PRINCIPLES OF TOXICOLOGY

174 PULMONOTOXICITY: TOXIC EFFECTS IN THE LUNG
Figure 9.5 Electron micrograph of an alveolar septum, showing the various tissue layers through which oxygen
and carbon dioxide must move during the process of diffusion. The surface of the alveolar spaces (AS) is lined by
continuous epithelium (EP). The capillary containing red blood cells (RBC) is lined by endothelium (E). Both layers
rest on basement membranes (BM) that appear fused over the “thin” portion of the membrane and that are separated
by an interstitial space (IS) over the “thick” portion of the membrane. [Reproduced with permission from Murray
(1976) (see Figure 9.4 source note).]
Physiologic Differences between Inhalation and Ingestion
Following inhalation, the chemical goes directly into the bloodstream without being first processed by the
gastrointestinal system. This can result in an extremely rapid uptake of an industrial chemical from the air.
For some chemicals, this also results in an extremely rapid onset of toxicity following inhalation of the agent.
Inhalation of a chemical might also result in a higher degree of toxicity than if the compound were
ingested. This is because a chemical absorbed from the gut will go first to the liver, which is the primary
metabolizing organ of the body. The liver thus has the opportunity to eliminate the compound before
it exerts its effect in some other target organ. This is called the first-pass effect. When the chemical is
inhaled, it bypasses the liver and the toxin has the opportunity to reach a specific target organ and exert
some degree of toxicity before the liver has the opportunity to eliminate it.
Particulates
Many chemical and radionuclide agents are deposited in the respiratory tract in the form of solid
particles or droplets, also referred to as aerosols, meaning a population of particles that remain