Control of Ventilation Flashcards
Describe a negative feedback loop
- Sensor to detect parameter
- Internal standard or set point= what is too high?
= output signal in response to change in parameter - Output signal activates effector and removes stimulus
Describe a negative feedback loop in relation to the lungs
- Sensors- chemoreceptors and mechanoreceptors
- Central control- pons and medulla
- Effector- respiratory muscles
What is the equation for minute ventilation?
Tidal volume x respiratory rate
Want is the equation of alveolar ventilation?
(Tidal volume- dead space) x respiratory rate
What does the alveolar ventilation equation suggest?
This means slow and deep breaths improves ventilation much more than increasing breath
= Also means that care should be taken to avoid increasing dead space
What is tachypnoea?
High respiratory rate and (hyperventilation are different)
What is the role ventilation plays?
Removal of 13 moles of carbon dioxide and respiratory acid
What is the main stimulus of ventilation?
Carbon dioxide
What is the location of the central chemoreceptor?
- Lies in the ventral surface of the medulla
What is the central chemoreceptor?
- It is a H+ sensor and rate of firing is proportional to the H+ which is linked to the PaCO₂
What lies between the chemoreceptor sensor and an artery?
Blood-brain barrier
Why is the CSF highly sensitive to pH changes?
Contains little protein so the buffer capacity is much less than plasma
What can and cannot cross the blood brain barrier (in relation to the central chemoreceptor)?
- Plasma H+ cannot
- Plasma CO₂ can
What is the significance of Plasma CO₂ being able to pass through the BBB?
CSF pH is proportional to the PaCO₂ (at least in the short term
What is chronic hypocapnia?
CSF does not remain acidic forever, despite a constantly high carbon dioxide
- pH of CSF does not change, desensitisation of central chemoreceptor
What are the normal levels of PaCO₂?
4.8-6.1 kPa
What is the PaCO₂ a good indicator of?
How well a person is ventilating
What would lead to an increased PaCO₂?
Issues that lead to carbon dioxide not being remove
What does it mean for the patient if PaCO₂ is above normal?
- Hypoventilating
- Can lead to respiratory acidosis
What does it mean for the patient if PaCO₂ is below normal?
- Hyperventilating
- Only to the extent where CO₂ is being removed
What are the two peripheral chemoreceptors and where are they found?
- Aortic bodies just above the aortic arch
- The carotid body at the bifurcation of the common carotid artery
Which peripheral chemoreceptor is more important in humans?
Carotid
What do peripheral chemoreceptors respond to?
- Changes in PaCO₂, PaO₂ and pH/H+
What do carotid bodies sense?
Blood as it flows via the heart up to the vein
What do PaCO₂/pH peripheral chemoreceptors detect?
- CO₂ but minimal importance due to limited impact (very limited long-term control)
What will a high plasma and H+ result in?
- Increase the respiration rate
- Increase CO₂ loss
What do carotid bodies and the central chemoreceptor have in common?
- Have same H+ sensor
How long-term are peripheral chemoreceptors?
- NO long term role with CO₂
- Only rapid response
When is oxygen sensing higher?
- At lower PaO₂s
- There is some firing of nerves but not enough to produce a noticeable impaired
At what level of PaO₂ is there activity from peripheral chemoreceptor cells?
> 13kPa
- From ~8 kPa, firing increase exponentially as PaO₂ falls
What does increasing firing of nerves lead to?
Increases rate and depth of ventilation
Are central peripheral and central chemoreceptors mutually exclusive?
- No
- As PaO₂ sensors start firing
- Nothing changes with central chemoreceptors (work alongside)
What do mechanoreceptors respond to?
Changes in stretch
What are pulmonary stretch mechanoreceptors?
- Lie within smooth muscle
- Discharge in response to distension of lung
What are irritant mechanoreceptors?
- Lie between epithelial cells are stimulated by cold air, noxious gases
What are J/Juxtacapillary mechanoreceptors and Bronchial C Fibres?
- In the alveolar walls close to pulmonary/bronchial circulation
- Respond to increases in fluid volume (Capillaries or interstitial) with increases in respiratory rate
- Dyspnoea
What are nose and upper airway mechanoreceptors?
- An extension of the irritant receptors producing coughing, sneezing etc.
What are joint and muscle mechanoreceptors?
- Pain may lead to hyperventilation as may falling blood pressure
Is breathing part of the ANS and where does it arise from?
- Not part of AND but it does have an autonomic nature
- Arises from brainstem in medulla
- Can be modulated from higher brain (cortex) and sensors
- Breath holding, speaking etc.
How can central control breathing be modulated?
- Higher brain (cortex) and sensors
- E.g. breath holding, speaking etc.
What are the two medullary centres?
- Dorsal respiratory group (DRG)
- Ventral Respiratory group (VRG)
What does the DRG do?
- Mainly causes inspiration (expiration is passive)
What does the VRG do?
- Some say expiration, but more likely involved in inspiration and expiration at increased demand
Which nerve (respiratory) do neutrons spontaneously stimulate after all other stimuli have been removed?
- Phrenic nerve- somatic nerve (diagram) at regular nintervals
Where is the pneumotaxic centre?
- Upper pons
What is the role of pneumotaxic centre?
- Importnat role in limiting inspiration
How does the pneumotaxic centre limit inspiration?
Controls filling of lungs and rate and depth of breathing
- When to breathe in and out
What can override DRG and VRG?
Higher brain regions
How can PaCO₂ be halved?
By hyperventilation
How can PaCO₂ be increased?
Hypoventilation
What is the effect of other areas of the brain such as the iambic system inputting into the medulla?
- Leads to changes in respiratory rate in states such as rage and fear etc.
What does the DRG have to act on in order to produce a change in ventilation?
Respiratory muscles controlling inspiration
- And expiration in non-passive situations
- Several respiratory muscles all of which must work together
What ensures that respiratory muscles will work together when acted on by the DRG?
- The role of the central control in the brain set
- Increase thoracic space and drop pleural pressure
What are the inspiratory muscles?
- Diaphragm, external intercostals
- Accessory muscles: sternomastoid and scalene
What are the expiratory muscles?
- Abdominal wall
- Rectus abdominis
- Obliques (internal and external)
- Transversus abdominus
- Internal intercostal muscles
