Surface tension of alveoli and surfactant 1 Surface tension of the alveoli Figure

- results from the attractive forces between molecules of liquid lining the alveoli.

- creates a collapsing pressure that is directly proportional to surface tension and inversely proportional to alveolar radius (Laplace's law), as shown in the following equation:

where:

P = collapsing pressure on alveolus (or pressure required to keep alveolus open) [dynes/cm2] T = surface tension (dynes/cm) r = radius of alveolus (cm)

a. Large alveoli (large radii) have low collapsing pressures and are easy to keep open.

b. Small alveoli (small radii) have high collapsing pressures and are more difficult to keep open.

- In the absence of surfactant, the small alveoli have a tendency to collapse (atelectasis).

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