Drug Targeting Organ-Specific Strategies

3.2 The Respiratory Tract
3.2 The Respiratory Tract 55
The human respiratory tract is a branching system of air channels with approximately 23 bifurcations
from the mouth to the alveoli [8,18,19]. In Figure 3.1 a schematic representation of
the human airways as described by Weibel [20] is shown. Furthermore, this figure shows some
typical geometric features of the lung. The major task of the lungs is gas exchange, by adding
oxygen to, and removing carbon dioxide from the blood passing the pulmonary capillary bed
[21]. This task is facilitated by inhaling certain quantities of fresh air into the lungs at regular
intervals and exhaling similar volumes of used air in between. The muscles that are responsible
for this task can be divided into inspiratory and expiratory muscles. During inhalation,
the chest is expanded mainly longitudinally by contraction of the main inspiratory muscle:
the dome shaped diaphragm in the lower part of the chest.This enlargement of the chest volume
creates an underpressure in the lungs which is the driving force for an airflow, entering
through the mouth or nose. Expiration during quiet breathing occurs passively as a result of
recoil of the lung. Only during heavy breathing are expiratory muscles, which depress the
ribs, activated.
conducting zone
transitional and
respiratory zones
Generation
diameter
(cm)
length
(cm) Number
total cross
sectional
area (cm 2 )
Trachea 0 1.80 12.0 1 2.54
Bronchi
1 1.22 4.8 2 2.33
2 0.83 1.9 4 2.13
3 0.56 0.8 8 2.00
Bronchioles
4 0.45 1.3 16 2.48
5 0.35 1.07 32
Terminal bronchioles
↓
16
↓
0.06
↓
0.17
↓
6·10 4
3.11
↓
180.0
Respiratory
Bronchioles
17
18
19 0.05 0.10 5·10 5
10 3
20
Alveolar ducts
21
22
Alveolar sacs 23 0.04 0.05 8·10 6
10 4
Figure 3.1. Schematic representation of the lung according to the model described by Weibel [20].
3.2.1 Lung Capacities and Pulmonary Ventilation
The inhaled air volume (V in L) depends on the extent of chest enlargement. During normal
breathing, the inhaled and exhaled volumes (tidal volume) are only part of the total lung volume
[8,21]. The different parameters that describe pulmonary ventilation are shown in Figure
3.2.Table 3.1 presents a definition of the different parameters. Normal adults have a tidal