Pulmonary Physiology, Introduction, Dr. D. Penney

Pulmonary Physiology - Introduction (continued...)

Diffusion: Oxygen and carbon dioxide move between air and blood by simple diffusion. Fick's Law of Diffusion states that the amount of gas which moves across a sheet of tissue is proportional to the area of the sheet but inversely proportional to its thickness. The blood-gas barrier is exceedingly thin and has an area of 50 - 100 m2.

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.

Estimates of the dimensions of the pulmonary membrane.

Membrane Dimensions
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.

Last Changed 05/01/00

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