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Re: pulmonary surfactant



If the alveoli were to behave like soap bubbles (with surface
tension independent of area) then they would be unstable, and
that instability would be in such a direction as to collapse
them completely, just like the smaller bubble in your example.
With the surfactant working as your books say, the alveoli would
act more like they should, their backpressure going to ambient
atmospheric when collapsed, and would increase with increasing
area. If the surface tension did not increase, the result would
be runaway expansion. The surface tension in the alveoli is
*increased at elevated pressure* by the presence of the
surfactant. It appears the term "surfactant" does not always
mean a surface agent that decreases surface tension.

Leigh

I must add that I've never thought about this problem before,
but what the books say makes sense to me. I'm not a doctor and
I've never even played one on television.

I am puzzled about the description and role of the pulmonary surfactant in
the lungs given in two textbooks. I quote:

The Jones & Childers text says, "Experiments have shown that the surface
tension of the pulmonary surfactant increases (the word increases is in
italics!) with its area, in contrast with the behavior of water and most
other liquids." The Sternheim & Kane text says, "Nature solves this
problem by having the surfactant present. Its long molecules prefer to lie
nearly beside each other, making the membrane highly elastic. During
inhalation, as the radius increases, the molecules are pulled apart and the
wall tension increases. As the radius and change in pressure increases, so
does gamma (the surface tension). During exhalation the molecules slide
back together, and the wall tension decreases. Thus the surfactant serves
to change the wall tension so that equilibrium is maintained."

Are these textbooks just plain wrong? As I understand the problem, the
surface tension on the alveoli (the tiny air sacks in the lungs) is reduced
by the presense of the surfactant. If the surface tension is appreciable,
smaller alveoli would collapse while larger ones get bigger -- just like
the demonstration in which two different size soap bubbles, connected
together, do not reach equilibrium by both becoming the same size. The
surface area of one large bubble is less than that of two bubbles with the
same total volume and thus the surface energy (elastic potential energy) is
reduced by having the smaller bubble blow up the larger one.

I would also assume that adding a surfactant to a liquid (soap and water)
is different from covering a solid membrane with a surfactant. What is the
difference in the two mechanisms? Is the latter as Sternheim & Kane
describe it?

I will breath easier if I can sort all this out. Wolfgang