452
Chapter 13
Lung volume (ml)
Normal
compliance
Increased
compliance
Compliance =
=
Decreased
compliance
Transpulmonary pressure
(
P
tp
)
(
P
alv
P
ip
)
P
tp
V
(
P
alv
P
ip
)
Lung volume
0
Figure 13–16
A graphical representation of lung compliance. Changes in lung volume
and transpulmonary pressure are measured as a subject takes progressively
larger breaths. When compliance is lower than normal (the lung is
stiffer), there is a lesser increase in lung volume for any given increase in
transpulmonary pressure. When compliance is increased, as in emphysema,
small decreases in
P
tp
allow the lung to collapse.
Figure 13–16
physiological
inquiry
Premature infants with inadequate surfactant have decreased lung
compliance (respiratory distress syndrome of the newborn). If
surfactant is not available to administer for therapy, what can be done
to infl ate the lung?
Answer can be found at end of chapter.
set of intercostal muscles and the abdominal muscles, which
actively
decreases thoracic dimensions. The
expiratory
inter-
costal muscles (again a functional term, not an anatomical
one) insert on the ribs in such a way that their contraction
pulls the chest wall downward and inward, thereby decreas-
ing thoracic volume. Contraction of the abdominal muscles
increases intra-abdominal pressure and forces the relaxed dia-
phragm up into the thorax.
Lung Compliance
To repeat, the degree of lung expansion at any instant is propor-
tional to the transpulmonary pressure,
P
alv
P
ip
. But just how
much any given change in transpulmonary pressure expands
the lungs depends upon the stretchability, or compliance, of
the lungs.
Lung compliance (
C
L
)
is defi ned as the magni-
tude of the change in lung volume (
V
L
) produced by a given
change in the transpulmonary pressure:
C
L
=
V
L
/
P
tp
(13–4)
Thus, the greater the lung compliance, the easier it
is to expand the lungs at any given change in transpulmo-
nary pressure (
Figure 13–16
). Compliance can be consid-
ered the inverse of stiffness. A low lung compliance means
that a greater-than-normal transpulmonary pressure must be
developed across the lung to produce a given amount of lung
expansion. In other words, when lung compliance is abnor-
mally low (increased stiffness), intrapleural pressure must be
made more subatmospheric than usual during inspiration to
achieve lung expansion. This requires more vigorous contrac-
tions of the diaphragm and inspiratory intercostal muscles.
Thus, the less compliant the lung, the more energy is required
to produce a given amount of expansion. Persons with low
lung compliance due to disease therefore tend to breathe
shallowly and must breathe at a higher frequency to inspire an
adequate volume of air.
Determinants of Lung Compliance
There are two major determinants of lung compliance. One
is the stretchability of the lung tissues, particularly their elas-
tic connective tissues. Thus, a thickening of the lung tissues
decreases lung compliance. However, an equally important
determinant of lung compliance is not the elasticity of the
lung tissues, but the surface tension at the air-water interfaces
within the alveoli.
The surface of the alveolar cells is moist, so the alveoli
can be pictured as air-fi
lled sacs lined with water. At an air-
water interface, the attractive forces between the water mol-
ecules, known as
surface tension,
make the water lining
like a stretched balloon that constantly tends to shrink and
resists further stretching. Thus, expansion of the lung requires
energy not only to stretch the connective tissue of the lung,
but also to overcome the surface tension of the water layer lin-
ing the alveoli.
Indeed, the surface tension of pure water is so great that
were the alveoli lined with pure water, lung expansion would
require exhausting muscular effort, and the lungs would tend
to collapse. It is extremely important, therefore, that the type
II alveolar cells secrete the detergent-like substance mentioned
earlier, known as
surfactant,
which markedly reduces the
cohesive forces between water molecules on the alveolar sur-
face. Therefore, surfactant lowers the surface tension, which
increases lung compliance and makes it easier to expand the
lungs (
Table 13–4
).
Surfactant is a mixture of both lipids and proteins, but
its major component is a phospholipid that forms a monomo-
lecular layer between the air and water at the alveolar surface.
The amount of surfactant tends to decrease when breaths are
small and constant. A deep breath, which people normally
intersperse frequently in their breathing pattern, stretches the
previous page 480 Vander's Human Physiology The Mechanisms of Body Function read online next page 482 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off