Conversely changes in oxygen concentration in the

Conversely, changes in oxygen concentration
in the blood are detected by peripheral chemore-
ceptor regions in the aorta (aortic body in the aor-
tic arch) and in the fork of the common carotid
artery (the carotid body). These, in turn, send
impulses to the medulla when the blood oxygen
level is dropping. When oxygen levels are low,
these same chemoreceptors are also able to detect
high carbon dioxide levels. Although every cell in
the body must have oxygen to live, it is the body’s
need to rid itself of carbon dioxide that is the most
important stimulus for breathing. A decrease in the
oxygen level becomes an important stimulus only
when the level is dangerously low.
Homeostatic Imbalance 13.11
In people who retain carbon dioxide, such as peo-
ple with emphysema, chronic bronchitis, or other
chronic lung diseases, the brain no longer recog-
nizes an increased level of carbon dioxide as impor-
tant. In such cases, a dropping oxygen level becomes
the respiratory stimulus. This interesting fact explains
why such patients are always given a low level of
oxygen; it helps to maintain the prevailing stimulus
for breathing (a low oxygen level).
_____________ ✚
The respiratory system in healthy individuals has
homeostatic mechanisms. As carbon dioxide or other
sources of acids begin to accumulate in the blood and
tissues, and pH starts to drop, you begin to breathe
more deeply and more rapidly. Hyperventilation is
an increase in the rate and depth of breathing that

Chapter 13: The Respiratory System 455
13
Lung Cancer
Lung cancer is the leading cause of cancer death
for both men and women in North America, caus-
ing more deaths than breast, prostate, and colorec-
tal cancer combined. This is tragic, because lung
cancer is largely preventable—nearly 90 percent of
lung cancers result from smoking. Lung cancer is
aggressive and metastasizes rapidly and widely, so
most cases are not diagnosed until they are well
advanced. The cure rate for lung cancer is notori-
ously low; most victims die within 1 year of diag-
nosis. The overall 5-year survival of people with
lung cancer is about 17 percent.
Ordinarily, nasal hairs, sticky mucus, and the
action of cilia do a fine job of protecting the lungs
from irritants, but smoking overwhelms these cleans-
ing devices, and they eventually stop functioning.
Continuous irritation prompts the production of
more mucus, but smoking slows the movements of
cilia that clear this mucus and depresses lung mac-
rophages. One result is a pooling of mucus in the
lower respiratory tract and an increased frequency
of pulmonary infections, including pneumonia and
COPD. However, it is the irritating effects of the
“cocktail” of toxic chemicals in tobacco smoke that
ultimately lead to lung cancer.
The three most common types of lung cancer
are (1) adenocarcinoma (40 percent of cases),
which originates as solitary nodules in peripheral
Homeostatic Imbalance 13.13
The respiratory system is particularly vulnerable to
infections because it is open to airborne patho-
gens. We have already considered some of these
inflammatory conditions, such as rhinitis and ton-
sillitis. Now we will turn our attention to the most
disabling respiratory disorders, the group of dis-
eases collectively referred to as chronic obstructive
pulmonary disease (COPD) and lung cancer . These
disorders are “living proof” of cigarette smoking’s
devastating effects on the body. Long known to