Trying to understand alveolar surface tension

[samy4408]

TL;DR Summary

I need a satisfactory explanation about alveolar surface tension

Hello, I learned recently about alveolar surface tension, and the explanation provided in the course was not satisfactory, it said that it is due to the force that pushes water molecules of the outer layer to the inner layers, I don't understand why this force that pushes water toward the cell walls is the origin of another force that pusher water in the opposite direction.
can someone give me a satisfactory explanation, tanks.

 

[Borek]

I am not sure I follow, but typically when people have problems like the one you are describing they forget they should be comparing two situations, not analyze them separately.

Try to not think in terms of "what this force does" but in terms of "what will happen when I remove the other force". You probably should be comparing two situations: first, with the forces that act in all directions and cancel each other (so the net effect is zero), second, where half of the forces were removed and the net effect is opposite to what the left forces suggest.

 

[lofi_lemons]

I really want an answer to this question. I'm a med student and in my physiology textbook it only says that surface tension (ST) is, "The force pulling surface liquid molecules towards each other at an air-liquid interface. Water molecules at surface of alveoli are strongly attracted to each other than to air molecules." Then it goes on to say that ST in alveoli produce an inward force.. tending to decrease alveolar size... How is this possible.. If surface tension is caused by forces of water, shouldn't it pull water molecules away from the air water interface? Then shouldn't it cause alveolar expansion? ( As water forces should pull surface water molecules towards each other - more force is put on surface molecules towards the other water molecules present farther away from surface and away from the air...)

 

[Borek]

Water molecules at surface of alveoli are strongly attracted to each other than to air molecules.

And the net effect is the surface tension works more or less like in a balloon - surface "wants" to get as small as possible.

Surface tension forces act parallel to the surface. But forces are vectors, when the surface is curved and you add these vectors, sum will have a component that is perpendicular to the surface. In which direction the force perpendicular to the surface works depends on the curvature of the surface. In the case of small droplets in the air surface tension will act to make them more spherical, in the case of "holes" in the liquid (like alveoli) the force will make them spherical, but it will also compress the air inside, so if they are further open the air will be pushed out.

 

[lofi_lemons]

And the net effect is the surface tension works more or less like in a balloon - surface "wants" to get as small as possible.

Surface tension forces act parallel to the surface. But forces are vectors, when the surface is curved and you add these vectors, sum will have a component that is perpendicular to the surface. In which direction the force perpendicular to the surface works depends on the curvature of the surface. In the case of small droplets in the air surface tension will act to make them more spherical, in the case of "holes" in the liquid (like alveoli) the force will make them spherical, but it will also compress the air inside, so if they are further open the air will be pushed out.

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Ooh thanks a lot! This really helped!!