2. This term means stability of the size of different alveoli
It is produced by two factors: the surfactant and the property of
alveolar interdependence.
3. ROLE OF THE SURFACTANT
The surfactant maintains alveolar stability by keeping a nearly
equal pressure in various alveoli which occurs by Laplace law
It states that the pressure developed in the alveoli (P) equals
twice the surface tension (T) divided by the radius (r).
If the (T) was the same in all alveoli, the alveoli having small (r)
will have higher (P) than that in the large alveoli
This would shift air from the small to the large alveoli, resulting
in collapse of the smaller alveoli & overdistension of the larger
ones
4. Because the surfactant is more effective in the small alveoli, the
(T) is lower in these alveoli, so the (P) in them is not increased
and is maintained nearly equal to that in the large alveoli
This results in insignificant shift of air among the various alveoli,
which maintains alveolar stability
5. ALVEOLAR INTERDEPENDENCE
This is a property in the alveoli by which they support each
other. So that a change in the volume of a certain group is
opposed by the surrounding groups
For example the tendency of a group of alveoli to collapse is
antagonized by development of strong expanding forces in the
surrounding groups
Such property obviously helps in maintenance of alveolar
stability
8. ALVEOLAR INTERDEPENDENCE
The lungs and chest wall are elastic structures. Each has a
resting position at which it is neither stretched nor compressed,
and they always tend to acquire this position.
The volume at the resting position is called the relaxation
volume, and in normal adult persons, that volume of the lungs is
about one litre while that of the thoracic cavity is about 3.5 litres.
In the midthoracic position, each of the volume of the lungs as
well as that of the thoracic cavity equals the FRC (functional
residual capacity) i.e about 2.2 litres.
9. This is called the relaxation volume of the respiratory system
and at such volume, the lungs are stretched while the thoracic
cavity is compressed
Accordingly the lungs tend to recoil while the chest wall tends
to expand creating the negativity of the IPP
In other words, the tendency of the lungs to recoil is balanced
by a tendency of the chest wall to recoil by an equal degree but in
the opposite direction (i.e. outwards)
10.
11. Interaction between the recoil forces of the lungs & chest wall
This is shown in the pulmonary relaxation pressure curve. The
relaxation pressure at a certain volume is the intrapulmonary
pressure recorded at that volume while the respiratory muscles
are relaxed.
The relaxation pressures of either the lungs or the chest wall
results from the recoil force of each, while that of the whole
respiratory system is the resultant of the recoil forces of both the
lungs and chest wall together (the middle solid curve in the
diagram above)
12. The relaxation pressure curves show the following :
1. At the midthoracic position (when the lungs contain the FRC),
the relaxation pressure of the lungs alone is about + 5 H20 (as
they tend to recoil) while that of the chest wall alone is about - 5
H20 (as it tends to expand)
Since both forces are equal but opposite, they cancel each
other and the relaxation pressure of the whole respiratory system
becomes zero.
2. At volumes exceeding the FRC , the relaxation pressure of the
lungs becomes more +ve [because their recoil force increases]
while that of the chest wall becomes less -ve [because its recoil
force decreases as it approaches its resting volume (about 3.5
litres)]
13. Consequently, the relaxation pressure of the whole respiratory
system becomes positive
3. At a volume of about 3.5 litres (70% of the vital capacity), the
relaxation pressure of the lungs becomes more and more +ve
[because their recoil force further increases] while that of the
chest wall becomes zero [because its recoil force disappears as it
reaches its resting volume]
Consequently, the relaxation pressure of the whole respiratory
system becomes more positive
14. 5. At volumes less than the FRC, the relaxation pressure of the
lungs becomes less +ve [because their recoil force decreases)
while that of the chest wall becomes more -ve [because its
tendency to expand increases]
4. At volumes exceeding 3.5 litres, both the lungs and the chest
wall tend to recoil inwards, thus they operate additively producing
a further increase in the relaxation pressure of the whole
respiratory system (which reaches a maximum of 25-30 cm H2O
at 100 % vital capacity)
Consequently, the relaxation pressure of the whole respiratory
system becomes negative ( about -20 cmH20 at the residual
volume)
**The relaxation pressure of the lungs becomes zero only at lhe
minimal volume and it becomes negative at lower volumes
