19. Control of Breathing

Question 1

How is Breathing Controlled?

Answer 1

• Unaware: until something goes wrong - dyspnea
  ○ e.g. high altitude / disease
• Aware: scuba diving, partners to sleepy snorers

Question 2

Describe the Local Control P(O2) and P(CO2) In Active Tissues.

Answer 2

Increased P(CO2) = Increased CO2 removal + vasodilation 
= Increase Blood Flow

Decreased P(O2) = Increased O2 delivery

Question 3

Describe the Local Control of Gas Transport (Lung Persuasion)

Answer 3

Decreased P(O2) = vasoconstriction = Decreased blood flow

→ direct blood to areas of higher PO2

Question 4

Describe the Local Control of Gas Transport (Alveolar ventilation)

Answer 4

Increased P(CO2) = bronchodilator = Increased air flow

→ direct airflow to areas of higher PCO2

• improve efficiency of gas transport

Question 5

Describe the process of Central Control of Ventilation.

Answer 5

1. Sensors Detect Change: Central & Peripheral Chemoreceptors, Mechanoreceptors
2. Central Controller coordinates response: Respiratory Centres in the pons and medulla
3. Effectors cause change: Muscles of ventilation

Question 6

Role of Central chemoreceptors.

Answer 6

• Located just beneath the ventral surface of the Medulla.
• Change in pH
  ○ Stimulated by acidic pH in the cerebrospinal fluid
• Hypercapnia -> CO2 levels in blood too high
  ○ Stimulated by high PCO2 in the cerebrospinal fluid
• No effect of hypoxia
• Not affected by oxygen
• Close to entry of VIII & XI cranial nerves

Question 7

Describe the location of Peripheral chemoreceptors.

Answer 7

• Outside brain
• Carotid body at bifurcation of carotid arteries
• Innervated by carotid sinus nerve (CSN) → glossopharyngeal
• Aortic bodies above and below aortic arch
• Innervated by the vagus

Question 8

Role of Mechanoreceptors.

Answer 8

Lung receptors

Respond to stretch
○ rapidly adapting receptors
○ slowly adapting receptors
○ C-fibres receptors

Question 9

Define Dyspnoea

Answer 9

Difficult or laboured breathing

Question 10

Factors influencing rate & depth of breathing

Answer 10

• Changing body demands, e.g. exercise
• Altitude – acute mountain sickness
• Disease
• In the arterial blood changing levels of;
  ○ CO2
  ○ H+
  ○ O2
  → Only a problem when PO2 of alveolar gas and arterial blood falls below 60mmHg

Question 11

Which gas has no effect on central chemoreceptors?

Answer 11

↓ PO2

Question 12

Function of Peripheral Chemoreceptors

Answer 12

• Peripheral chemoreceptors are vital for response to ↓ PO2

(no effect on central chemoreceptors)

Question 13

What stimuli effect the Peripheral Chemoreceptors?

Answer 13

Stimulated by ↓PO2, ↑ PCO2, ↓ pH in arterial blood

• hypoxia
• hypercapnia

Question 14

What happens if P(O2) changes?

Answer 14

• If PO2 goes up, e.g. breathing oxygen-rich gas mixtures
  ○ Generate free radicals leading to coma and death
• If PO2 goes down
  ○ Arterial PO2 must drop below 60 mmHg before ventilation is increased
  ○ Central chemoreceptors switch off
  ○ Peripheral chemoreceptors increase breathing rate

Question 15

What happens if P(CO2) decreases?

Answer 15

CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3-

Equation pushes left
= ↓ H+ = pH increases = alkaline = Respiratory alkalosis

Question 16

What happens if P(CO2) increases?

Answer 16

CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3-

Equation pushes right
= ↑H+ = pH decreases = acidic = Respiratory acidosis

Question 17

Name the 3 types of Mechanoreceptors in Lung Tissue and Airways.

Answer 17

1. Slowly adapting Stretch Receptors (bronchopulmonary stretch receptors)
2. Rapidly adapting Stretch Receptors (Irritant receptors)
3. C-fibres receptors (J receptors)

Question 18

Describe the role of Slowly adapting Stretch Receptors.

bronchopulmonary stretch receptors

Answer 18

Hering-Breuer reflex
= prevents the lungs from over-inflating on inspiration

• Stretch receptors in the visceral pleura, bronchioles & alveoli

OverInflation = ↑discharge = inhibition of respiratory centres

Question 19

Describe the role of Rapidly adapting Stretch Receptors

Irritant receptors

Answer 19

• In airway epithelia (close to mucosa)
• Shape the ventilatory pattern and protecting the airway
• Initially fire rapidly but then soon decrease their firing rate

Noxious gases, Smoke/dust, Cold air = ↑discharge = Bronchoconstriction (COUGHING REFLEX)

Question 20

Describe the role of

C-fibres receptors (J receptors)

Answer 20

• In alveoli wall (close to the capillaries) and conducting airways (bronchial mucosa)
• In alveoli wall (close to the capillaries) and conducting airways (bronchial mucosa)

Chemical/ mechanical stimuli = ↑discharge = Bronchoconstriction (rapid shallow breathing, mucus secretion)

Question 21

What are the 2 central controllers of ventilation?

Answer 21

Rhythmicity centre in the medulla oblongata

Pneumotaxic and Apneustic centres in the pons

Question 22

What is the Rhythmicity centre in the medulla oblongata?

Answer 22

• Controls automatic breathing (sets pace for respiratory movements)
• interacting neurons that fire either during :
  ○ inspiration (I neurons) or
  ○ expiration (E neurons).

Question 23

What is the Pneumotaxic and Apneustic centres in the pons?

Answer 23

• Modify firing pattern of medullary centres
• Apneustic centre in the pons increases the intensity of inhalation
• Pneumotaxic centre in the pons stops inhalation and promotes exhalation

Question 24

Role of inspiration (I neurons) in the Rhythmicity centre in the medulla oblongata.

Answer 24

“I” neurons in Dorsal Respiratory Group (DRG) regulate activity of phrenic nerve allows it to set rhythm & stimulate muscles of quiet inspiration.

Question 25

Role of Expiratiory (E neurons) in the Rhythmicity centre in the medulla oblongata.

Answer 25

“E” neurons in Ventral Respiratory Group (VRG) → Passive process.
○ Activity of E neurons inhibit I neurons.

Question 26

What happen is the Apneustic Centre?

Answer 26

• promotes inspiration, stimulates the I neurons

- ↑ intensity of inhalation during respiratory cycle

Question 27

What happens in the Pneumotaxic centre?

Answer 27

• Inhibits apneustic centre = stops inhalation & promotes exhalation

Question 28

How do Central neurons determine ventilation rate (VR)?

Answer 28

Central neurons determine ventilation rate (VR) by regulating tidal volume (TV) and respiratory rate (f)
VR = TV x f

• Steep rise in VR with ↑ PCO2
• Mainly due to central chemo-receptors
• ↓ PO2 = ↑ slope (↑sensitivity)
  ○ Due to peripheral chemoreceptors