Chapter 22
The Respiratory System
837
22
even if P
CO
2
is normal. In this way, the peripheral chemorecep-
tor system can maintain ventilation even though the brain stem
centers are depressed by hypoxia.
Influence of Arterial pH
Changes in arterial pH can modify
respiratory rate and rhythm even when CO
2
and O
2
levels are
normal. Because H
1
does not cross the blood brain barrier, the
increased ventilation that occurs in response to falling arterial
pH is mediated through the peripheral chemoreceptors.
Although changes in P
CO
2
and H
1
concentration are inter-
related, they are distinct stimuli. A drop in blood pH may reflect
CO
2
retention, but it may also result from metabolic causes,
such as accumulation of lactic acid during exercise or of fatty
acid metabolites (ketone bodies) in patients with poorly con-
trolled diabetes mellitus. Regardless of cause, as arterial pH de-
clines, respiratory system controls attempt to compensate and
raise the pH. Tey do this by increasing respiratory rate and
depth to eliminate CO
2
(and carbonic acid) from the blood.
Summary of Interactions of P
CO
2
, P
O
2
, and Arterial pH
Al-
though every cell in the body must have O
2
to live, the body’s
need to rid itself of CO
2
is the most important stimulus for
breathing in a healthy person. However, CO
2
does not act in
isolation, and various chemical factors enforce or inhibit one
another’s effects. Tese interactions are summarized here:
Rising CO
2
levels are the most powerful respiratory stimulant.
As CO
2
is hydrated in brain tissue, liberated H
1
acts directly
on the central chemoreceptors, causing a reflexive increase in
breathing rate and depth. Low P
CO
2
levels depress respiration.
Under normal conditions, blood
P
O
2
affects breathing only in-
directly by influencing peripheral chemoreceptor sensitivity to
changes in
P
CO
2
. Low P
O
2
augments P
CO
2
effects, and high P
O
2
levels diminish the effectiveness of CO
2
stimulation.
When arterial
P
O
2
falls below 60 mm Hg, it becomes the ma-
jor stimulus for respiration, and ventilation is increased via
reflexes initiated by the peripheral chemoreceptors.
Tis may
increase O
2
loading into the blood, but it also causes hypo-
capnia (low P
CO
2
blood levels) and an increase in blood pH,
both of which inhibit respiration.
Changes in arterial pH resulting from CO
2
retention or meta-
bolic factors act indirectly through the peripheral chemorecep-
tors to alter ventilation, which in turn modifies arterial
P
CO
2
and pH.
Arterial pH does not influence the central chemore-
ceptors directly.
Influence of Higher Brain Centers
Hypothalamic Controls
Acting through the hypothalamus
and the rest of the limbic system, strong emotions and pain send
signals to the respiratory centers, modifying respiratory rate and
depth. For example, have you ever touched something cold and
clammy and gasped? Tat response was mediated through the
hypothalamus. So too is the breath holding that occurs when we
are angry and the increased respiratory rate that occurs when
we are excited. A rise in body temperature raises the respiratory
rate, while a drop in body temperature produces the opposite
effect. Sudden chilling (a dip in the North Atlantic Ocean in late
October) can stop your breathing (apnea)—or at the very least,
leave you gasping.
Cortical Controls
Although the brain stem respiratory cen-
ters normally regulate breathing involuntarily, we can also exert
conscious (voluntary) control over the rate and depth of our
breathing. We can choose to hold our breath or take an extra-
deep breath, for example. During voluntary control, the cerebral
motor cortex sends signals to the motor neurons that stimulate
the respiratory muscles, bypassing the medullary centers.
Our ability to voluntarily hold our breath is limited, however,
because the brain stem respiratory centers automatically reini-
tiate breathing when the blood concentration of CO
2
reaches
critical levels. Tat explains why drowning victims typically
have water in their lungs.
Brain
Sensory nerve fiber in cranial nerve
IX (pharyngeal branch
of glossopharyngeal)
External carotid artery
Internal carotid artery
Carotid body
Common carotid artery
Cranial nerve X (vagus nerve)
Sensory nerve fiber in
cranial nerve X
Aortic bodies
in aortic arch
Aorta
Heart
Figure 22.26
Location and innervation of the peripheral
chemoreceptors in the carotid and aortic bodies.
previous page 871 Human Anatomy and Physiology (9th ed ) 2012 read online next page 873 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off