739
CHAPTER NINETEEN
Respiratory System
D
amage to the respiratory system from cig-
arette smoking is slow, progressive, and
deadly. A healthy respiratory system is
continuously cleansed. The mucus produced by the
respiratory tubules traps dirt and pathogens, which
cilia sweep toward the mouth, where they can be
eliminated. Smoking greatly impairs this house-
keeping. With the first inhalation of smoke, the
beating of cilia slows. With time, the cilia become
paralyzed and, eventually, disappear. The loss of
cilia leads to “smoker’s cough.” The cilia no longer
eF
ectively remove mucus, which must be coughed
up. Coughing is usually worse in the morning
because mucus has accumulated during sleep.
To make matters worse, smokers produce
excess mucus and it accumulates, clogging the air
passageways. Pathogens normally removed now
have easier access to the respiratory surfaces, and
the resulting lung congestion favors their growth.
This is why smokers have respiratory infections
more often than nonsmokers. A lethal chain reac-
tion begins. Cough leads to chronic bronchitis,
and increased mucus production and bronchial
lining thickening compromises breathing. The
smaller airways lose elasticity and are no longer
able to absorb the pressure changes of coughing.
A cough can increase the air pressure in the alve-
oli (microscopic air sacs) enough to rupture their
delicate walls. This is the beginning of smoking-
induced
emphysema.
The burst alveoli worsen the
cough, fatigue, wheezing, and impaired breathing.
Emphysema is ±
fteen times more common among
individuals who smoke a pack of cigarettes a day
than among nonsmokers.
Simultaneous with the structural changes
progressing to emphysema may be cellular
changes leading to lung cancer. ²irst, cells in
the outer border of the bronchial lining begin
to divide more rapidly than usual. Eventually,
these cells displace the ciliated cells. Their nuclei
begin to resemble those of cancerous cells—
large and distorted with abnormal numbers of
chromosomes. Up to this point, the damage can
be repaired if smoking ceases. If smoking contin-
ues, these cells may eventually break through the
basement membrane and begin dividing within
the lung tissue, forming a tumor with the poten-
tial of spreading throughout lung tissue (figs.
19A and 19B) and beyond, such as to the brain or
bones. While more than 80% of lung cancer cases
are due to cigarette smoking, only 20% of smok-
ers develop the cancer. Genetics may explain
this seeming discrepancy. A DNA sequence on
chromosome 15 affects part of the receptor for
nicotine on neurons. A certain variant of that
sequence increases the risk of developing the
most common type of lung cancer—but only in
smokers. Inheriting this DNA sequence does not
aF
ect cancer risk unless a person smokes.
It pays to quit. Much of the damage to the
respiratory system can be repaired. Cilia are
restored, and the thickening of alveolar walls due
to emphysema can be reversed. But ruptured
alveoli are gone forever. The nicotine in tobacco
smoke causes a powerful dependency by bind-
ing to certain receptors on brain cells.
19.1
CLINICAL APPLICATION
The Effects of Cigarette Smoking on the Respiratory System
Normal lung tissue
Cancerous lung tissue
Tumor
FIGURE 19B
Lung cancer may begin
as a tiny tumor growing in an alveolus, a
microscopic air sac (125×). This image is
falsely colored.
FIGURE 19A
The lung on the left is healthy. A cancerous tumor has invaded the lung on the
right, taking up nearly half of the lung space.
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