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What happens when you try to exhale
below the FRC?
As you know, the FRC is composed of two volumes, the
expiratory reserve volume and the residual volume. There is a good reason
why the two of these are separated.
Below FRC, expiration is active, the diaphragm and chest wall must press
down on lung tissue to express gas. There comes a point where, no matter
how much pressure is applied, it is not possible to push any more air out
of the lungs: this is the residual volume, it is approximately 1 liter in
adults. What this effectively means is that there is a liter of air
trapped in the lungs at the end of forced expiration.
Why does this happen? Forced expiration increased the pressure in the
lungs above atmospheric, this is applied to the alveoli and to the
conducting airways. We know that the pressure in the mouth, which is the
end of the conducting airways, is atmospheric. Thus there must be a
pressure gradient between the alveoli and the mouth. Alveoli are difficult
to compress, as they are filled with air and coated with surfactant.
Airways are compressible.
Thus there will come a point where the pressure in the thorax is greater
that that which splints open the conducting airways, and these are
compressed, and air is trapped in the alveoli. We call this “dynamic
airway collapse”.
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