Alveoli alveolar sacs

The transition zone consists of the respiratory bronchioles (generations 17 to 19), which contain alveoli. At the terminal end, the respiratory zone is composed of parenchyma that contains the alveolar ducts and about 300 million alveoli (alveolar sacs) to provide the gas-exchange surface. Since the surface area expands to such a large extent within the very last generations of bifurcations, the inhalation airflow rapidly slows down to zero velocity so that the movement of gas molecules and the exchange occurs entirely by diffusion (Stocks and Hisloop 2002). 

Figure 3.10 The structure of the mammalian respiratory system (A) trachea, (B) bronchiole, (C) alveolar sac with blood supply, (D) arrangement of blood vessels around alveoli, (E) arrangement of cells and airspaces in alveoli showing the large surface area available for absorption, (F) cellular structure of alveolus showing the close association between (G) the endothelial cell of the capillary (H) with erythrocytes and (I) the epithelial cell of the alveolar sac. The luminal side of the epithelial cell is bathed in fluid, which also facilitates absorption and gaseous exchange. Source From Ref. 1.

The lung. The middle diagram is a close-up view of the end of a bronchiole. Each bronchiole terminates in many alveolar sacs. The diagram on the right is a close-up view of an alveolus. The cell membrane of each alveolus is very thin and surrounded by many blood capillaries, thereby allowing for rapid exchange of oxygen for carbon dioxide, as well as absorption of other substances. 

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