Control of breathing

Question 1

Where is the respiratory centre located in the brain?

What types of neurons are located there?

What are their roles?

Answer 1

Brainstem: pons and medulla

Pons: neurons in the pons contribute to the pneumotaxic centre

2 groups of neurons in the medulla:

• Dorsal respiratory group contains inspiratory neurons
• Ventral respiratory group contains inspiratory and expiratory neurons
• Both of these generate the respiratory pattern

Question 2

Describe the respiratory feedback cycle

Answer 2

Regular, cyclical output from the central controller → peripheral NS → effector muscles → indirect effect on chemoreceptors (pCO₂ & pO₂ changes) → central controller

Other control of the effector muscles: voluntary control and involuntary (e.g. cough reflex)

Question 3

What influences the respiratory centres?

Which are the principal regulators of respiration?

Answer 3

Principal regulators:

Arterial blood gases: measured by central and peripheral chemoreceptors

• ​Measure arterial pO₂, pCO₂ and pH

Other regulators:

Hering-Breuer reflex: stretch receptors in the lung

• Less important reflex but comes into play when lungs are overstretched, i.e in deep inspiration.

Higher centres

Voluntary control

Involuntary control, non rhythmic control:

• E.g. cough reflex

Question 4

Where are central chemoreceptors?

Describe how they regulate respiration

Answer 4

Principal sensors of pCO₂

Located in the medulla adjacent to CSF, respond to pCO₂ generated changes in pH of CSF.

pH of CSF determined by CO₂ and HCO₃^-:

• CO₂ can diffuse across the blood brain barrier so CO₂ in the blood will be the same as CSF.
• HCO₃^- enters CSF via the choroid plexus, cannot diffuse across BBB.

Send information to respiratory centres to regulate ventilation rates.

Question 5

Describe the Hering-Breuer reflex

Answer 5

1. Stretch receptors in airways detect overstretching of lungs
2. Communicate afferent information to respiratory centre in medulla which inhibits inspiratory neurones
3. Efferent information sent to respiratory effector muscles to cease inspiration permitting exhalation.

Question 6

How do blood gases regulate respiration?

Answer 6

The need to provide tissues with oxygen is met by a hypoxic drive to respiration

Respiratory responses to CO₂ faciliate the need to excrete CO₂

The need to defend acid-base balance means that responses to CO₂ levels are stronger than responses to O₂ levels

Question 7

Define:

Hypercapnia

Hypocapnia

Hypoxia

Answer 7

Hypercapnia: rises in p_aCO₂ (arterial partial pressure of CO₂)

Hypocapnia: falls in p_aCO₂

Hypoxia: falls in p_aO₂

Question 8

What is the effect of hyperventilation and hypoventilation on O₂ and CO₂ levels if there is no change in O₂ and CO₂ usage?

Answer 8

Hyperventilation:

• Fall in p_aCO₂
• Rise in p_aO₂

Hypoventilation:

• Rise in p_aCO₂
• Fall in p_aO₂

Question 9

What effect will an increase in perfusion have on alveolar CO₂?

Answer 9

Increase in p_ACO₂

Question 10

At what oxygen saturation does ventilation quickly increase?

Answer 10

90% (8kPa)

Question 11

Where are peripheral chemoreceptors located?

How do they regulate respiration?

Answer 11

Primarily detect p_aO₂ but also detect p_aCO₂, pH, hypotension and some chemicals.

• Located in the carotid bodies
• Receive blood at a very high rate, thus very little difference between aortic and venous pO₂.
• Rapid response (1-3 seconds)
• Detect p_aO₂ levels and send information to medullary respiratory centres via CNIX.
• Can increase and decrease both rate and depth of respiration.
  
  • Also increase BP and secretion of corticosteroids from adrenal glands.

Question 12

How are central chemoreceptors moderated?

Answer 12

Response of central chemoreceptors to changes in pCO₂ are regulated by levels of O₂:

• If p_aO₂ is low when increased p_aCO₂ is high, response will be rapid.
• If p_aO₂ is high when p_aCO₂ is high, response will be slower and will require a higher level of CO₂ to increase ventilation.

Question 13

How do central chemoreceptors control respiratory compensation?

Answer 13

Metabolic acidosis: systemic HCO₃^- will fall, meaning less is secreted into the CSF by the choroid plexus.

This causes pH of CSF to fall.

This is detected by central chemoreceptors which stimulate the respiratory centres to initiate hyperventilation to excrete more CO₂ and correct pH.

Metabolic alkalosis: systemic HCO₃^- will rise, so more will be secreted by the choroid plexus into CSF.

pH of CSF rises which is detected by central chemoreceptors which stimulate the respiratory centres to initiate hypoventilation.

Question 14

What is the base excess on an ABG?

What base excess indicates metabolic acidosis and alkalosis?

Answer 14

The amount of strong acid that must be added or removed for each litre of fully oxygenated blood to return the pH to 7.4 at a temperature of 37°C and pCO₂ of 5.3kPa

• By correcting for the respiratory component, the base excess represents the metabolic component.
• Positive BE = metabolic alkalosis
• Negative BE = metabolic acidosis

Question 15

What are 5 causes of metabolic acidosis?

Answer 15

Lactic acidosis (sepsis/ischaemia)

Ketoacidosis (diabetes)

Acute renal failure

Excessive loss of HCO₃^- (e.g. raised plasma chloride)

Poisons (e.g. aspirin)