Control Of Ventilation

Question 1

Describe how ventilatory control is stimulated

Answer 1

Through stimulation of the skeletal muscles on inspiration via the phrenic and intercostal nerves.

Question 2

Is ventilatory control subconscious?

Answer 2

Yes, but can be subject to voluntary modulation

Question 3

State the origin of the phrenic nerve

Answer 3

C3-5

Question 4

Describe the function of the respiratory centres

Answer 4

Set an automatic rhythm of breathing through co-ordinating the firing of smooth and repetitive bursts of action potentials in the Dorsal Respiratory Group.

Can adjust this rhythm in response to stimuli

Question 5

State the primary output of the Dorsal Respiratory group

Answer 5

To inspiratory muscles

Question 6

What muscles does the ventral respiratory group innervate?

Answer 6

Muscles for expiration and larynx & pharynx

Question 7

Describe the pneumotaxic centre

Answer 7

Higher brain centre in the pons that provides inhibitory impulses on inspiration ie helps maintain alternately recurrent inspiration and expiration

Question 8

Describe how the rhythm of the respiratory centres are modulated

Answer 8

• emotion, via the limbic system in the brain
• voluntary over-ride, via higher centres in the brain
• mechano-sensory input from the thorax, eg stretch reflex
• chemical composition of the blood, detected by chemoreceptors

Question 9

Where are central chemoreceptors located?

Answer 9

The medulla

Question 10

What do central chemoreceptors respond directly to?

Answer 10

[H+], which is wholly derived from CO2, therefore directly reflects PCO2

Question 11

Describe how central chemoreceptors play a role in the control of ventilation

Answer 11

Are the primary ventilatory drive. Detect changes in [H+] in the CSF around the brain, and cause reflex stimulation of ventilation followed by a rise in [H+], driven by raised PCO2

Question 12

State the name of the condition caused by raised PCO2

Answer 12

Hypercapnia

Question 13

Why do chemoreceptors not respond to direct changes in plasma [H+]?

Answer 13

Because of the blood-brain barrier. CO2 crosses the barrier not the H+, only once crossed and converted to HCO3- and H+ in the CSF do the chemoreceptors respond.

Question 14

Where are peripheral chemoreceptors located?

Answer 14

In carotid and aortic bodies

Question 15

What do peripheral chemoreceptors respond to?

Answer 15

Changes in arterial PO2 (not oxygen content) and [H+]

Question 16

Describe how peripheral chemoreceptors play a role in the control of ventilation

Answer 16

Cause a reflex stimulation of ventilation following a rise in [H+] or a significant fall in arterial PO2 (eg Hb dissociation or at high altitude)

Question 17

Describe how changes in plasma pH can alter ventilation via the peripheral chemoreceptors pathways

Answer 17

• If plasma pH falls ([H+] increases), ventilation will be stimulated (drives CO2/HCO3- equation to the left) lowering [H+]
• if plasma pH increases ([H+] falls) ventilation will be inhibited (drives CO2/HCO3- equation to the right) increasing [H+]

Question 18

Describe how breath holding cannot override involuntary stimuli

Answer 18

As if breath is held long enough consciousness is lost, as is the ability to make decisions so the brain takes over and breaths for you again.

Question 19

State some common drugs that affect the respiratory centres and how

Answer 19

• barbiturates and opioids depress respiratory centre
• most gaseous anaesthetic agents, increase RR but decrease TV
• nitrous oxide, blunts peripheral chemoreceptors response to falling PaO2.

Question 20

Why should nitrous oxide not be given to a patient with chronic lung disease

Answer 20

Can send patient into hypoxic drive where the administration of O2 will aggravate the situation by tricking brain into inhibiting ventilation - causes CO2 retention