PRINCIPLES OF TOXICOLOGY

9.1 LUNG ANATOMY AND PHYSIOLOGY 173
Figure 9.4 Photomicrograph of lung tissue, showing the relationship of a terminal bronchiole (TB) and its
accompanying blood vessel, the pulmonary artery (PA), to the alveoli. (Reproduced with permission from J. F.
Murray, The Normal Lung. The Basis for Diagnosis and Treatment of Pulmonary Disease, Saunders, Philadelphia,
1976.)
to the alveolus, and the alveolar epithelium. In many instances, the red blood cells are just barely able
to fit through the small capillaries, so the blood cell wall is often in very close proximity to this
membrane complex with the alveolus.
Figure 9.5 illustrates how the remarkable design discussed above facilitates gas exchange. Carbon
dioxide and oxygen readily cross this membrane complex in a process of simple diffusion. Many
inhaled airborne industrial chemicals will also readily cross this membrane and will enter the
bloodstream. These potential toxins thus enter the blood circulatory system in a manner analogous to
someone receiving an intravenous infusion of a drug. A unique view of the alveoli is provided in Figure
9.6. The small holes, called pores of Kohn, provide for some ventilation between adjacent alveoli.
Toxicologic insult to the lung as well as various disease states can result in a functional derangement
of this membrane system. Exposure to some chemicals may result in an increase in fluid in the
interstitial space. If sufficient fluid accumulates, a condition known as pulmonary edema, gas exchange
can be hindered sufficiently to result in severe difficulty in breathing and even in death. Damage to the
membrane itself can result in scarring, which may increase the thickness of the membrane or decrease
the elasticity of the lung tissue, or both. As with pulmonary edema, an increase in the thickness of the
membrane can deleteriously affect pulmonary gas exchange. Alterations in elasticity make the work
of breathing harder, which can decrease the volume of respiration as the individual tires from the
increased effort required. Of course, whenever gas exchange or the volume of respiration is sufficiently
decreased, the amount of oxygen pressure in the circulatory system will also decline. If this decline
proceeds to a sufficient extent, affected individuals can become seriously compromised in their health
status.