Biomedical Engineering Reference
In-Depth Information
3
Small Airways Disease in COPD
James C. Hogg
St. Paul's Hospital
=
University of British Columbia, The James Hogg
iCAPTURE Centre for Cardiovascular & Pulmonary Research,
Vancouver, British Columbia, Canada
I.
INTRODUCTION
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 bronch-
ioles 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 alveo-
lar 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 air-
ways branch up to 12 times over this distance (3). The parallel branching
pattern of the smaller airways rapidly enlarge the total cross-sectional area
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