3. Control of Ventilation Flashcards
How are skeletal muscles stimulated during inspiration ?
Phrenic nerve to diaphragmIntercostal nerves to external intercostal muscles Entirely dependant on signalling from the brain
What part of the brain is responsible for ventilatory control ?
Ill defined centres located in the pons and medulla (respiratory centres)
What is the activity of respiratory centres ?
Fire repetitive short bursts of action potentials in DRG which travel to the inspiratory musclesThis sets an autonomic rhythm of breathingThey adjust their rhythm according to the stimuli
What modulates rhythm of respiratory centres ?
Emotion (via limbic system in brain)Voluntary over-ride (via higher centres in brain)Mechano-sensory input from thorax ( e.g. stretch reflex)Chemical composition of blood (PCO2, PO2, pH)- detected by chemoreceptors
What does DRG stand for ?
Dorsal respiratory group of neurons
What does VRG stand for ?
Ventral respiratory group of neurons
What does PRG stand for ?
Pontine respiratory group - pneumotaxic area
What does NTS stand for ?
Nucleus tractis soolitaris
Which respiratory centre is located in the pons ?
PRG
What does VRG do ?
Maintains basal tone
What does VRG supply ?
TonguePharynxLarynxExpiratory muscles
What does DRG supply ?
Inspiratory muscles, via phrenic and intercostal nerves
What is the most significant input ?
Chemoreceptor input
Where are the central chemoreceptors located ?
Medulla
What chemical change does the medulla detect from the CSF ?
Respond directly to H+, directly reflects PCO2Primary ventilatory drive
Where are the peripheral chemoreceptors located ?
Carotid and aortic bodies
What chemical change do the peripheral chemoreceptors detect ?
Primarily to plasma [H+] and PO2 (not oxygen content) (less so to PCO2)Second ventilatory drive
What do central chemoreceptors in medulla cause ?
Reflex stimulation of ventilation following rose in [H+] ( driven by hypercapnea)
What is hypercapnea ?
Raised PCO2
What change in blood PCO2 does hyperventilation cause ?
Reduces PCO2 - reduces CSF H+Inhibits ventilation
does the medulla respond to direct changes in the plasma [H+] ?
No
When PaCO2 increases, what crosses the blood-brain barrier ?
CO2 not H+
What responds to H+ ?
Receptors on bicarbonate and H+ when they are formed
What does feedback via the respiratory centres increases ?
Increases ventilation in response to increased PaCO2
What does decreased PaCO2 do ?
Slows ventilation rate
A 10% increases in PCO2 causes what ?
100% increase in ventilation
What are we very sensitive to changes in ?
PCO2, so small changes in CO2 will stimulate ventilation in order to get rid of CO2 and remove the toxic gas from our blood
In chronic lung disease what can become chronically elevated ?
PaCO2
What do individuals with chronic lung disease become desensitised to ?
PCO2
What do people with chronic lung disease rely on to stimulate ventilation ?
Changes in PO2Peripheral chemoreceptors
What kind of respiratory drive are people with chronic lung disease said to be on ?
Hypoxic drive
What do the peripheral chemoreceptors cause ?
Reflex stimulation of ventilation following significant fall in arterial PO2 or increasing levels of plasma [H+]
When do the peripheral chemoreceptors kick in ?
When PO2 drops below 60mmHg
Which direction does increased ventilation shift the equilibrium ? CO2 + H20 H2CO3 HCO3- + H+
Increased ventilation drives this equation to the left (by blowing off CO2) and lowers [H+]
Which direction does decreased ventilation shift the equilibrium ? CO2 + H20 H2CO3 HCO3- + H+
Decreased ventilation drives this equation to the right (by retaining CO2) and increases [H+]
What will happen to respiration rate in an anaemic patient with normal lung function, who has a blood oxygen content half the normal value ?
It will stay the same, because peripheral chemoreceptors are responding to PO2 and not total O2 content If the lungs are working normally, diffusion will take place normally and therefore the amount of oxygen in solution in the plasma (PaO2) will be normal
Which type of chemoreceptors afferent sensory neurones ?
Carotid and aortic chemoreceptors
What do the somatic motor neurones for inspiration and expiration innervate ?
Inspiration - Scalene and sternocleidomastoid muscles, external intercostals, diaphragmExpiration - Internal intercostalsAbdominal muscles
What do most gaseous anaesthetic agents do ?
Increase RR but decrease TV, so decrease AV
What type of drugs depress respiratory centres ?
Barbiturates and opioids
What is nitrous oxide ?
A common sedative/ light anaesthetic agent that blunts peripheral chemorecptors to failing PaCO2
When is nitrous oxide problematic ?
Chronic lung disease cases where individuals often on hypoxic drive
What happens when you administer O2 to patients with chronic lung disease ?
Aggravates situation
When will changes in plasma pH alter ventilation via peripheral chemoreceptor pathways ?
If plasma pH falls ([H+] increases) ventilation will be stimulated (acidosis)If plasma pH increases ([H+] falls) e.g. Vomiting (alkalosis) ventilation will be inhibited
Is CO2 capable of changing ECF pH ?
Yes
What will alter plasma PCO2 and therefore plasma [H+] ?
HyperventilationHypoventilation
What will hypoventilation do ?
Cause CO2 retention, leads to increased [H+] bringing about respiratory acidosis
What will hyperventilation do ?
Blow off more CO2, leads to decreased [H+] bringing about respiratory alkalosis
What allows us to have a large degree of voluntary control over breathing ?
Descending neural pathways from cerebral cortex to respiratory motor neurones
Can cerebral cortex neural pathways override involuntary stimuli such as PCO2 or H+ ?
No
What are examples of voluntary control of ventilation ?
Breath-holdingHyperventilation
What is the neurotransmitter that responds to low PO2 in the peripheral chemoreceptors ?
Dopamine
Why is respiration inhibited during swallowing ?
To avoid aspiration of food or fluids into the airways
Why is swallowing followed by an expiration ?
In order that any particles are dislodged outwards from the region of the glottis
