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INTRODUCTION The airways represent a unique organ system in the body. Their structure allowing air to come into close contact with blood is one of the principal adaptations permitting the existence of terrestrial life. This adaptation also makes the airways useful as a route of administration of drugs in the inhaled, or aerosol, form. This chapter provides an overview of the physiology of the airways, excluding the nasopharyngeal regions of the airways. Aspects considered relevant to the practical and theoretical application of inhaled substances are emphasized. ANATOMY OF THE AIRWAYS The airways (constituting the lungs) may be viewed as a series of dividing passageways originating at the trachea and terminating at the alveolar sacs.