PSIO 532 Study Guide - Midterm Guide: Deflate, Transpulmonary Pressure, Functional Residual Capacity
PSL – Respiratory Physiology
Gravity’s Affect on Pressures
• Surfactant molecules form a film that lines the inside of the lung. This film of
surfactant
molecules at the air-liquid interface lowers the attractive forces between surface
molecules of
the liquid. The amount of surface tension lowering generated by a lung surfactant film is
dependent on the nature of the concentration of the surfactant on the surface.
Therefore, surface tension is reduced the most when a monolayer film of surfactant
molecules is squeezed close together in expiration. The surfactant film is produced
when molecules enter the surface, a process called "adsorption." During inspiration, the
molecules in the surfactant film separate as the alveolar surface expands. During
expiration, the molecules in the film get squeezed together as tightly as possible, and in
fact, some are "squeezed out" of the film because there is no room for them. However,
these "squeezed out" molecules remain "associated" with the film and they re-enter the
film during the next inspiration, a process called "respreading."
• At the end of inspiration, there will be more surface tension because the surfactant
molecules are more spread out → this will help DEFLATE the alveolus
• At the end of expiration, the alveolus is reduced in size and the surfactant molecules
are more concentrated (surface tension is reduced) → helps us inflate the alveolus
The calculation for average lung compliance is
shown in the figure. Note that the change in volume in
this example was 1000 ml for a 5 cmH2O change in
pressure, leading to an average compliance value of
0.2 L/cmH2O
Graphs helps us understand the total lung
compliance in humans
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