The Human Respiratory System - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 33

Fig. 2.11 Schematic of the tracheobronchial airway showing the subdivisions in the first three
generations and where the branches lead into the segments of the lung, subsequently called the
bronchopulmonary segments. The right lung has three lobes and approximately ten segments. The
left lung has two lobes and approximately eight segments

The lobar bronchi further divides into segmental bronchi ( tertiary bronchi ), which
supply the bronchopulmonary segments of each lobe. Figure 2.12 and 2.13 show the
first three generations of the tracheobronchial tree in the left and right lung respec-
tively. A bronchopulmonary segment may be defined as an area of distribution of any
bronchus (Jackson and Huber 1943). Technically there are ten bronchopulmonary
segments in each lung, however in the left lung some of these segments fuse and there
are as few as eight bronchopulmonary segments. The bronchi continually divide into
smaller and smaller bronchi up to about 23-24 generations of divisions from the main
bronchi. As the bronchi become smaller, their structure changes:

• the cartilaginous rings that support the branches turn into irregular plates of car-

tilage and eventually disappear by the time bronchioles are reached (

1mm in
diameter). When the bronchi eventually lose all support (usually between gen-
erations 12-15) the airways are then referred to as bronchioles (Vanpeperstraete
1974);

∼

• the epithelium changes from pseudo stratified columnar to columnar and then to

cuboidal in the terminal bronchioles. There are no cilia or mucous producing cells