Physiology: control of ventilation Flashcards
What does breathing depend on?
Cyclical activation of the phrenic and intercostal nerves which stimulate contraction of the diaphragm and external intercostal muscles - muscles of inspiration. Neural activity triggered by medullary inspiratory neurones.
How long do inspiration and expiration take?
Inspiration 2 seconds and expiration 3 seconds. More and more inspiratory neurones then climax, then shutting off. Still a basal level of inspiratory neurones during expiration so that it is smooth.
Where does ventilatory control occur?
Ventilation centres in pons and medulla - respiratory centres of brain. Phrenic innervates diaphragm which provides 70% breathing capacity. Breathing can be either subconscious or voluntary.
What affect would severing the spinal cord above C3-C5 have on breathing?
Breathing would cease as this is the origin of the phrenic nerve.
What is the job of respiratory centres?
To set an automated rhythm through co-ordinating firing of smooth and repetitive bursts of AP’s in DRG (dorsal respiratory group of neurones) which travel to inspiratory muscles. Can be modulated by stimuli. VRG keep basal tone during expiration.
Name 4 things that can modulate rhythm of respiratory centres.
Chemical composition of blood (most important, PO2, PCO2 and pH), emotion, voluntary override and mechanosensors –> all 4 regulate VRG and DRG.
What are the 2 types of chemoreceptors?
Central: medulla, respond directly to [H+] in CSF so reflect PCO2, primary ventilatory drive. Peripheral: come from aortic and carotic bodies, respond to [H+] in the plasma and PO2 (less so PCO2), secondary ventilatory drive.
Describe the primary ventilatory drive.
Central chemoreceptors respond to changes in [H+] in the CSF, therefore reflecting changes in PCO2. A rise in [H+] caused by a rise in PCO2 (hypercapnia) will stimulate primary ventilatory drive.
What would happen if there was a 10% increase in PCO2?
There would be a 100% increase in ventilation to get rid of CO2 as it is toxic.
What would happen if there was a decrease in PaCO2?
Ventilation would be inhibited by a decrease in [H+] - hyperventilation.
Describe peripheral chemoreceptors.
Aortic and carotid bodies that respond to a decrease in PaO2 and increase in [H+]. This is the secondary ventilatory drive and is used in patients with chronic lung disease as their primary will burn out and stop responding appropriately to get rid of CO2.
When would the secondary ventilatory drive be activated?
When PO2 is below 66mgHg (equivalent to altitude of 3000m).
How does plasma pH affect ventilation?
Low plasma pH (acidosis) - ventilation stimulated. High plasma pH (alkalosis) - ventilation inhibited.
Name 5 other aspects of control of breathing.
Neural pathways that allow voluntary control, involuntary stimuli (PCO2 or [H+], breath-holding, hyperventilation, swallowing (followed by an expiration).
Which patients should not be given NO?
Common anaesthetic. Not safe for chronic lung disease patients as they will be using hypoxic drive. NO blunts peripheral chemoreceptor response to falling PaO2.
