The basic rhythm of breathing is controlled by respiratory centers located in
This rhythm is modified in response to
input from sensory receptors and from other regions of the brain.
To understand how the respiratory centers control breathing to maintain
To examine how PCO2, pH, PO2, and other factors affect ventilation.
To understand the relationship between breathing and blood pH.
To explore the factors which stimulate increased ventilation during exercise.
Page 3. Homeostasis and
the Control of Respiration
Fill out the chart to the right as you proceed through this page.
The control of respiration is tied to the principle of homeostasis.
Recall that the body maintains homeostasis through homeostatic control
mechanisms, which have three basic components:
2. control centers
The principal factors which control respiration are chemical factors in
Changes in arterial PCO2,
PO2 and pH are monitored by sensory receptors called
The chemoreceptors send sensory input to respiratory centers in the
brainstem, which determine the
appropriate response to the changing variables.
These centers then send nerve impulses to the effectors, the respiratory
muscles, to control the force and frequency of contraction.
This changes the ventilation, the rate and depth of breathing.
Ventilation changes restore the arterial blood gases and pH to their
Page 4. Inspiratory
Label the diagram to the right.
The basic rhythm of breathing is controlled by respiratory centers
located in the medulla and pons of the brainstem.
Within the medulla, a paired group of neurons known as the inspiratory
center, or the dorsal respiratory group, sets the basic rhythm by
automatically initiating inspiration.
inspiratory center sends nerve impulses along the phrenic nerve to the diaphragm
and along the intercostal nerves to the external intercostal muscles.
nerve impulses to the diaphragm and the external intercostal muscles continue
for a period of about 2 seconds. This stimulates the inspiratory muscles to
contract, initiating inspiration.
neurons stop firing for about 3 seconds, which allows the muscles to relax. The
elastic recoil of the lungs and chest wall leads to expiration.
automatic, rhythmic firing produces the normal resting breathing rate, ranging
between 12 and 15 breaths per minute.
Page 8. Predict the
Effect of Increased PCO2
Fill in the diagram to the right:
What will happen to the breathing rate and depth if the arterial PCO2
An increase in the PCO2
in the blood leads to an increase in hydrogen ions in the cerebrospinal
fluid, decreasing the pH.
The central chemoreceptors fire more frequently, sending more nerve
impulses to the respiratory centers, which in turn send more nerve
impulses to the respiratory muscles.
This results in an increased breathing rate and depth, allowing more
carbon dioxide to be exhaled, returning the blood PCO2
to normal levels.
Page 9. Peripheral
Chemoreceptors: Effect of pH Changes
peripheral chemoreceptors also respond to pH changes caused by PCO2 changes, however they directly monitor changes in the
arterial blood, not the cerebrospinal fluid as the central chemoreceptors do.
• The role
of the peripheral chemoreceptors:
Increased carbon dioxide levels in the arterial blood result in decreased blood pH, which stimulates the peripheral
They respond by sending more nerve impulses to the respiratory centers, which
stimulate the respiratory muscles, causing faster and deeper breathing.
More carbon dioxide is exhaled, which
drives the chemical reaction to the left and returns the PCO2
and pH to normal levels.
Fill in the diagram to the right:
The peripheral chemoreceptors also respond to acids such as lactic acid,
which is produced during strenuous exercise:
• Active muscles
produce lactic acid, which enters the blood, releases hydrogen ions, and
lowers the pH.
• The decreased pH
stimulates the peripheral chemoreceptors to send more nerve impulses to
the respiratory centers, which stimulate the respiratory muscles to
increase the breathing rate and depth.
• More carbon
dioxide is exhaled, lowering the PCO2
in blood, driving the chemical reaction to the left, and lowering hydrogen
Page 11. Hyperventilation
What changes will occur if a person hyperventilates, that is, breathes
deeper and faster than necessary for normal gas exchange?
During hyperventilation, carbon dioxide is exhaled, lowering the PCO2.
This drives the chemical reaction to the left, decreasing the hydrogen ion
concentration, and increasing pH:
Since the PCO2
is low, the central chemoreceptors send fewer impulses to the respiratory
Since the pH is high, the peripheral chemoreceptors also send fewer
impulses to the respiratory centers, which send fewer nerve impulses to
the respiratory muscles, thereby further decreasing breathing rate and
depth and returning the arterial gases and pH to normal levels.
Hyperventilation does not normally cause an increase in the oxygen levels in the
blood, because oxygen is poorly soluble in blood and normally hemoglobin in
arterial blood is saturated with oxygen already.
Page 12. Hypoventilation
Now predict what changes will occur if a person hypoventilates.
