Respiratory Physiology
453
type II cells, which stimulates the secretion of surfactant. This
is why patients who have had thoracic or abdominal surgery
and are breathing shallowly because of the pain must be urged
to take occasional deep breaths.
The
Law of Laplace
describes the relationship between
pressure (
P
), surface tension (
T
), and the radius (
r
) of an alve-
olus shown in
Figure 13–17
:
P
= 2
T
/
r
(13–5)
As the radius inside the alveolus decreases, the pressure increases.
Now imagine two alveoli next to each other sharing an alveolar
duct (see Figure 13–17). The radius of alveolus a (
r
a
) is greater
than the radius of alveolus
b
(
r
b
). If surface tension (
T
) were
equivalent between these two alveoli, alveolus
b
would have a
higher pressure than alveolus
a
by the Law of Laplace. If
P
b
is
higher than
P
a
,
air would fl ow from alveolus
b
into alveolus a,
and alveolus
b
would collapse. Therefore, small alveoli would be
unstable and would collapse into large alveoli. Another impor-
tant property of surfactant is that it stabilizes alveoli of different
sizes by altering surface tension, depending on the surface area
of the alveolus. As an alveolus gets smaller, the molecules of
surfactant on its inside surface are less spread out, thus reducing
surface tension. The reduction in surface tension helps to main-
tain a pressure in smaller alveoli equal to that in larger ones.
This gives stability to alveoli of different sizes.
A striking example of what occurs when surfactant is defi -
cient is the disease known as
respiratory distress syndrome
of the newborn
.
This is a leading cause of death in premature
infants, in whom the surfactant-synthesizing cells may be too
immature to function adequately. Respiratory movements in
the fetus do not require surfactant because the lungs are fi lled
with amniotic fl uid, and the fetus receives oxygen from the
maternal blood. Because of low lung compliance, the affected
infant can inspire only by the most strenuous efforts, which may
ultimately cause complete exhaustion, inability to breathe, lung
collapse, and death. Therapy in such cases is assisted breathing
with a mechanical ventilator and the administration of natural
or synthetic surfactant given through the infant’s trachea.
Airway Resistance
As previously stated, the volume of air that fl ows into or out
of the alveoli per unit time is directly proportional to the
pressure difference between the atmosphere and alveoli and
is inversely proportional to the resistance to fl ow of the air-
ways (see equation 13–2). The factors that determine airway
resistance are analogous to those determining vascular resis-
tance in the circulatory system: tube length, tube radius, and
interactions between moving molecules (gas molecules, in this
case). As in the circulatory system, the most important factor
by far is the radius of the tube—airway resistance is inversely
proportional to the fourth power of the airway radii.
Table 13–4
Some Important Facts About
Pulmonary Surfactant
1. Pulmonary surfactant is a mixture of phospholipids and
protein.
2. It is secreted by type II alveolar cells.
3. It lowers the surface tension of the water layer at the alveolar
surface, which increases lung compliance, thereby making
the lungs easier to expand.
4. Its surface tension is lower in smaller alveoli, thus stabilizing
alveoli.
5. A deep breath increases its secretion by stretching the type
II cells. Its concentration decreases when breaths are small.
6. Production in the fetal lung occurs in late gestation.
No surfactant
a
b
Air flow
r
a
r
b
With surfactant
a
b
r
a
r
b
If
T
a
=
T
b
then
P
a
<
P
b
and air flows from
b
to
a
;
b
collapses into
a
P
a
P
a
T
a
T
a
T
b
T
b
P
b
P
b
r
a
>
r
b
=
2
r
P
T
then
P
a
=
P
b
and there is no flow from
b
to
a
;
smaller alveoli do not
collapse into bigger alveoli
If
T
a
>
T
b
(due to unique
property of surfactant)
Figure 13–17
Stabilizing effect of surfactant.
P
is pressure
inside the alveoli,
T
is a surface tension, and
r
is
the radius of the alveolus. The Law of Laplace is
described by the equation in the box.
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