urn to their unexpanded size. The role of the passageways in the lung depends on their location. The larger passageways such as bronchi and bronchioles serves as conduction pathways for air. The smallest passageways such as alveolar ducts and respiratory bronchioles serves as areas of gaseous exchange between the blood and the air.
7.3 The alveoli are the terminal air spaces of the respiratory system where the exchange of gases takes place between the air and the blood (Ross, Romrell and Kaye, 1995, 544). Each alveolus is a polyhedral chamber with a very thin wall. Each alveolus is confluent with an alveolar duct which leads directly to a respiratory bronchiole. Alveolar ducts are elongated airways with almost no walls, their peripheral boundaries being alveoli, and rings of smooth muscle in the interalveolar septa. The alveolar sacs are spaces surrounded by clusters of alveoli which open into these spaces. Alveoli are surrounded by, and separated from each other by, a thin layer of connective tissue.
The alveolar epithelium consists mainly of two types of cells, with occasional brush cells (Ross, Romrell and Kaye, 1995, 544). Type I alveolar cells (type I pneumocytes) are extremely thin squamous cells lining most of the surface of the alveoli, joined to each other, and to other cells of the alveolar epithelium, type II alveolar cells (type II pneumocytes) or septal cells and the brush cells, by zonulae occludentes. These tight junctions form an occlusive barrier between the air space and the components of the septal wall. Type II alveolar cells are secretory cells, which tend to concentrate at septal junctions. Their cytoplasm contains stacks of lamellae which contain phospholipids, among which is a surfactant. Exocytosis releases the surfactant onto the surface of the alveolar epithelium, reducing the surface tension at the air/epithelium interface, and preventing the alveoli from collapsing on exhalation....