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Lecture 13

PHGY 210 Lecture 13: Lecture 13 NTC

Course Code
PHGY 210
Melissa Vollrath

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lecture #:
February 1, 2017
Dr. Lauzon
phgy 210
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PHGY 210 Lecture 13 Dr. Lauzon
Do not redistribute. Page 2 of 5 Written by: Yingshuo Song
Surface Tension (Image A)
The surface tension of the liquid film lining the lungs contributes to the mechanical properties of the
respiratory system.
Surface tension is the tension that occurs at the surface of the liquid
This tension arises because the molecules in the surface of the film tend to arrange themselves in the
configuration involving the lowest energy.
Being more attracted to themselves than to air, they like to “hold hands” rather than freely associate with air
o The attraction between water molecules is stronger than the forces of attraction between water
molecules and the air
o e.g. mosquitos can stand on water
This causes a tension to be generated across the film surface.
Alveolar epithelial type II cells secrete surfactant that decreases surface tension
Surface Tension in Alveoli (Image B)
If the surface is curved, such as on the inside of an alveolus or airway, the surface
tension can produce a pressure.
Use a soap bubble-alveoli analogy
The previous surface tension vectors are now in a sphere
If added together, they generate a pressure inward toward the center (P) of the bubble that
tends to collapse the soap bubble
In pulmonary physiology, these inward pressures will tend to collapse the alveoli.
o This surface tension is actually an enormous force.
Compliance = need to inflate the lungs to work against the elastic properties of the lungs
o But we also have to work against the surface tension of the alveoli
Surface Tension (Image C)
Alveoli can be modeled as a collection of soap bubbles.
The pressure P inside a bubble of radius R, from a surface tension T, is given by LaPlace’s Law: P = 4T/r
o This means the pressure due to surface tension is greater in a small alveolus than a large alveolus
If you breathe to TLC, all the alveoli will reach the same final volume
At the beginning of the breath, not all of them are at the same volume
o This makes it difficult to inspire because now we have to generate a lot of force
to overcome the surface tension to open up the alveoli
o And we also have a system where small alveoli which have a greater pressure
would empty into big alveoli (because big alveoli have less pressure inside) and
so on that would create an unstable system, so lungs can’t behave that way
o This is why we have surfactant
Pulmonary Surfactant
Secretion of pulmonary surfactant by alveolar type II cells prevents small alveoli collapsing into large ones.
o Surfactant behaves like a detergent = it decreases the surface tension
o Surfactant molecules will insert themselves between water molecules and decrease the force of water
attraction decreases surface tension
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