It consists of 3 to 10 acini enclosed by an interlobular septum of connective tissue and measures 1 to 2.5 cm. The secondary pulmonary lobule (SPL) is the smallest functioning subunit of lung. Canals of Lambert provide direct connections between some membranous bronchioles and adjacent alveoli, channels of Martin provide direct interbronchiolar connections, and pores of Kohn provide interalveolar connections. Collateral ventilation occurs via small accessory channels connecting various portions of the distal small airways. This network is particularly important for maintaining small airway patency during expiration. An acinus is formed by a terminal bronchiole-with its first-order respiratory bronchioles, their branching alveolar ducts, alveolar sacs, and alveoli-and is a functional unit of the lung in which all airways participate in gas exchange.īronchioles maintain their mechanical support, structure, and patency through a complex network of elastic fibers attaching bronchioles to each other and to neighboring alveoli. Respiratory bronchioles can arise from terminal bronchioles or from other respiratory bronchioles and then branch into multiple alveolar ducts, alveolar sacs, and alveoli. Terminal bronchioles, the final generation of membranous bronchioles, mark the end of the conducting division of airflow in the lungs and give rise to respiratory bronchioles, which are the beginning of the respiratory division where gaseous exchange occurs. Few goblet cells and seromucinous glands may be present. They are lined by ciliated columnar epithelium and nonciliated Clara cells, surrounded by a layer of smooth muscle, which diminishes distally, and a layer of adventitia. Membranous bronchioles measure between 0.5 and 1 mm in diameter and are characterized by the absence of cartilage. Those include membranous bronchioles, respiratory bronchioles, and alveolar ducts. Small airways by definition are airways having a diameter of less than 2 mm. Therefore, in this chapter, we use a classification system based on histologic features in the clinical and radiologic contexts ( Table 59.2 ).ĭiffuse idiopathic pulmonary neuroendocrine cell hyperplasiaĪnatomic Features of the Small Airways and Secondary Pulmonary Lobule Given the nonspecific nature of histologic findings, interpretation in the context of the clinical and radiologic findings is crucial. Histologic classification into cellular (proliferative) or constrictive (fibrotic) bronchiolitis is valuable as it correlates most directly with the imaging features of small airways disease. Multiple classification systems have been proposed, some of which are based on the clinical features along with the presumed etiology and the pulmonary or systemic diseases with which it is associated, some on computed tomography (CT) findings and others on histologic features. Respiratory bronchiolitis–associated interstitial lung diseaseīronchiolitis is the most common form of small airways disease. Bronchiolitis Associated With Large Airways Diseaseīronchiolitis Associated With Parenchymal and Interstitial Lung DiseaseĬhronic bronchitis/chronic obstructive pulmonary disease
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