ABSTRACT
This paper reviews the mechanisms and physiological processes that act when drugs or chemicals are administered into the lower airways and lungs. Administration is usually by aerosol. Agents can be given, for example, either to treat pulmonary diseases such as asthma, or the test for airways' responsiveness or other functions, or as a means of access of a drug to the systemic circulation. The first barrier to absorption is the airway surface liquid, including mucus. The thickness of this layer will determine the concentration of the drug in solution, and therefore its rate of entry into the tissue. The agent must then penetrate the airway epithelium, the strongest barrier for hydrophilic agents. Agents must then diffuse through the epithelial basement membrane and the interstitium. Finally, the agent may be taken up into the mucosal vasculature, and changes in blood flow will influence its uptake and distribution. If the drug is to reach a target organ, such as airway smooth muscle or glands, these barriers have first to be traversed.