11 Control of Respiration Flashcards
Describe the control of
respiration.
> Ventilation is regulated in order to maintain homeostasis of pH, PaO2 and PaCO2 in the blood.
> The respiratory centre is located
in the brainstem and
is composed of a
group of nuclei
within the medulla and pons.
> Three major brainstem respiratory
neuronal areas have been identified:
• Dorsal respiratory group (DRG) of neurons –
located in the medulla
and controls inspiration
• Pneumotaxic area –
located in the pons and
assists in regulating inspiration
• Ventral respiratory group (VRG) of neurons – located in the medulla and regulates expiration.
> The respiratory centre receives input from higher CNS structures, peripheral and central chemoreceptors, and mechanoreceptors in the lungs and chest wall.
Brainstem
What is located here
How does it fire
Via what
What else has input
Expiration during normal quiet
> DRG are mainly
inspiratory neurons
and
control inspiration.
This area has intrinsic automaticity
and
exhibits a ramp effect
of increasing action potential frequency
to the diaphragm (via the phrenic nerve, C3, 4, 5)
and to the inspiratory muscles
of the chest and abdomen
(via intercostal nerves).
They are responsible for basic ventilatory rhythm.
> Inspiration may be terminated prematurely
by inhibitory impulses
from the pneumotaxic centre.
In effect, the pneumotaxic centre
‘fine-tunes’ inspiration.
> During normal quiet breathing
expiration is passive.
However, during exercise, for example the VRG of neurons are stimulated and drive the expiratory muscles.
Peripheral chemoreceptors
> Located in the aortic
(near aortic arch)
and
carotid bodies
(bifurcation of the common carotid artery).
Cranial nerves X and IX
link the receptors to the brainstem.
> The peripheral chemoreceptors primarily respond to hypoxia and respond to partial pressure of oxygen in the arterial blood rather than oxygen content of the blood.
Thus patients with reduced blood oxygen content due to anaemia or carboxyhaemoglobin do not have respiratory stimulation via the peripheral chemoreceptors.
> The chemoreceptors are
composed of glomus cells,
which contain dopamine.
> Each carotid body receives
an extremely high blood flow
equivalent to 2 L /100 g tissue per minute.
> The aortic bodies respond to reductions in
PaO2 and rises in PaCO2
by stimulating the inspiratory centre
to increase respiratory rate.
> The carotid bodies respond not only
to reductions in PaO2
and rises in PaCO2,
but also to pH changes.
> Hypotension may also result in
stimulation of the peripheral
chemoreceptors probably via
stagnant hypoxia.
> The respiratory stimulant doxapram acts via the peripheral chemoreceptors.
> Volatile anaesthetics abolish the
peripheral chemoreceptor
response to hypoxia.
Central chemoreceptors
> Situated in the ventral medulla,
in an area that is extremely sensitive
to hydrogen ions.
> Chemoreceptors are surrounded
by extracellular fluid.
> When blood PaCO2 rises,
CO2 diffuses across the blood–brain barrier
into the CSF generating hydrogen ions
[CO2 + H2O ⇋ H2CO3 ⇋ H+ + HCO3−].
> pH thus falls
(note that CSF has less protein
than blood and therefore
less buffering capacity)
and this stimulates the inspiratory area.
> Hypercarbia provides an acute drive
to increase ventilation
for up to 48 hours;
after this period CSF compensation
occurs via increased HCO3 −
transport into the CSF in order to correct pH.
Voluntary control of respiration
> Cerebral cortex can override
brainstem control,
within limits,
e.g. voluntary hyperventilation.
> Limbic system and hypothalamus –
emotions such as rage and fear may
also alter respiratory pattern.
Voluntary control of respiration
pg 36
Describe the response to inhalation of 5% CO2 in oxygen.
Page 37 diagram Fig. 11.2 Feedback systems involved in the control of respiration
Over a short period of time, PaCO2 will increase, which in turn will lead to an increase in CSF hydrogen ion concentration and thus a fall in pH
which will be detected
by the central chemoreceptors,
leading to an increase in respiratory rate.
> An additional stimulus to respiration
will come from the peripheral
chemoreceptors,
which will detect both the rise in PaCO2
and the fall in pH,
leading to input to the DRG of neurons
via cranial nerves IX and X.
> Note that the ventilatory response
to CO2 is reduced by opiates,
increasing age and sleep.
Describe the effects of raised CO2 on the body.
> Hypercarbia stimulates respiration via activation of peripheral and central chemoreceptors.
> CVS –
systemic vasodilatation,
myocardial depression
and
arrhythmias.
> Pulmonary circulation –
increased pulmonary vascular resistance.
Respiratory acidosis.
> CNS –
stimulates respiration but at
high levels causes narcosis.
Increases cerebral blood flow
and intracranial pressure.
> Renal –
slower compensation via
bicarbonate retention and
urinary hydrogen ion excretion.
Describe the ventilatory response to hypoxia.
> Hypoxaemia stimulates ventilation
through its effects
n the carotid and aortic bodies
(peripheral chemoreceptors).
Hypoxaemia is not a stimulus
for the central chemoreceptors,
although prolonged hypoxia will
cause cerebral acidosis,
which in turn can stimulate respiration.
> Isocapnic (holding CO2 constant)
oxygen curves illustrate the effect of
changing PaO2 on alveolar ventilation.
The main stimulation of respiration
through hypoxia occurs at
PaO2 < 8 kPa.
Hypercarbia augments the
ventilatory response to hypoxia.
> Increasing PaCO2 by 0.1 kPa results
in an increase in alveolar ventilation
of approximately 1–2 L /min.
In the same way a reduction in PaCO2 results
in a reduction in alveolar ventilation
up until a PaCO2 of 4 kPa
below which there is no effect.
Hypoxia produces a
higher alveolar ventilation
for any given PaCO2.
