Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

lung surfactants fwd from P.



Paul Zitzewitz forwarded recent PHYS-L postings regarding lung
surfactants to a colleague, who writes this in reply:


You are quite right about your three major points. First, surface tension
produces an instability in the alveoli so that there is a tendency for
smaller ones to collapse in favor of the larger ones. Second, the lungs are
coated with a surfactant that significantly reduces the overall surface
tension, allowing the alveoli to expand and contract under much less
pressure than would otherwise be needed. Third, the particular surfactant
has the property that the surface tension increases with area. This third
fact is simply that as the alveoli expand, it dilutes the surfactant so it
has less effect.

I think you may be confusing two factors. First, internal pressure in a
bubble is inversely related to diameter -- as the alveoli expand, the
pressure goes down. Second, the relation depends on the "constant" gamma,
which is definitely not constant -- as the alveoli expand, gamma increases.
Therefore, when put together, the decrease in pressure in the alveoli with
increasing diameter is greatly reduced. In other words, the surfactant
helps reduce the instability and therefore helps to stabilize the lungs.

Incidentally, adding surfactant to a solution vs adding it to a surface is
irrelevant. The surfactant only works at the phase interface and it makes
no difference how it is applied to that interface.

So it is really biology that lets physicists breathe easier!

Richard Norman
Associate Professor of Biology
Department of Natural Sciences
Universtiy of Michigan - Dearborn
Dearborn MI 48128 USA
rnorman@umich.edu