765
CHAPTER NINETEEN
Respiratory System
Breathing becomes dif
cult. Untreated, pneumo-
nia can kill.
Tuberculosis
is a diF
erent type o± lung in±ec-
tion, caused by the bacterium
Mycobacterium
tuberculosis
. ²ibrous connective tissue develops
around the sites o± in±ection, ±orming structures
called
tubercles
g. 19E)
.
By walling oF
the bacte-
ria, the tubercles help stop their spread. Sometimes
this protective mechanism ±ails, and the bacteria
´
ourish throughout the lungs and may spread to
other organs. In the later stages o± in±ection, other
types o± bacteria may cause secondary in±ections.
As lung tissue is destroyed, the sur±ace area ±or gas
exchange decreases. In addition, the widespread
³
brous tissue thickens the respiratory membrane,
±urther restricting gas exchange. A variety o± drugs
are used to treat tuberculosis, but in recent years,
strains resistant to drugs have arisen, and these
can be swi±tly deadly.
Another type o± condition that impairs gas
exchange is
atelectasis.
This is the collapse o± a
lung, or some part o± it, with the collapse o± the
blood vessels that supply the a±±ected region.
Obstruction o± a respiratory tube, such as by an
inhaled ±oreign object or excess mucus secre-
tion, may cause atelectasis. The air in the alveoli
F
ive-year-old Carly became ill very quickly.
A±ter twelve hours o± ±lulike symptoms,
her temperature soared, her chest began
to hurt, and her breathing became rapid and
shallow. A chest radiograph indicated that she
had
pneumonia.
The bacteria that had caused a
mild upper respiratory in±ection in her sisters had
in±ected her lower respiratory structures.
Antibiotics treated Carly’s pneumonia.
Many o± the 50 million people who died in the
1918 in´
uenza pandemic were not so lucky, in
that era be±ore antibiotics. Although viruses can
cause pneumonia, most o± the ´
u deaths were
actually caused by secondary bacterial in±ec-
tions. Bacteria that normally inhabit upper respi-
ratory structures easily moved downward to the
lungs made vulnerable by the viral in±ection.
²ungi can also cause pneumonia, such as the
±ungus
Pneumocystis jirovecii
that aF
ects people
with AIDS.
²or all types o± pneumonia, events in the
in±ected lung are similar: alveolar linings swell
with edema and become abnormally permeable,
allowing ±luids and white blood cells to accu-
mulate in the air sacs. As the alveoli ³
ll, the sur-
±ace area available ±or gas exchange diminishes.
beyond the obstruction is absorbed, and as the
air pressure in the alveoli decreases, their elastic
walls collapse, and they can no longer ±unction.
²ortunately, a±ter a portion o± a lung collapses,
the ±unctional regions that remain are o±ten able
to carry on enough gas exchange to sustain the
body cells.
In
acute respiratory distress syndrome
(ARDS),
which is a special ±orm o± atelectasis, alveoli col-
lapse. Causes include pneumonia and other
in±ections, near drowning, shock, sepsis, aspira-
tion o± stomach acid into the respiratory system,
or physical trauma to the lungs ±rom an injury or
surgical procedure. Anesthetic drugs can cause
ARDS by suppressing sur±actant production. Until
sur±actant levels return to normal—in a day or
two—the respiratory membrane that separates
the air in the alveoli ±rom the blood in the pulmo-
nary capillaries is damaged enough to allow pro-
tein-rich ´
uid to escape ±rom the capillaries and
´
ood the alveoli. In response the tiny air sacs col-
lapse. Blood vessels and airways narrow, greatly
elevating blood pressure in the lungs. Delivery
o± oxygen to tissues is seriously impaired. ARDS is
±atal about 60% o± the time. (It was until recently
called adult respiratory distress syndrome.)
19.6
CLINICAL APPLICATION
Disorders That Impair Gas Exchange: Pneumonia, Tuberculosis, and Adult
Respiratory Distress Syndrome
Tuberculosis
Healthy lungs
FIGURE 19E
Healthy lungs appear dark and clear on a radiograph. Lungs with tuberculosis have cloudy areas where ³
brous tissue grows,
walling oF
in±ected areas.
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