Download Regulation of Respiration During Exercise

Regulation of breathing
 What makes the
inspiratory muscles
contract and relax
rhythmically?
 How could the
respiratory activity be
modified?
 How could the
expiratory muscles be
called on during
active expiration?
 How could the arterial
PO2 and PCO2 be
maintained within
narrow limits?
 What is the role of the
respiratory system in
regulating blood H+
concentration?
What is
This
Lecture
About?
To answer these questions we need to understand:
The Neural
&
Chemical
Control of Respiration
Neural control
• The motor neurons that stimulate the
respiratory muscles are controlled by two
major descending pathways: one that
controls voluntary breathing and another
that controls involuntary breathing.
• The unconscious rhythmic control of
breathing is influenced by sensory
feedback from receptors sensitive to the
PCO , pH, and PO of arterial blood.
2
2
• Inspiration and expiration are produced by
the contraction and relaxation of skeletal
muscles in response to activity in somatic
motor neurons in the spinal cord.
• The activity of these motor neurons is
controlled, by descending tracts from
neurons in the respiratory control centers
in the medulla oblongata and from
neurons in the cerebral cortex.
Neural control of Respiration
anterior
The Rhythm:
inspiration followed
by expiration
Fairly normal ventilation
retained if section above
medulla
Ventilation ceases if section
below medulla
 medulla is major
rhythm generator
Neural control of Respiration
It is believed that the
breathing rhythm is
generated by a network of
neurons called the PreBrotzinger complex.
These neurons display
pacemaker activity. They
are located near the upper
end of the medullary
respiratory centre
What gives rise to inspiration?
Dorsal respiratory
group neurones
(inspiratory)
PONS
Fire in bursts
Firing leads to
contraction of
inspiratory muscles
- inspiration
When firing stops,
passive expiration
MEDULLA
SPINAL CORD
What about “active” expiration
during hyperventilation?
Increased firing
of dorsal
neurones excites
a second group:
Ventral
respiratory
group neurones
In normal quiet
breathing, ventral
neurones do not
activate expiratory
muscles
Excite internal
intercostals,
abdominals etc
Forceful expiration
The rhythm generated in the
medulla can be modified by
neurones in the pons:
“pneumotaxic
centre” (PC)
Stimulation
terminates inspiration
PC stimulated when
dorsal respiratory
neurones fire
Inspiration inhibited
+
-
The “apneustic centre”
Apneustic centre
Impulses from
these neurones
excite
inspiratory
area of
medulla
Prolong inspiration
Conclusion?
Rhythm generated in
medulla modified by inputs
from pons
Reflex modification of breathing
Pulmonary stretch receptors
Activated during inspiration, afferent discharge inhibits
inspiration - Hering-Breuer reflex
Do they switch off inspiration during normal respiratory cycle?
Unlikely - only activated at large >>1 -1.5 litre tidal volumes
Maybe important in new born babies
May prevent over-inflation lungs during hard exercise?
Chemical Control of Respiration
An example of a negative feedback control
system
Chemoreceptors sense the values of the gas
tensions
central and
peripheral
 The activity of the respiratory centers is
regulated by the O2, CO2 and H+ content of the
blood.
Chemical regulation of respiration
Central chemoreceptors
• Carbon dioxide and H+ are most important.
•
CO2 dissolves in cerebrospinal fluid (CSF) which bathes receptors
sensitive to H+ on the ventral aspect of the medulla. Stimulation of these
receptors is responsible for about 70% of the increase in the rate and
depth of respiration in response to increased CO2.
 Respond to the [H+] of the cerebrospinal fluid (CSF)
 CSF is separated from the blood by the blood-brain barrier
 Relatively impermeable to H+ and HCO3 CO2 diffuses readily
 CSF contains less protein than blood and hence is less buffered
than blood
Increase in CO2 increases
H+ concentration in CSF
(CO2 + H2O in CSF
H2CO3
Stimulates H+
receptors
HCO3– + H+)
Increases rate and
depth of breathing
Stimulates
RESPIRATORY
CENTERS
Fall in blood CO2 slightly depresses breathing
• Сarotid and aortic bodies are responsible
for the other 30% of the response to raised
to CO2.
• They also increase ventilation in response
to a rise in H+ or a large drop in PaO2 ( to
below 60 mmHg).
• Arterial PaO2, normally 100 mmHg, has to fall to 60
mmHg to stimulate chemoreceptors.
• Severe lack of O2 depresses respiratory center.
• CHEMO-REFLEXES:
in addition to the effect of CO2 and O2 on center,
rise in H+ of blood stimulates carotid and aortic
bodies.
Lack of O2
Stimulates
CHEMORECEPTORS
(‘Oxygen-lack’ receptors)
in carotid body
and aortic body
Reflexly
stimulates
respiration
• The chemical and nervous means of regulating the
activity of respiratory centers act together to adjust
rate and depth of breathing to keep the PaCO2 close
to 40 mmHg. This automatically sets the PaO2 to an
appropriate value depending on the partial pressure
of O2.
• For example, exercise causes increased
requirement for O2 and the production of more CO2.
ventilation is increased to get rid of the extra CO2
and keep the alveolar PaCO2 at 40 mmHg. More
oxygen is used by the tissues. The alveolar PO2 and
PCO2 both remain constant.
