478
Chapter 13
Finally, note that the responses to high altitude are
essentially the same as the responses to hypoxia from any
other cause. Thus, a person with severe hypoxia from lung dis-
ease may show many of the same changes—increased hemato-
crit, for example—as a high-altitude sojourner.
Nonrespiratory Functions
of the Lungs
The lungs perform a variety of functions in addition to their
roles in gas exchange and regulation of H
+
concentration. Most
notable are the infl
uences they have on the arterial concen-
trations of a large number of biologically active substances.
Many substances (neurotransmitters and paracrine agents, for
example) released locally into interstitial fl uid may diffuse into
capillaries and thus make their way into the systemic venous
system. The lungs partially or completely remove some of these
substances from the blood and thereby prevent them from
reaching other locations in the body via the arteries. The cells
that perform this function are the endothelial cells lining the
pulmonary capillaries.
In contrast, the lungs may also produce new substances
and add them to the blood. Some of these substances play
local regulatory roles within the lungs, but if produced in
large enough quantity, they may diffuse into the pulmonary
capillaries and be carried to the rest of the body. For example,
infl ammatory responses (Chapter 18) in the lung may lead,
via excessive release of potent chemicals such as histamine, to
alterations of systemic blood pressure or fl ow. In at least one
case, the lungs contribute a hormone, angiotensin II, to the
blood (Chapter 14).
Finally, the lungs also act as a sieve that traps small blood
clots generated in the systemic circulation, thereby prevent-
ing them from reaching the systemic arterial blood where they
could occlude blood vessels in other organs.
This chapter has already described respiratory distress
syndrome of the newborn, asthma, chronic obstructive
pulmonary disease (COPD), and emphysema.
Acute
respiratory distress syndrome
(
ARDS
)
and
sleep apnea
are
fascinating disorders of lung function and breathing that
illustrate important characteristics of respiratory physiology.
Acute Respiratory Distress Syndrome (ARDS)
ARDS occurs mostly in adults, hence its alternate name,
adult respiratory distress syndrome. As opposed to respiratory
distress syndrome of the newborn, a defi ciency in alveolar
surfactant is the result of, not the cause of, ARDS. The
hallmark of ARDS is the leaking of protein from the
pulmonary blood vessels, which leads to an increase in the
movement of liquid into the lung. ARDS can be caused by a
wide variety of conditions, including pulmonary and systemic
infections, aspiration of stomach contents, and severe trauma.
The protein and fl uid leakage into the lung invariably leads
to a decrease in oxygen diffusion in the lung, leading to a
decrease in oxygen in the blood (hypoxemia). Patients with
ARDS are usually treated with supportive therapy, including
mechanical pulmonary ventilation and supplemental oxygen
therapy. Mortality from ARDS is very high (>50 percent),
although this appears to be declining due to improved
recognition and treatment.
Sleep Apnea
Sleep apnea is characterized by periodic cessation of
respiration during sleep, which results in hypoxia and
hypercapnia (asphyxia). In severe cases, this may occur
more than 20 times an hour. There are two general types
of sleep apnea.
Central sleep apnea
is primarily due to
a decrease in neural output from the respiratory center
in the medulla to the phrenic motor nerve output to the
diaphragm.
Obstructive sleep apnea
is caused by increased
ADDITIONAL CLINICAL EXAMPLES
airway resistance because of narrowing or collapse of the
upper airways (primarily the pharynx) during inspiration
(
Figure 13–44
). Obstructive sleep apnea may occur in as
much as 4 percent of the adult population and with a greater
frequency in the elderly and in men. Signifi cant snoring may
be an early sign of the eventual development of obstructive
sleep apnea. Obesity is clearly a contributing factor because
the excess fat in the neck decreases the diameter of the upper
airways. A decrease in the activity of the upper airway dilating
muscles, particularly during REM sleep, also contributes to
airway collapse. Finally, anatomical narrowing and increased
compliance of the upper airways contributes to periodic
inspiratory obstruction during sleep.
There are many serious consequences of untreated sleep
apnea, including hypertension of the pulmonary arteries
(pulmonary hypertension) and added strain on the right
ventricle of the heart. This can lead to heart failure and
abnormal heart rhythm, leading to increased mortality. The
periodic arousal that occurs during these apneic episodes
results in serious disruption of normal sleep patterns and
can lead to sleepiness during the day
(
daytime somnolence
).
Increased catecholamine release during these frequent arousals
can also contribute to the development of high blood pressure.
There are a variety of treatments for obstructive sleep
apnea. Surgery such as laser-assisted widening of the soft palate
and uvula can sometimes be of benefi t. Weight loss is often
quite helpful. However, the mainstay of therapy is
continuous
positive airway pressure
(
CPAP
)
(
Figure 13–45
). The
patient wears a small mask over the nose during sleep, which is
attached to a positive pressure-generating device. By increasing
airway pressure greater than
P
atm
, the collapse of the upper
airways during inspiration is prevented. Although the CPAP
nasal mask may seem obtrusive, many patients sleep much
better with it, and many of the symptoms resolve with this
treatment.
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