Control of ventilation

Question 1

How does the stimulation of the skeletal muscles of inspiration occur?

Answer 1

This occurs via the phrenic (to diaphragm) and intercostal nerves (to external intercostal muscles).

Question 2

Where does ventilatory control reside?

Answer 2

Resides within ill defined centres located in the pons and medulla (Respiratory Centres).

Question 3

How do respiratory centres set an automatic rhythm of breathing?

Answer 3

through co-coordinating the firing of smooth and repetitive bursts of action potentials in DRG (dorsal respiratory group) - travel to inspiratory muscles.

Question 4

How do respiratory systems have their rhythm modulated?

Answer 4

1. Emotion (via limbic system in the brain)
2. Voluntary over-ride (via higher centres in the 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 5

What is the most significant out of the following inputs:
Emotional

Voluntary

Chemoreceptor

Mechano-sensory receptor

Answer 5

Chemoreceptor input is the most significant

Question 6

What are the 2 main chemoreceptors?

Answer 6

Central and Peripheral chemoreceptors

Question 7

Features of central chemoreceptors

Answer 7

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

Question 8

Features of peripheral chemoreceptors

Answer 8

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

Question 9

What is hypercapnia?

Answer 9

Raised PCO2
- It is when you have too much carbon dioxide (CO2) in your bloodstream. It usually happens as a result of hypoventilation, or not being able to breathe properly and get oxygen into your lungs

Question 10

How is ventiliation reflexly inhibited?

Answer 10

Ventilation is reflexly inhibited by a decrease in arterial PCO2 (reduces CSF [H+])
(hyperventilation)
- It doesn’t respond to changes in plasma (H+).

Question 11

What chemical movement occurs when arterial PCO2 increases?

Answer 11

CO2 crosses the blood-brain barrier but H+ does not.

Question 12

What type of chemoreceptors monitor the PCO2 indirectly in the cerebrospinal fluid?

Answer 12

Central chemoreceptors

Question 13

How does decreased arterial PCO2 affect ventilation rate?

Answer 13

It slows ventilation rate

Question 14

What action do peripheral chemoreceptors carry out following significant fall in arterial PO2 (consider haemoglobin dissociation) or a rise in [H+] ?

Answer 14

Cause reflex stimulation of ventilation

Question 15

What occurs to ventilation if plasma pH falls (H+) increases?

Answer 15

Ventilation will be stimulated (acidosis)

Question 16

What occurs to ventilation if plasma pH increases (H+) decreases?

Answer 16

Ventilation will be inhibited (alkalosis) e.g. vomiting

Question 17

What do Descending neural pathways from cerebral cortex to respiratory motor neurons allow?

Answer 17

They allow a large degree of voluntary control over breathing.

Question 18

What mechanism occurs in Hyperventilation?

Answer 18

Ventilation is reflexly inhibited by an increase in arterial PO2 or a decrease in arterial PCO2/[H+]

Question 19

Why is respiration inhibited during swallowing?

Answer 19

To avoid aspiration of food or fluids into the airways.
Swallowing is followed by an expiration in order that any particles are dislodged outwards from the region of the glottis.

Question 20

Examples of common drugs that affect respiratory centres

Answer 20

• Barbiturates and opioids depress respiratory centre
• Most gaseous anaesthetic agents increase RR but decrease TV so decrease AV
• Nitrous oxide