558 Chapter 16
ventilation to acclimatize to high altitude, where a beneficial
hyperventilation occurs.
Conversely, persistently elevated arterial P^ CO 2 enhances the
sensitivity of the carotid bodies to a fall in P^ O 2. Breathing in
response to lowered P^ O 2 is important in emphysema, where the
chemoreceptor response to chronically elevated carbon dioxide
becomes blunted. This blunting results from increased secretion
of bicarbonate by the choroid plexus (chapter 8), buffering the
fall in pH. People with emphysema are consequently stimulated
to breathe by a hypoxic drive (due to low P^ O 2 ) rather than by the
elevated P^ CO 2. Over a long period of time, however, the hypoxic
drive in people with emphysema can also become blunted.
The effects of changes in the blood P^ CO 2 , pH, and P^ O 2 on
chemoreceptors and the regulation of ventilation are summa-
rized in table 16.6.
produce an initial apnea, followed by rapid, shallow breath-
ing when a person eats these peppers or inhales pepper spray.
Irritant receptors in the wall of the larynx, and receptors in
the lungs identified as rapidly adapting receptors, can cause
a person to cough in response to components of smoke and
smog, and to inhaled particulates. The rapidly adapting recep-
tors in the lungs are stimulated most directly by an increase
in the amount of fluid in the pulmonary interstitial tissue.
Because the same chemicals that stimulate the unmyelinated C
fibers can cause increased pulmonary interstitial fluid (due to
extravasation from pulmonary capillaries), a person may also
cough after eating hot peppers.
The Hering-Breuer reflex is stimulated by pulmonary
stretch receptors. The activation of these receptors dur-
ing inspiration inhibits the respiratory control centers, mak-
ing further inspiration increasingly difficult. This helps to
prevent undue distension of the lungs and may contribute to
the smoothness of the ventilation cycles. A similar inhibitory
reflex may occur during expiration. The Hering-Breuer reflex
appears to be important in maintaining normal ventilation inTable 16.6 | Sensitivity of Chemoreceptors to Changes in Blood Gases and pH
Stimulus Chemoreceptor Comments↑ P^ CO 2 Medullary chemoreceptors;
aortic and carotid bodiesMedullary chemoreceptors are sensitive to the pH of cerebrospinal fluid (CSF). Diffusion
of CO 2 from the blood into the brain lowers the pH of brain interstitial fluid by forming
carbonic acid. Similarly, the aortic and carotid bodies are stimulated by a fall in blood pH
induced by increases in blood CO 2.↓pH Aortic and carotid bodies Peripheral chemoreceptors are stimulated by decreased blood pH independent of the
effect of blood CO 2. Chemoreceptors in the medulla are not affected by changes in
blood pH because H^1 cannot cross the blood-brain barrier.↓ P^ O 2 Carotid bodies Low blood P^ O^2 (hypoxemia) augments the chemoreceptor response to increases in blood
P^ CO 2 and can stimulate ventilation directly when the P^ O 2 falls below 70 mmHg.CLINICAL APPLICATION
Sudden infant death syndrome ( SIDS ) is the sudden
death of an infant under one year old that cannot be other-
wise explained. Sometimes called crib death, it most often
strikes infants between the ages of two and four months.
The cause of SIDS is unknown, but evidence suggests it
may be due to a failure of the central or peripheral chemore-
ceptors to detect a rise in carbon dioxide. The incidence of
SIDS has been falling significantly since the American Acad-
emy of Pediatrics recommended that parents put infants to
sleep on their backs rather than on their stomachs, and that
infants sleep on a firm surface. However, SIDS still remains
the leading cause of death in infants younger than one year.Effects of Pulmonary Receptors
on Ventilation
The lungs contain various types of receptors that influence the
brain stem respiratory control centers via sensory fibers in the
vagus nerves. Unmyelinated C fibers are sensory neurons in
the lungs that can be stimulated by capsaicin, the chemical in
hot peppers that creates the burning sensation. These receptors
CLINICAL APPLICATION
Obstructive sleep apnea is a condition in which there are
15 or more periods of apnea during sleep caused by tem-
porary blockage of the upper airway. It is more common in
men than women, and obesity is a significant risk factor.
The upper airway of the oropharynx is normally kept open
by contractions of over 20 different skeletal muscles. When
the pharyngeal air passages narrow during sleep to cause
apnea, the arterial PO 2 and oxyhemoglobin saturation fall and
the arterial PCO 2 rises to stimulate chemoreceptor reflexes,
which can cause the apnea to end with a gasp and jerk.
Snoring, sleepiness and fatigue during the day, and a morn-
ing headache caused by cerebral vasodilation, are some of
the other symptoms. Dangers include pulmonary hyperten-
sion that results in right ventricle hypertrophy. People with
obstructive sleep apnea must often wear CPAP ( continuous
positive airway pressure ) devices when they sleep to keep
the oropharynx air passage open.