ABSTRACT
The tracheobronchial tree of the human lung provides an unequal branching system of conducting airways that can lead to the gas exchanging surface in as few as eight or as many as 23 branches (Fig. 1A and B) depending on the pathway followed (1,2). The term small airways refers to the bronchi and bronchioles less than 2mm in maximum internal diameter that are located from the 4th to the 14th generation of airway branching in the adult human lung (Fig. 1C). These purely conducting airways end in many thousands of terminal bronchioles that range from 400 to 600 mm in diameter and supply a gas exchanging unit of lung termed the acinus (2,3). Figure 2A shows a group of several terminal bronchioles surrounded by connective tissue septa that defines a larger unit of lung termed the secondary lobule. Within each acinus (Fig. 2B) the terminal bronchioles branch into respiratory bronchioles that are defined by the appearance of alveolar openings in their walls. The numbers of these openings increase with each successive generation of branching until the bronchiolar epithelium is completely replaced by alveolar openings in the alveolar ducts. The ducts branch several more times and finally end blindly as alveolar sacs (3). On average the distance from the first respiratory bronchiole to alveolar sacs is approximately 9mm and the airways branch up to 12 times over this distance (3). The parallel branching pattern of the smaller airways rapidly enlarge the total cross-sectional area
at each successive generation of airway branching (Fig. 1D) which lowers the resistance to the flow in the peripheral lung and shifts the movement of gas from bulk flow to diffusion in the gas exchanging regions.
