Control Of Breathing

Question 1

What are the two key tasks of breathing control?

Answer 1

1. Establish automatic rhythm
2. Adjust the rhythm to accommodate
   - metabolic (arterial blood gases and pH)
   - mechanical (postural changes)
   - episodic non-ventilatory behaviours (speaking, sniffing, eating)

Question 2

How is breathing controlled?

Answer 2

Changes in blood flow and oxygen delivery → local

Changes in depth and rate of respiration → central

This involves a complex interaction between local and central mechanisms.

Question 3

What is the balance achieved between O2 and CO2 in normal conditions?

Answer 3

O2: rate of absorption is matched to delivery; CO2: rate of generation is matched to removal

This balance is crucial for maintaining homeostasis in the body.

Question 4

What happens to P_O2 and P_CO2 in active tissue?

Answer 4

↓P_O2 —> increases O2 delivery and blood flow

↑P_CO2 —>↑CO_2 removal and ↑blood flow (vasodilation)

Question 5

What effect does ↓P_O2 have on lung perfusion?

Answer 5

Vasoconstriction, ↓ blood flow
directs blood to areas of higher P_O2

Question 6

What do central chemoreceptors respond to?

Answer 6

Located in medulla

• change in pH
• Hypercapnia (increased CO2)
• no effect of hypoxia

Question 7

Which gas has no effect on central chemoreceptors?

Answer 7

P_O2

Central chemoreceptors do not respond directly to changes in oxygen levels.

Question 8

What are peripheral chemoreceptors sensitive to?

Answer 8

Hypoxia, hypercapnia, and changes in pH

Located in the aortic and carotid bodies, they play a crucial role in ventilatory responses.

Question 9

What is the Hering-Breuer reflex?

Answer 9

A defense mechanism that inhibits respiratory centers in response to over-inflation

Involves slowly adapting stretch receptors in the lungs.

Question 10

What happens during respiratory acidosis?

Answer 10

Increased P_CO2 (>43 mmHg) leads to more H+ released and a drop in pH (pH < 7.35)

Equation shifts to the right

CO2 + H2O <—> H2CO3 <—> H+ + HCO3-

This condition can arise from limited gas exchange, such as in emphysema.

Question 11

What is the response when P_O2 drops below 60 mmHg?

Answer 11

Ventilation is increased

This response is mediated by peripheral chemoreceptors.

Question 12

What are the three types of mechanoreceptors in lung tissue?

Answer 12

Characterised by response to lung inflation

1. Slowly adapting stretch receptors = Hering Breuer reflex
2. Rapidly adapting stretch receptors = cough reflex
3. C-fibre receptors = defence mechanism

Question 13

What is the role of the rhythmicity center in the medulla?

Answer 13

Controls automatic breathing

It consists of interacting neurons that fire during inspiration and expiration.

Question 14

What do the pneumotaxic and apneustic centers in the pons do?

Answer 14

Modify firing patterns of medullary centers to regulate respiratory rate and depth

(Adjust rate & depth at which we breathe)

Question 15

How does the body respond to increased levels of CO2?

Answer 15

Increased ventilation rate and depth

CO2 and H+ are key regulators of breathing.

Question 16

Fill in the blank: The equation for ventilation rate (VR) is VR = ______.

Answer 16

TV x f

Where TV is tidal volume and f is respiratory rate.

Question 17

True or False: Central chemoreceptors mainly detect changes in O2 levels.

Answer 17

False

Central chemoreceptors primarily respond to CO2 levels.

Question 18

What effect does high P_CO2 have on alveolar ventilation?

Answer 18

Bronchodilation = increases air flow
Directs airflow to areas of higher P_CO2

Question 19

What factors influence the rate and depth of breathing?

Answer 19

• change in body demands eg. Exercise
• altitude - acute mountain sickness
• disease
• changing levels of CO2, H+ and O2

Only a problem when pO2 of alveolar gas and arterial blood falls below 60 mmHg

Question 20

What are mechanoreceptors sensitive to?

