756
UNIT FIVE
striction. Breathing becomes increasingly dif
cult,
and inhalation produces a characteristic wheezing
sound as air moves through narrowed passages.
A person with asthma usually F
nds it harder
to ±orce air out o± the lungs than to bring it in. This
is because inspiration uses power±ul breathing
muscles, and, as they contract, the lungs expand,
opening the air passages. Expiration, on the other
hand, is a passive process due to elastic recoil o±
stretched tissues. Expiration also compresses the
tissues and constricts the bronchioles, ±urther
impairing air movement through the narrowed
air passages.
Increase in the prevalence o± asthma in the
United States may be due to a too-clean environ-
ment, especially ±or children. Many studies have
shown that children who are with others and
contract minor respiratory in±ections, as well as
children raised with cats or dogs, are less likely
to develop asthma than are children who do
not have these exposures. This association o± a
primed immune system with lower risk o± devel-
oping asthma is called the hygiene hypothesis.
I
njuries to the respiratory center or to spinal
nerve tracts that transmit motor impulses
may paralyze breathing muscles. Paralysis
may also be due to a disease that a²
ects the cen-
tral nervous system and injures motor neurons,
such as
poliomyelitis.
Sometimes, other muscles,
by increasing their responses, can compen-
sate ±or ±unctional losses o± a paralyzed muscle.
Otherwise, mechanical ventilation is necessary.
More common disorders that decrease ventila-
tion are bronchial asthma and emphysema.
Bronchial asthma
is usually an allergic reac-
tion to ±oreign antigens in the respiratory tract,
such as ±rom inhaled pollen or material on dust
mites. Cells o± the larger airways secrete abundant
mucus, which traps allergens. Ciliated columnar
epithelial cells move the mucus up and out o± the
bronchi, then up and out o± the trachea, clear-
ing the upper respiratory structures. However, in
the lower respiratory areas, mucus drainage plus
edematous secretions accumulate because ±ewer
cells are ciliated. The allergens and secretions irri-
tate smooth muscles, stimulating bronchocon-
Emphysema
is a progressive, degenerative
disease that destroys many alveolar walls. As a
result, clusters o± small air sacs merge into larger
chambers, which decreases the total sur±ace area
o± the alveolar walls. At the same time, the alveolar
walls lose their elasticity, and the capillary networks
associated with the alveoli diminish (F
g. 19D).
A person with emphysema ±inds it increas-
ingly dif
cult to ±orce air out o± the lungs because
o± the loss o± tissue elasticity. Abnormal muscular
orts are required to compensate ±or the lack o±
elastic recoil that normally contributes to expira-
tion. Only 3% o± the 2 million people in the United
States who have emphysema inherit the condition;
the majority o± the other cases are due to smoking
or exposure to other respiratory irritants.
An experimental treatment ±or severe
emphysema is lung volume reduction surgery.
As its name suggests, the procedure reduces lung
volume, which opens collapsed airways and eases
breathing. So ±ar, it seems to noticeably improve
lung ±unction (as measured by distance walked in
six minutes) and quality o± li±e.
19.3
CLINICAL APPLICATION
Respiratory Disorders That Decrease Ventilation: Bronchial Asthma
and Emphysema
Current evidence suggests that the basic rhythm of
breathing arises from the
ventral respiratory group
. Two dif-
ferent populations of neurons in this area have been impli-
cated in maintaining inspiration and expiration, depending
The
medullary respiratory center
includes two bilateral
groups of neurons that extend throughout the length of the
medulla oblongata. They are called the ventral respiratory
group and the dorsal respiratory group.
FIGURE 19D
Comparison o± lung tissues. (
a
) Normal lung tissue (100×). (
b
) As emphysema develops, alveoli coalesce, ±orming larger
chambers (100×).
(a)
(b)
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