pneumonia, a. This photomicrograph from the lung of an individual with emphysema shows the partial destruction of interalveolar septa, resulting in permanent enlargement of the air spaces. Note that the changes in the lung parenchyma are accompanied by thickening of the wall of the pulmonary vessels (arrows) and the presence of numerous cells within the air spaces. These cells are the alveolar macrophages and are shown at higher magnification in Figure 18.21. X240. b. This photomicrograph from the lung of an individual in the early stages of acute pneumonia (inflammation of the lung). Note that the air spaces are filled with exudate containing white blood cells (mainly neutrophils), red blood cells, and fibrin. The capillaries in the alveolar septum are enlarged and congested

Alveolar Septum

with red blood cells. Pathologists recognize this stage as the red hepatization stage of the pneumonia. At this stage, the affected portion of the lung on gross examination appears red (because of enlarged capillaries), firm (because of the lack of air spaces), and heavy (because of the presence of exudate within the alveoli); the term hepatization stems from the tissue's resemblance to the liver. x240. inset. Part of an alveolus at a higher magnification. Note the enlarged, congested capillary within the alveolar septum. The air space is filled with neutrophils and red blood cells. The lower right corner shows early organization of the intra-alveolar exudate; observe that the developing fibrin network contains entrapped neutrophils and several red blood cells. x420.

Alveolar macrophages remove inhaled particulate matter from the air spaces and red blood cells from the septum

Alveolar macrophages are unusual in that they function both in the connective tissue of the septum and in the air space of the alveolus (Fig. 18.21). In air spaces, they scavenge the surface to remove inhaled particulate matter, e.g., dust and pollen, thus giving them one of their alternate names, dust cells. Alveolar macrophages are derived from blood monocytes and belong to the mononuclear phagocytotic system (page 144). They phagocytize red blood cells that may enter the alveoli in heart failure (see Fig. 18.21). 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 as well as smokers will usually show many alveolar and septal macrophages filled with carbon particles, anthracotic pigment, and birefringent needle like particles of silica. Alveolar macrophages also phago-cytose 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, and other infections or conditions that damage alveolar macrophages can lead to 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 Table 18.1.

TABLE 18.1. Divisions of the Bronchial Tree and Summary of Its Histologic Features goblet ciliated hyaline smooth elastic epithelium ce|!s ce,|s glands cartj|age muscle fibers goblet ciliated hyaline smooth elastic epithelium ce|!s ce,|s glands cartj|age muscle fibers

Alv Oles Pulmonaires Svt

type I alveolar cell

M v FIGURE 18.21

Photomicrograph of alveolar macrophages. This high-magnification photomicrograph shows the structure of the alveolar septum and the lumen of an alveolus containing alveolar macrophages and red blood cells. The cytoplasm of the alveolar macrophages, when they are present in significant numbers, often contains the brown pigment hemosiderin from phagocytosed red blood cells. These hemosiderin-laden macrophages (often called "heart failure cells") are typically found in heart disease, mostly left ventricular failures that cause pulmonary congestion and edema. This results in enlargement of the alveolar capillaries and small hemorrhages into the alveoli. x560.

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