Area: How is it possible to pack such a large area for diffusion inside the thoracic cavity? By wrapping the small blood vessels around an enormous number of small air sacs called alveoli.
The airways consist of a series of branching tubes which become narrower, shorter and more numerous as they penetrate deeper into the lung. The trachea divides into right and left main bronchi. This process continues down to the terminal bronchioles, which further divide into ocassional alveoli and the alveolar ducts completely lined with alveoli, as seen at the right. This is known as the Respiratory Zone.
At Right -> Electron micrograph showing a pulmonary capillary (C) in the alveolar wall. Note the extremely thin blood gas barrier of less than 0.5 microns (1/2000 mm). The arrow indicates the diffusion path from alveolar gas to the interior of the ertythrocyte (EC) and includes the layer of surfactant (not shown), alveolar epithelium (EP), interstitium (IN), capillary endothelium (EN) and plasma. Parts of structural cells called fibroblasts (FB), basement membrane (BM), and a nucleus of an endothelial cell are also seen.
|Number of alveoli||300 x 106|
|Alveolar diameter||280 microns (0.28 mm)|
|Total alveolar area||70 m2|
|Membrane thickness||0.36 - 2.5 microns|
|Capillary length||10.3 x 10-3 mm|
|Capillary volume||140 ml|
|Blood transit time|
|..... Rest||0.75 sec.|
|..... Exercise||0.34 sec.|
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