PRINCIPLES OF TOXICOLOGY - Biology East Borneo

172 PULMONOTOXICITY: TOXIC EFFECTS IN THE LUNGFigure 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 toappear (at about the nineteenth or twentieth division) and increase in frequency with proximity to thelung. The bronchi in this region are known as respiratory bronchioles. It is in these alveoli that gasexchange between the inhaled air and the blood circulatory system occurs.Pulmonary System and Gas ExchangeThe number of alveoli in the lungs number in the hundreds of millions, although the size of eachindividual alveolus is quite small. The total surface area of the human lung, which results from thesummation of these alveoli, approximates that of about one-third of the square footage of an averageAmerican home.In each alveolus, a thin wall separates the blood in the capillary vessels from the inhaled air in thealveolus. In Figure 9.4, the terminal bronchiole and the many surrounding alveoli can be seen inrelationship to the pulmonary blood supply. The wall between the blood vessel and the alveolus is acombination of the capillary endothelium, a basement membrane adjacent to the capillary, the spacebetween the capillary and the alveolus (known as the interstitial space), a basement membrane adjacent