8. Control of Ventilation

Question 1

What does ventilatory control require?

Answer 1

Stimulation of the (skeletal) muscles of inspiration

Occurs via the phrenic (to diaphragm) and intercostal nerves (to external intercostal muscles)

Question 2

Where does ventilatory reside?

Answer 2

Ill defined centres in the pons and medulla (respiratory centres)

Question 3

Ventilatory control is normally _______ and subject to _______

Answer 3

Subconscious

Voluntary modulation

Question 4

What does the Phrenic nerve supply?

Answer 4

The diaphragm

Question 5

What is ventilatory control entirely dependent on?

Answer 5

Signalling from the brain

Question 6

Why is C3,4,5 significant?

Answer 6

If severed above this level breathing ceases

Question 7

Which muscles does the Dorsal Respiratory group go to?

Answer 7

Muscles for inspiration

Question 8

Which muscles does the Ventral Respiratory group go to?

Answer 8

Muscles for expiration

Question 9

What do respiratory centres have their rhythm modulated by?

Answer 9

1. Emotion (limbic system)
2. Voluntary over-ride (higher centres in brain)
3. Mechano-sensory input from the thorax (e.g. stretch reflex)
4. Chemical composition of the blood (PCO2, PO2 and pH) - detected by chemoreceptors

Question 10

Where do Central chemoreceptors come from and what do they respond to?

Answer 10

• Medulla
• Respond directly to H+ (directly reflects PCO2)
• Primary ventilatory drive

Question 11

Where do Peripheral chemoreceptors come from and what do they respond to?

Answer 11

• Carotid and aortic bodies
• Respond primarily to plasma [H+] and PO2 (less so to PCO2)
• Secondary ventilatory drive

Question 12

Following the rise in [H+], what do central receptors cause?

Answer 12

Reflex stimulation of ventilation

driven by raised PCO2 = Hypercapnea

Question 13

What buffer equation can be made for the effect of [H+] in the brain?

Answer 13

CO2 + H2O H2CO3 H+ + HCO3-

Question 14

What is ventilation reflexly inhibited by?

Answer 14

A decrease in PCO2 (reduces CSF [H+]) HYPERVENTILATION

Question 15

What causes an increase in the rate and depth of breathing?

Answer 15

Increase [H+]

Question 16

What happens when arterial PCO2 increases?

Answer 16

CO2 crosses the blood-brain barrier and not H+

Question 17

What do central chemoreceptors indirectly do in the cerebrospinal fluid?

Answer 17

Monitor PCO2

Question 18

What two products are formed?

Answer 18

Bicarbonate and H+

Question 19

What does feedback via the respiratory centres increase and why?

Answer 19

Increases ventilation in response to increased arterial PCO2

Question 20

What does decreased PCO2 cause?

Answer 20

Slowed ventilation rate

Question 21

What do Peripheral Chemoreceptors detect changes in and then cause?

Answer 21

Arterial PO2 and [H+]
Cause reflex stimulation of ventilation following significant fall in arterial PO2 (consider haemoglobin dissociation) or a rise in [H+]

Question 22

Below which PO2 level would stimulation ventilation have the greatest effect?

Answer 22

Below 60mmHg

Question 23

What doe Peripheral chemoreceptors respond to?

Answer 23

Arterial PO2 not O2 content

Question 24

What does increased [H+] accompany?

Answer 24

A rise in arterial PCO2

CO2 + H2O H2CO3 HCO3- + H+

Question 25

Will changes in Plasma pH alter ventilation and if so how?

Answer 25

Yes they will alter ventilation via the peripheral chemoreceptors pathways

Question 26

What is acidosis?

Answer 26

Plasma pH falls ([H+] increases) and ventilation will be stimulated

Question 27

What is alkalosis?

Answer 27

Plasma pH increases ([H+] falls-) (e.g. vomiting) and ventilation will be inhibited

Question 28

What allows a large degree of voluntary control over breathing?

Answer 28

Descending neural pathways from cerebral cortex to respiratory motor neurons

Question 29

What cannot be overridden?

Answer 29

Involuntary stimuli such as arterial PCO2 or [H+]

e.g. Breath-holding

Question 30

Which drugs depress the respiratory centre?

Answer 30

Barbiturates and Opiods

Question 31

What does an overdose of barbiturates and opioids often result in?

Answer 31

Death as a result of respiratory failure

Question 32

What do most gaseous anaesthetic agent do?

Answer 32

Increase RR but decrease TV and so decrease AV

Question 33

What is Nitrous Oxide and what does in do?

Answer 33

A common sedative/light anaesthetic agent

Blunts peripheral chemoreceptor response to falling PaO2

Question 34

Is Nitrous Oxide safe all the time?

Answer 34

Very safe in most individuals, problematic in chronic lung disease cases where individual often on “hypoxic drive”
Administering O2 to these patients aggravates situation