PRINCIPLES OF TOXICOLOGY - Biology East Borneo

9.1 LUNG ANATOMY AND PHYSIOLOGY 173Figure 9.4 Photomicrograph of lung tissue, showing the relationship of a terminal bronchiole (TB) and itsaccompanying 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 ableto fit through the small capillaries, so the blood cell wall is often in very close proximity to thismembrane complex with the alveolus.Figure 9.5 illustrates how the remarkable design discussed above facilitates gas exchange. Carbondioxide and oxygen readily cross this membrane complex in a process of simple diffusion. Manyinhaled airborne industrial chemicals will also readily cross this membrane and will enter thebloodstream. These potential toxins thus enter the blood circulatory system in a manner analogous tosomeone receiving an intravenous infusion of a drug. A unique view of the alveoli is provided in Figure9.6. 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 derangementof this membrane system. Exposure to some chemicals may result in an increase in fluid in theinterstitial space. If sufficient fluid accumulates, a condition known as pulmonary edema, gas exchangecan be hindered sufficiently to result in severe difficulty in breathing and even in death. Damage to themembrane itself can result in scarring, which may increase the thickness of the membrane or decreasethe elasticity of the lung tissue, or both. As with pulmonary edema, an increase in the thickness of themembrane can deleteriously affect pulmonary gas exchange. Alterations in elasticity make the workof breathing harder, which can decrease the volume of respiration as the individual tires from theincreased effort required. Of course, whenever gas exchange or the volume of respiration is sufficientlydecreased, the amount of oxygen pressure in the circulatory system will also decline. If this declineproceeds to a sufficient extent, affected individuals can become seriously compromised in their healthstatus.