chitosan and plga microspheres as drug delivery ... - UniCA Eprints

1.3. Lung Anatomy1. General IntroductionThe lung is the body’s organ of respiration. The respiratory tract can be divided into upperand lower airways, with the line of division being the junction of the larynx and trachea (61).Figure 1.2: The Anatomy Of Human LungsThe upper airways or nasopharyngeal region consists of the nose, mouth, larynx, and pharynx.Below the contours of the nasopharyngeal region, the lower airways resemble a series of tubesundergoing regular dichotomous branching (62). Successive branching from the trachea to thealveoli reduces the diameter of the tubes, but markedly increases the surface area of theairways, which allows gas exchange (63, 64). The lower airways can be divided into 3physiologic zones: conducting, transitional, and respiratory zones (62, 65). The conductingzone consists of the larger tubes responsible for the bulk movement of air and blood. In thecentral airways, air flow is rapid and turbulent and no gas exchange occurs. The transitionalzone plays a limited role in gas exchange. The epithelial layer of the trachea and main bronchiis made up of several cell types, including ciliated, basal, and goblet. On the surface of theepithelium of the proximal respiratory tract, ciliated cells predominate. A large number ofmucus-producing and serum-producing glands are located in the submucosa.The human lung consists of 5 lobules and 10 bronchopulmonary segments. Arranged adjacentto each segment are lung lobules composed of 3–5 terminal bronchioles. Each bronchiolesupplies the smallest structural unit of the lung, the acinus, which consists of alveolar ducts,alveolar sacs, and alveoli. Alveolar epithelial type I cells represent the principle cell typelining the surface of the alveoli. The major functions of these cells, which cover 93% of the16