Control of Ventilation Flashcards by Nisha Akella

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

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Describe a negative feedback loop in relation to the lungs

Sensors- chemoreceptors and mechanoreceptors
Central control- pons and medulla
Effector- respiratory muscles

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What is the equation for minute ventilation?

Tidal volume x respiratory rate

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Want is the equation of alveolar ventilation?

(Tidal volume- dead space) x respiratory rate

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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

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What is tachypnoea?

High respiratory rate and (hyperventilation are different)

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What is the role ventilation plays?

Removal of 13 moles of carbon dioxide and respiratory acid

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What is the main stimulus of ventilation?

Carbon dioxide

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What is the location of the central chemoreceptor?

Lies in the ventral surface of the medulla

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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

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What lies between the chemoreceptor sensor and an artery?

Blood-brain barrier

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Why is the CSF highly sensitive to pH changes?

Contains little protein so the buffer capacity is much less than plasma

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What can and cannot cross the blood brain barrier (in relation to the central chemoreceptor)?

Plasma H+ cannot

- Plasma CO₂ can

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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

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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

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What are the normal levels of PaCO₂?

4.8-6.1 kPa

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What is the PaCO₂ a good indicator of?

How well a person is ventilating

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What would lead to an increased PaCO₂?

Issues that lead to carbon dioxide not being remove

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What does it mean for the patient if PaCO₂ is above normal?

Hypoventilating

- Can lead to respiratory acidosis

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What does it mean for the patient if PaCO₂ is below normal?

Hyperventilating

- Only to the extent where CO₂ is being removed

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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

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Which peripheral chemoreceptor is more important in humans?

Carotid

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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