450
Chapter 13
inspiratory intercostal muscles located between the ribs. The
adjective
inspiratory
here is a functional term, not an anatomi-
cal one. It denotes the several groups of intercostal muscles that
contract during inspiration. The diaphragm is the most impor-
tant inspiratory muscle that acts during normal quiet breath-
ing. When activation of the
phrenic nerves
to the diaphragm
causes it to contract, its dome moves downward into the abdo-
men, enlarging the thorax (
Figure 13–14
). Simultaneously,
activation of the intercostal nerves to the inspiratory inter-
costal muscles causes them to contract, leading to an upward
and outward movement of the ribs and a further increase in
thoracic size.
The crucial point is that contraction of the inspiratory
muscles, by
actively
increasing the size of the thorax, upsets
the stability set up by purely elastic forces between breaths. As
the thorax enlarges, the thoracic wall moves ever so slightly
End of expiration
No flow
P
ip
= – 4
P
alv
= 0
P
atm
= 0
End of inspiration
No flow
P
ip
= – 7
P
alv
= 0
P
atm
= 0
Mid-inspiration
Air flow
Air flow
P
ip
= – 6
P
alv
= – 1
P
atm
= 0
Mid-expiration
P
ip
= – 5
P
alv
= 1
P
atm
= 0
4
2
3
1
P
tp
P
alv
2
P
atm
4
2
3
1
1
0
V
a
rio
us
pre
ssu
re
s
d
u
ring
b
re
a
thing (mmHg)
–2
–4
–6
0
4 s
Time
0.5
P
ip
Bre
a
th vol
u
me (L)
Inspiration Expiration
Elastic recoil
force
Inspiratory
muscle force
P
tp
= 4
P
tp
= 5
P
tp
= 7
P
tp
= 6
Figure 13–13
Summary of alveolar (
P
alv
), intrapleural (
P
ip
), and transpulmonary (
P
tp
) pressure changes and air fl ow during a typical respiratory cycle. At the
end of expiration
1
,
P
alv
is equal to
P
atm
and there is no air fl ow. At mid-inspiration
2
, the chest wall is expanding, lowering
P
ip
and making
P
tp
more positive. This expands the lung making
P
alv
negative and results in an inward air fl
ow. At end-inspiration
3
, the chest wall is no longer
expanding but has yet to start passive contraction. Because lung size is not changing and the glottis is open to the atmosphere,
P
alv
is equal to
P
atm
and there is no air fl ow. As the respiratory muscles relax, the lungs and chest wall start to passively collapse due to elastic recoil. At mid-
expiration
4
, the lung is collapsing, thus compressing alveolar gas. As a result,
P
alv
is positive relative to
P
atm
, and air fl ow is outward. The cycle
starts over again at the end of expiration. Notice that throughout a typical respiratory cycle with a normal tidal volume,
P
ip
is negative relative
to
P
atm
. In the graph on the left, the difference between
P
alv
and
P
ip
at any point along the curves is equivalent to
P
tp
.
Figure 13–13
physiological
inquiry
How do the changes in
P
tp
between each step (
1
4
) explain whether the volume of the lung is increasing or decreasing?
Answer can be found at end of chapter.
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