Trying to understand alveolar surface tension

[samy4408]

TL;DR

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.

View attachment 348306

Ooh thanks a lot! This really helped!!

 

[Tipaza]

H20 is a dipole. Patm is the atmospheric pressure. When the dipoles (i.e. the molecules of water of the very superficial layer of an alveolus – the layer which line the inner surface of the alveolus) are distorted by an external force like Patm, the arrangement of the dipoles changes.

The dipoles are no longer parallel to the surface but are slanted. This distortion causes the dipoles to exert forces on each other due to their electric fields. Consider the two slanted dipoles on either side of the central dipole. Each dipole has a positive and negative charge. The positive charge of one dipole will attract the negative charge of the adjacent dipole, and vice versa. This attraction creates a force that has both horizontal and vertical components.
The vertical components of these forces from the two slanted dipoles will add up to create a net upward force on the central dipole (towards the center of the alveolus).

This net upward force(which in this case represent the resulting vector of the surface tension ) opposes the downward external force (i.e. the Patm or Palv ) applied to the surface , and is directed towards the center of the alveolus)
The dipoles attract each other due to the electrostatic forces between the positive and negative charges. This attraction creates a cohesive force that resists the separation of the dipoles.
When an external force tries to penetrate the surface, the dipoles resist this penetration due to their mutual attraction.

This resistance to separation is a simple model for surface tension. Surface tension is the force that acts on the surface of a liquid, causing it to behave like a stretched elastic membrane.