Biomedical Engineering Reference
In-Depth Information
Figure 1
(A) Bronchogram from a normal human lung showing the nature of the
branching system of the tracheobronchial tree. (B) Shows that when the trachea is
designated 0 the number of branches to the terminal bronchioles varies from as
few as eight to as many as 24 depending on the pathway that is followed. (C) Shows
that the small conducting airways less than 2mm in diameter are found from the 4th
to the 14th generation of branching in the tracheobronchial tree (D) Shows the
remarkable increase in cross-sectional area that occurs beyond the small airways.
(Figures B-D are modified from Refs. 1, 2.)
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.
The total resistance to airflow of a healthy person breathing through
the mouth varies with lung volume and is approximately 2 cm H
2
O
=
L
=
sec
during quiet breathing (4). The resistance to flow below the larynx accounts
for about 50% of this value (5,6) and the small airways about 10% of total
resistance to the bulk flow of gas in the peripheral lung (6,7). For this rea-
son, the small airways have been referred to as the lung's ''quiet zone'',
where considerable disease may develop before there is any effect on total
airway resistance (8). The realization that the small airways offer little resis-
tance in health (6,7) and become the major site of airway obstruction in dis-
ease (6-8) is consistent with the fact that patients with COPD have had a
