Control of ventilation

Question 1

Which parts of the brain does respiratory stimulation come from?

Answer 1

Pons and medulla, in brain stem

Question 2

What nerves do skeletal muscles receive stimulation from for inspiration?

Answer 2

• Phrenic (C3,4,5, to diaphragm)

- Internal intercostal nerves (to external intercostal muscles)

Question 3

What are the two respiratory groups and what do they do?

Answer 3

• Dorsal respiratory group of neurons and ventral respiratory group of neurons
• They work together to allow for breathing

Question 4

What does the DRG specifically do?

Answer 4

• Goes to inspiratory muscles via phrenal and intercostal nerves
• Alllows for slow, controlled breath out in expiration

Question 5

What does the VRG specifically do?

Answer 5

• Goes to tongue, pharynx, larynx and expiratory muscles

- Operates to allow breathing as the tongue, pharynx and larynx need to comply

Question 6

What does the respiratory centre do?

Answer 6

1. Set an automatic rhythm of breathing through co-ordinating the firing of smooth and repetitive bursts of action potentials in DRG
2. Adjust this rhythm in response to stimuli

Question 7

What factors change the ‘respiratory drive’, rate and depth of breathing?

Answer 7

• Emotion -> limbic system
• Voluntary override -> higher centres in the brain -> e.g. speaking
• Mechano-sensory input from thorax -> tells body to stop inspiring and start expiring so that the alveoli don’t get damaged -> stretch reflex
• Chemical composition of the blood -> PCO2, PO2 and pH

Question 8

Which factor is most significant in altering respiratory drive?

Answer 8

Chemical composition of blood

Question 9

What are two main groups of chemoreceptors?

Answer 9

Central chemoreceptors and peripheral chemoreceptors

Question 10

Where are the central chemoreceptors?

Answer 10

• Medulla

Question 11

What do central chemoreceptors respond to?

Answer 11

Respond directly to pH which directly reflects PCO2

Question 12

Where does primary ventilatory drive come from?

Answer 12

Partial pressure of CO2

Question 13

How do the central chemoreceptors respond to a change in PCO2?

Answer 13

• Increases ventilation in response to PaCO2

- Decreased PaCO2 slows ventilation rate

Question 14

When would the body respond not respond to central chemoreceptors?

Answer 14

• In a pathology with chronic PCO2, the receptors become less receptive over time
• Patients shift to hypoxic drive and their ventilation is determined by PO2 rather than PCO2

Question 15

Where are the peripheral chemoreceptors?

Answer 15

Carotid and aortic bodies

Question 16

What do peripheral chemoreceptors respond to?

Answer 16

Plasma H+ and PaO2

Question 17

Where does secondary ventilatory drive come from?

Answer 17

• Peripheral chemoreceptors

- e.g. patient with chronic lung disease in hypoxic drive

Question 18

Why do chemical chemoreceptors respond to pH?

Answer 18

The blood/brain barrier is permeable to gas and not ions.

Means that CO2 can cross and lower the pH

Question 19

How do peripheral chemoreceptors work?

Answer 19

• Reflex stimulation of ventilation following a significant drop in arterial PO2 (haemoglobin disassciation)
• Can also respond to rise in [H+] as a response to metabolic acidosis

Question 20

What is metabolic alkalosis and how would peripheral chemoreceptors respond?

Answer 20

• Increase in pH can be due to someone vomitting for a prolonged period of time
• Decrease ventilation to retain [H+] ions

Question 21

What’s worse? A room with normal PO2 and high PCO2 or a room with low PO2 and no PCO2? Why?

Answer 21

• Room with normal PO2 and high PCO2

- Body is wholly programmed to get rid of CO2, this would be an extremely distressing experience

Question 22

What are some common drugs that affect the respiratory centres?

Answer 22

• Barbituates and opioids: OD often results in death as a result of respiratory failure
• Most gaseous anaesthetics increase RR but decrease tidal volume