818
UNIT 4
Maintenance of the Body
22
A good example is an inflated automobile tire. Te tire is
hard and strong enough to bear the weight of a car because air is
compressed to about one-third of its atmospheric volume inside
the tire, providing high pressure. Now let’s see how this relates
to inspiration and expiration.
Inspiration
Visualize the thoracic cavity as a gas-filled box with a single
entrance at the top, the tubelike trachea. Te volume of this box
is changeable and can be increased by enlarging all of its dimen-
sions, thereby decreasing the gas pressure inside it. Tis drop in
Sequence of events
Changes in anterior-posterior and
superior-inferior dimensions
Changes in lateral dimensions
(superior view)
Ribs are
elevated and
sternum flares
as external
intercostals
contract.
Diaphragm moves
inferiorly during
contraction.
External
intercostals
contract.
Ribs and
sternum are
depressed as
external
intercostals
relax.
External
intercostals
relax.
Diaphragm
moves
superiorly
as it relaxes.
Inspiration
Expiration
1
Inspiratory muscles contract
(diaphragm descends; rib cage
rises).
2
Thoracic cavity volume
increases.
3
Lungs are stretched;
intrapulmonary volume increases.
4
Intrapulmonary pressure
drops (to –1 mm Hg).
5
Air (gases) flows into lungs
down its pressure gradient until
intrapulmonary pressure is 0
(equal to atmospheric pressure).
1
Inspiratory muscles relax
(diaphragm rises; rib cage
descends due to recoil of costal
cartilages).
2
Thoracic cavity volume
decreases.
3
Elastic lungs recoil passively;
intrapulmonary volume
decreases.
4
Intrapulmonary pressure rises
(to +1 mm Hg).
5
Air (gases) flows out of lungs
down its pressure gradient
until intrapulmonary pressure is 0.
Figure 22.13
Changes in thoracic
volume and sequence of events during
inspiration and expiration.
The sequence
of events in the left column includes
volume changes during inspiration (top)
and expiration (bottom). The lateral views
in the middle column show changes in the
superior-inferior dimension (as the diaphragm
alternately contracts and relaxes, see
black arrows) and in the anterior-posterior
dimension (as the external intercostal muscles
alternately contract and relax). The superior
views of transverse thoracic sections in the
right column show lateral dimension changes
resulting from alternate contraction and
relaxation of the external intercostal muscles.
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