Hypoventilation occurs when the breathing rate and depth is too low to
maintain normal blood gas levels.
During hypoventilation, not enough oxygen is inhaled, so
decreases. In addition, carbon dioxide builds up in the blood, increasing
the PCO2. This drives the
chemical reaction to the right, increasing the H+ concentration and decreasing pH.
The PO2 drops, but not
enough to stimulate the peripheral chemoreceptors.
The high PCO2
stimulates the central chemoreceptors to send more impulses to the
A decrease in pH stimulates the peripheral chemoreceptors, which also send
more nerve impulses to the respiratory centers, which stimulate the
respiratory muscles, increasing the breathing rate and depth.
This allows oxygen to be inhaled, carbon dioxide to be exhaled, and drives
the chemical reaction to the left, returning the arterial gases and pH to
Page 13. Summary:
Effects of PO2,
pH, and PCO2
chart summarizes how the three major chemical factors - PO2,
pH, and PCO2 - modify breathing rate and depth.
• When the
drops below 60 millimeters of mercury, the peripheral chemoreceptors send
nerve impulses to the respiratory centers. The respiratory centers send nerve
impulses to the respiratory muscles, increasing ventilation. More oxygen is
inhaled, returning the PO2
to normal levels.
cells release acids into the blood,
the acids release hydrogen ions, which lower the pH. This stimulates the
peripheral chemoreceptors to send more nerve impulses to the respiratory
centers. They, in turn, send more nerve impulses to the respiratory muscles,
increasing ventilation. More carbon
dioxide is exhaled, which returns the pH to normal levels.
increase in PCO2
leads to a decreased pH in the blood, which stimulates the peripheral
chemoreceptors to send more nerve impulses to the respiratory centers. In
addition, the increased PCO2 leads to a
decreased pH within the cerebrospinal fluid of the fourth ventricle. This
stimulates the central chemoreceptors to send more nerve impulses to the
respiratory centers. The respiratory centers send more nerve impulses to the
respiratory muscles, which increase breathing rate and depth. More
carbon dioxide is exhaled, returning the PCO2 and pH to normal levels.
Page 14. Other Factors
Which Influence Ventilation
other factors influence ventilation. These factors include:
By sending signals from the cerebral cortex to the respiratory muscles, we can
voluntarily change our breathing rate and depth when holding our breath,
speaking, or singing.
However, chemoreceptor input to the respiratory centers will eventually override
conscious control and force you to breathe.
Pain and emotions.
Pain and strong emotions, such as fear and anxiety, act by way of the
hypothalamus to stimulate or inhibit the respiratory centers.
Laughing and crying also significantly alter ventilation.
Dust, smoke, noxious fumes, excess mucus and other irritants stimulate receptors
in the airways.
This initiates protective reflexes, such as coughing and sneezing, which
forcibly remove the irritants from the airway.
Stretch receptors in the visceral pleura and large airways send inhibitory
signals to the inspiratory center during very deep inspirations, protecting
against excessive stretching of the lungs. This is known as the inflation, or
Page 15. Exercise and
in ventilation during exercise:
Ventilation increases during strenuous exercise, with the depth increasing more
than the rate.
It appears that changes in PCO2
do not play a significant role in stimulating this increased ventilation.
the precise factors which stimulate increased ventilation during exercise are
not fully understood, they probably include:
Ventilation increases within seconds of the beginning of exercise, probably in
anticipation of exercise, a learned response.
Neural input from the motor cortex.
The motor areas of the cerebral cortex which stimulate the muscles also
stimulate the respiratory centers.
Receptors in muscles and joints.
in moving muscles and joints stimulate the respiratory centers.
Increased body temperature.
An increase in body temperature stimulates the respiratory centers.
Circulating epinephrine and norepinephrine.
Circulating epinephrine and norepinephrine secreted by the adrenal medulla
stimulates the respiratory centers.
changes due to lactic acid
Lactic acid, produced by exercising muscles, is another stimulus.
Page 16. Summary
basic rhythm of breathing is set by the inspiratory center, located in the
medulla. Other respiratory centers, located in the medulla and pons, also
Chemoreceptors monitor the PCO2,
pH, and PO2 of arterial blood
and alter the basic rhythm of breathing.
dioxide, reflected by changes in pH,
is the most important stimulus controlling ventilation.
changes due to metabolic acids also alter ventilation.
stimulates ventilation only when the blood PO2
is very low.
factors, such as voluntary control, pain and emotions, pulmonary irritants, and
lung hyperinflation, also play roles in controlling ventilation.
The control of ventilation during exercise, while complex and not fully
understood, involves multiple inputs including chemical and neural factors.