Control of ventilation Flashcards
What nerves are stimulated during inspiration?
Phrenic C3-5( diaphragm) and intercostal nerves (external intercostal muscles)
Where is the respiratory centres located?
DRG- medulla
PRG- pons
VRG- medulla
What is ventilatory control dependent on?
signalling from the brain
Function of respiratory centres ( PRG , VRG+ DRG)
- DRG(inspiratory centre)
- activates inspiratory muscles via the phrenic nerve(diaphragm) +intercostal nerves(ext intercostal )
- recieves signals from peripheral chemoreceptors and mechanoreceptors via vagus/glossopharyngeal nerve and will send this to PRG
PRG
-NF mechanism; too much DRG stimulation will inhibit DRG, hence inhibiting inspiration
VRG(expiratory mainly)
- stimulation of expiratory muscles ( int intercostal), pharynx, larnyx + tongue and few inspiratory, muscles.
- keeps airways open
What can affect respiratory centres?
- Emotion ( via limbic system in brain)
- Voluntary override ( via higher centres in brain)
- Mechano-sensory input from thorax ( stretch reflex)
- Chemical composition of the blood ( PCO2, PO2 and pH) detected by chemoreceptors
What does damage to the PRG and VRG lead to?
PRG- prolonged inspiration as it keeps DRG in line
VRG: sleep apnoea as airway opening not maintained
Where are central and peripheral chemoreceptors located and state their function.
Central:
- medulla
- detects changes in [H+] in CSF around the brain.
- Causes reflex stimulation of ventilation + increased [H+] ( driven by raised PCO2=Hypercapnea)
- primary ventilatory drive
Peripheral:
- Carotid and aortic bodies
- detects changes in [plasma H+] and PO2(needs to be major change)
- causes reflex stimulation of ventilation to decrease in PaO2/Rise in [H+] which causes rise in PCO2. Responds to arterial PO2 not O2 content!
- secondary ventilatory drive
Describe the compensatory mechanism when plasma pH falls/[H+ ] increases
- ACIDOSIS
- Ventilation will be stimulated
- Drives equation to the LHS as bowing off CO2 will lower [H+]
Describe the compensatory mechanism when plasma pH increases/[H+] decreases
- ALKALOSIS
- ventilation will be inhibited; decreased V will drive equation to RHS( by retaining CO2) and increases [H+]
- vomiting
Other mechanisms that control breathing
descending neural pathways from the cerebral cortex to respiratory motor neurones allow a large degree of voluntary control over breathing
- but cannot ovveride stimuli such as PaCO2/[H+]
- breath holding
- hyperventilation: ventilation inhibited by increase in PaPO2/decrease in PaCO2
When is respiration inhibited?
during swallowing to avoid aspiration of food/fluids in airways. Swallowing is followed by expiration.
Barbituarates and opiodes effect on respiratory centre
depresses RC: overdose results in death due to respiratory failure
Effect of gaseous anaesthetic agents on respiratory centre
Increases RR but decreases TV and so decreases AV
Effect of nitrous oxide on respiratory centre
- prevents peripheral chemoreceptor response to falling PaO2. Problematic in chronic lung diseases where individual is often on ‘hypoxic drive’. Administering O2 to these patients aggravates situation
Describe what is meant by a ‘hypoxic drive’
PCO2 is chronically elevated, within time individual becomes desensitised to PCO2 + instead rely on changed in PO2 to stimulate ventilation. Reliant on peripheral chemoreceptors instead.
-Hypoxic drive
-
