PRINCIPLES OF TOXICOLOGY

172 PULMONOTOXICITY: TOXIC EFFECTS IN THE LUNG
Figure 9.3 Schematic representation of the subdivisions of the conducting airways and terminal respiratory units.
(Reproduced with permission from E. R. Weibel, Morphometry of the Human Lung, Springer-Verlag, New York,
1963.)
conducting airway tubes. In the bronchi near the lung itself, very small air sacs, or alveoli, begin to
appear (at about the nineteenth or twentieth division) and increase in frequency with proximity to the
lung. The bronchi in this region are known as respiratory bronchioles. It is in these alveoli that gas
exchange between the inhaled air and the blood circulatory system occurs.
Pulmonary System and Gas Exchange
The number of alveoli in the lungs number in the hundreds of millions, although the size of each
individual alveolus is quite small. The total surface area of the human lung, which results from the
summation of these alveoli, approximates that of about one-third of the square footage of an average
American home.
In each alveolus, a thin wall separates the blood in the capillary vessels from the inhaled air in the
alveolus. In Figure 9.4, the terminal bronchiole and the many surrounding alveoli can be seen in
relationship to the pulmonary blood supply. The wall between the blood vessel and the alveolus is a
combination of the capillary endothelium, a basement membrane adjacent to the capillary, the space
between the capillary and the alveolus (known as the interstitial space), a basement membrane adjacent