Answer 20

Lung receptors

• respond to stretch
• rapidly adapting receptors
• slow adapting receptors
• C-fibres receptors

Question 21

What is the role of central chemoreceptors?

Answer 21

• located just beneath the ventral surface of the medulla
• close entry of VII & XI (type of) cranial nerves
• stimulated by acidic or high pCO2 in the cerebral spinal fluid (CSF)

High pCO2 —> lowers pH —> increases ventilation to exhale more CO2 —> lowers pCO2

CO2 crosses the blood-brain barrier = lipid soluble (CSF is only weakly buffered)

Question 22

What is the role of peripheral chemoreceptors?

Answer 22

• detect changes in pO2, pCO2 and pH
• located outside the brain in the carotid body at bifurcation (where it splits) of carotid arteries
• Innervated by carotid sinus nerve (CSN) which is a branch of the glossopharyngeal nerve
• aortic bodies above and below the aortic arch
• Innervated by the vagus

Question 23

What happens in respiratory alkalosis?

Answer 23

Decreased pCO2 (<37 mmHg) means less H+ and an increase in pH (>7.45)

Equation shifts to the left

CO2 + H2O <—> H2CO3 <—> H+ + HCO3-

E.g. hyperventilation (during panic attacks)

Question 24

What happens when pO2 changes?

Answer 24

pO2 goes up
- Generates free radicals leading to coma and death
- e.g. breathing oxygen-rich gas mixtures

pO2 goes down
- arterial pO2 must drop below 60 mmHg before ventilation is increased
- central chemoreceptors switch off (as don’t respond to pO2)
- Peripheral chemoreceptors increase breathing rate

Question 25

What is the role of slow adapting stretch receptors?

Answer 25

Known as bronchopulmonary stretch receptors
Located in the visceral pleura, bronchioles and alveoli

Over inflation —> increase discharge of receptors —> inhibition of respiratory centres

Known as the Hering-Breuer reflex = forces exhalation

Question 26

What is the role of rapidly adapting stretch receptors?

Answer 26

Known as irritant receptors
Located in airway epithelia (close to mucosa)

Irritating stimulants (i.e. noxious gases, smoke, dust or cold air) —> increases discharge of the irritant receptors —> bronchoconstriction = coughing reflex

Shape the ventilatory pattern and protecting the airway
Initially fire rapidly but then soon decrease their firing rate

Question 27

What is the role of C-fibres receptors?

Answer 27

Known as J receptors
Located in alveoli wall (close to the capillaries) and conducting airways (bronchial mucosa)

Chemical/ mechanical stimuli —> increase discharge of the receptors —> bronchoconstriction = rapid shallow breathing and mucus secretion

E.g. mechanical
Left heart failure causes engorgement of the pulmonary capillaries (due to backflow of blood)
Innervates the C-fibres receptors causing rapid shallow breathing

Question 28

What is the role of the rhythmicity centre?

Answer 28

Set pace for respiratory movements

• I neurons in DRG regulate activity of phrenic nerve = set rhythm and stimulate muscles of quiet inspiration
• E neurons in VRG is a passive process = activity of E neurons inhibit I neurons
• Pneumotaxic and Apneustic centres of the pons adjust the output of the rhythmicity centres

Question 29

What is the Dorsal Respiratory Group (DRG)?

Answer 29

Location in the medulla where I neurons (inspiratory neurons) are = inspiratory centre

Question 30

What is the Ventral Respiratory Group (VRG)?

Answer 30

Location in the medulla where E neurons (expiratory neurons) and I neurons (inspiratory neurons) are = Expiratory centre

Question 31

What is the role of the Pneumotaxic and Apneustic centres?

Answer 31

Modify firing pattern of medullary centres
Regulate respiratory rate and depth of respiration

Apneustic centre
- promotes inspiration, stimulates the I neurons
- increases intensity of inhalation during respiration cycle

Pneumotaxic centre
- inhibits Apneustic centre
- stops inhalation and promotes exhalation