Further histologic changes: This photo from another area of the lung shows two types of abnormally enlarged air spaces. The pleural surface is to the right. What is the difference between the space marked with one asterisk and that marked by two asterisks? Answer

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Answer: The single asterisk marks an emphysematous space in the lung. If it is larger than 1 cm, it is called a bulla. Note that the histologic hallmark of emphysema is apparent free-floating alveolar walls. This artifact of sectioning signifies that large holes have developed in the three-dimensional alveolar walls. The two asterisks mark an intrapleural bleb. How does it develop? Answer

Note that the free alveolar walls of emphysema do not show any reactive fibrosis where breaks have occurred.

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Answer: Blebs form by dissection of trapped air into the interstitium. It may dissect along bronchovascular bundles centrally to give pneumomediastinum, or along interlobular septa to the pleura to form a bleb. Rupture causes pneumothorax, which is characteristically accompanied by an influx of eosinophils, giving an eosinophilic pleuritis.

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