Control of Ventilation

Question 1

What kind of feedback does breathing involve?

Answer 1

Rapid positive feedback group

Cyclical - 500ml breathed in and 500ml breathed out

Question 2

Describe the process of inspiration in terms of nerve activation

Answer 2

Triggered by neural activity of medullary respiratory centres (but with voluntary override)
Cyclical activation of phrenic and intercostal nerves causes diaphragm and external intercostal muscles to contract

Question 3

Describe the process of expiration in terms of nerve inhibition

Answer 3

Motor signals to external intercostals and diaphragm cease

Question 4

What other muscles need to be activated for ventilatory control?

Answer 4

Tongue and pharynx muscles to prevent airway collapse

Question 5

What percentage of inspiration is carried out by the diaphragm vs the intercostal muscles?

Answer 5

70% vs 30%

Question 6

Where are the respiratory centres located?

Answer 6

Pons and medulla of brainstem

Question 7

Is breathing conscious or subconscious?

Answer 7

Normally subconscious but can be subject to voluntary control eg singing, talking, holding breath etc

Question 8

Does the breathing have pacemaker activity?

Answer 8

No, breathing is entirely dependent on brain signalling.

Question 9

Cutting above what region in the spinal cord would cause breathing to cease?

Answer 9

C3-5 (phrenic nerve)

Question 10

Where are central chemoreceptors and what is their function?

Answer 10

Located in medulla

Responsible for primary ventilatory drive

Question 11

Describe the process by which central chemoreceptors stimulate primary ventilator drive.

Answer 11

Respond to change in hydrogen ion concentration of CSF and thus directly reflect PaCO2 since H+ ions only cross the blood brain barrier if they have dissociated from H2CO3 (from when CO2 combines with water)
Cause reflex stimulation of ventilation following hypercapnia

Question 12

Do central chemoreceptors respond to changes in plasma hydrogen ion concentration?

Answer 12

No, central chemoreceptors only respond to changes to hydrogen ions in the CSF because only hydrogen ions from dissolved CO2 are able to cross the BBB.

Question 13

How is ventilation reflexly inhibited by central chemoreceptors?

Answer 13

Reduced PaCO2 and thus reduced CSF hydrogen ion concentration leads to the reflex inhibition of ventilation. This explains why you are unable to hyperventilate for a long time.

Question 14

Where are peripheral chemoreceptors located?

Answer 14

Carotid and aortic bodies

Question 15

What changes are peripheral chemoreceptors sensitive to?

Answer 15

Changes to PaO2 and plasma (NOT CSF AND THEREFORE NOT ONLY CARBON DIOXIDE) hydrogen ion concentration.

Question 16

When do peripheral chemoreceptors cause reflex stimulation of ventilation

Answer 16

At significant drop in PaO2 (haemoglobin dissociation at 60mmHg)

Question 17

What three events does in increase in plasma H+ concentration usually accompany

Answer 17

Increase in PaCO2
Lactic acid build up
Disease

Question 18

What is the function of respiratory centres?

Answer 18

Set automatic rhythm of breathing by co-ordinating the firing of smooth and repetitive bursts of action potentials in the DRG to travel to inspiratory muscles.
Adjust this rhythm in response to stimuli.

Question 19

What is the input to the PRG (pneumotaxic centre)?

Answer 19

Higher brain centres (voluntary override)

Question 20

What is the function of the PRG?

Answer 20

As the pneumotaxic centres, the PRG provides tonic input into medullary centres for a smooth breathing response.
Controls the DRG

Question 21

What is the main input to the DRG?

Answer 21

The PRG

Question 22

Aside from the PRG, what are the other inputs to the DRG?

Answer 22

Mechanosensory input from the thorax (eg stretch reflex)
Medullary chemoreceptors (PaCO2/CSF hydrogen ion concentration)
Chemosensory input (PaO2 and plasma hydrogen ion concentration)

Question 23

What is the function of the dorsal respiratory group (DRG)?

Answer 23

Fires APs to activate inspiratory muscles

Question 24

What is the main input to the VRG?

Answer 24

The PRG

Question 25

What is the function of the ventral respiratory group (VRG)?

Answer 25

Fires APs to activate expiratory muscles and muscles of the tongue and the pharynx

Question 26

What system has emotional input to respiratory centres?

Answer 26

Limbic system

Question 27

How does acid-base balance affect ventilation?

Answer 27

Changes in pH alter the peripheral chemoreceptor pathway and vary depending on the amount of dissolved CO2 that has dissociated according to the equation CO2 + H2O H2CO2 HCO3 + H+

Question 28

Describe the effect of acidosis on plasma pH, ventilation and the following equation: CO2 + H2O H2CO2 HCO3 + H+

Answer 28

Plasma pH falls leading to increased ventilation and shifting the equation to the left to get rid of the CO2

Question 29

Describe the effect of alkalosis on plasma pH, ventilation and the following equation: CO2 + H2O H2CO2 HCO3 + H+

Answer 29

Plasma pH is increased and hydrogen ion concentration is reduced. This inhibits ventilation and drives the equation to the right to increase CO2

Question 30

Explain hypoxic drive

Answer 30

Chronic lung disease leads to prolonged elevated CO2 levels leading to the desensitisation of central chemoreceptors. Patients depend on peripheral chemoreceptors meaning care must be taken if they are given oxygen because it is not detected that ventilation needs to increase in hypercapnia

Question 31

What involuntary stimuli cannot be overridden by voluntary control from descending neural pathways?

Answer 31

Arterial PCO2 or H+ content Why you can't hold your breath for too long without fainting or why you can't hyperventilate for too long (because you're no longer blowing off CO2)

Question 32

What happens to respiration during swallowing?

Answer 32

Respiration is inhibited during swallowing to prevent aspiration. It is then followed by an expiration to remove particle outwards from a closed glottis.

Question 33

Would it be more unpleasant to be in a chamber with high PCO2 and normal PO2 or a chamber with a low PO2 and no PCO2? Why?

Answer 33

It would be more unpleasant to be in a chamber with high PCO2 and normal PO2 or a chamber with a low PO2 and no PCO2 because the partial pressure gradient would be impaired. CO2 could not be removed from the pulmonary artery. This would also have an effect on the gradient pulling CO2 out of cells and would cause CO2 to be pulled out of cells.

Question 34

What is the effect of barbiturates and opioids on the respiratory system?

Answer 34

Depress respiratory centres. Can result in respiratory failure and death after overdose.

Question 35

What is the effect of gaseous anaesthetic agents on the respiratory system?

Answer 35

Increased respiratory rate but reduced tidal volume.

Question 36

What is the effect of nitrous oxide (gas and air) on the respiratory system?

Answer 36

Sedative/light anaesthetic. | Blunts peripheral chemoreceptors so should not be given to those on hypoxic drive.