Voluntary and reflex factor in the
regulation of respiration
• There is a separate voluntary system for the
regulation of ventilation. It originates in the cerebral
cortex and sends impulses to the nerves of the
respiratory muscles via the corticospinal tracts.
• In addition, ingoing impulses from many parts of the
body modify the activity of the respiratory centers
and consequently alter the outgoing impulses to the
respiratory muscles to coordinate rhythm, rate or
depth of breathing with other activities of the body.
• Proprioreceptors stimulated during muscle movements
send impulses to respiratory center ↑↑ rate and depth of
breathing.
• This occurs with active or passive movements of limbs.)
• Peripheral proprioceptors found in muscles, tendons, joints,
and pain receptors
• Movement stimulates hyperpnea.
• Moving limbs, pain, cold water all stimulate breathing in
patients with respiratory depression
Anesthesia
greatly depresses the respiratory center
J receptors
A few sensory nerve endings in the alveolar walls in
juxtaposition to the pulmonary capillaries They
are stimulated when the pulmonary capillaries
become engorged or when pulmonary edema
occurs as in congestive heart failure. (their
excitation may give the person a feeling of
dyspnea).
Irritant Receptors in the Airways
The epithelium of the trachea, bronchi, and
bronchioles is supplied with sensory nerve
endings called pulmonary irritant receptors
when stimulated cause coughing and
sneezing.
They may also cause bronchial constriction in
such diseases as asthma and emphysema.
Joint receptors
Impulses from moving limbs reflexly increase
breathing
Probably contribute to the increased
ventilation during exercise
Factors That May Increase Ventilation During
Exercise
Reflexes originating from body movement
Increase in body temperature
Adrenaline release
Impulses from the cerebral cortex
Later: accumulation of CO2 and H+ generated by
active muscles
Peripheral Chemoreceptors
Carotid bodies
Aortic bodies
Sense tension of oxygen and carbon dioxide;
and [H+] in the blood
Hypoxic Drive of Respiration
The effect is all via the peripheral chemoreceptors
Stimulated only when arterial PO2 fall less than
60mmHg
Is not important in normal respiration
May become important in patients with chronic
CO2 retention (e.g. patients with COPD)
It is important at high altitudes
The
+
H
Drive of Respiration
 The effect is via the peripheral chemoreceptors
 H+ doesn’t readily cross the blood brain barrier (CO2 does!)
 The peripheral chemoreceptors play a major role in
adjusting for acidosis caused by the addition of noncarbonic acid H+ to the blood (e.g. lactic acid during
exercise; and diabetic ketoacidosis)
 Their stimulation by H+ causes hyperventilation and
increases elimination of CO2 from the body (remember CO2
can generate H+, so its increased elimination help reduce
the load of H+ in the body)
 This is important in acid-base balance
• In summary
• very marked increase in ventilation caused
by an increase in Pco2 in the normal
range between 35 and75 mm Hg.
• By contrast, the change in respiration in
the normal blood pH range between 7.3
and 7.5 is less than one tenth as great.
• Changes in oxygen concentration have
virtually no direct effect on the respiratory
center itself to alter respiratory drive
(although oxygen changes do have an
indirect effect, acting through the peripheral
chemoreceptors
Abnormal Breathing Patterns
• Apnea
Absence of breathing. (Ap-knee-a)
• Eupnea
Normal breathing (Eup-knee-a)
• Orthopnea
Only able to breathe comfortable in upright position
(such as sitting in chair), unable to breath laying
down, (Or-thop-knee-a)
• Dyspnea
Subjective sensation related by patient as to
breathing difficulty.
Hyperpnea
• Increased volume with or without and increased
frequency (RR), normal blood gases present
Hyperventilation
"Over" ventilation - ventilation in excess of the
body's need for CO2 elimination. Results in a
decreased PaCO2, and a respiratory alkalosis
Hypoventilation.
Decreased rate (A) or depth (B), or some
combination of both.
"Under" ventilation - ventilation that is less than
needed for CO2 elimination, and inadequate to
maintain normal PaCO2. Results in respiratory
acidosis.
Tachypnea
• Increased frequency without blood gas
abnormality.
Cheyne-Stokes respirations
• Gradual increase in volume and
frequency, followed by a gradual decrease
in volume and frequency, with apnea
periods of 10 - 30 seconds between cycle
Regulation of Respiration During Exercise
In strenuous exercise, O2 consumption and
CO2 formation can increase as much as
20-fold. in the healthy athlete,alveolar
ventilation ordinarily increases almost
exactly in step with the increased level of
oxygen metabolism.
The arterial Po2, Pco2, and pH remain
almost exactly normal.
Chemical Factors and Nervous: in the
Control of Respiration During Exercise
Direct nervous signals stimulate the respiratory
center almost the proper amount to supply the
extra oxygen required for exercise and to blow
off extra carbon dioxide;
Then chemical factors play a significant role in the
final adjustment of respiration required to keep
the oxygen, carbon dioxide, and hydrogen ion
concentrations of the body fluids as nearly
normal as possible.
What causes intense ventilation during
exercise? The brain, on transmitting
motor impulses to the exercising muscles
at the same time collateral impulses into
the brain stem to excite the respiratory
center. This is analogous to the stimulation
of the vasomotor center of the brain stem
during exercise that causes a simultaneous
increase in arterial pressure.