Control of Breathing

Describe the control of breathing

Ventilation is controlled by a feedback loop involving:

• Inputs
• Integration and control centres
• Effectors

Inputs

Inputs to the respiratory centre comes from a number of sensors:

• Chemoreceptors
  Chemoreceptors act synergistically. Chemoreceptors are divided into:
  • Peripheral
  • Central
• Mechanoreceptors
• Other effects

Peripheral Chemoreceptors

Peripheral chemoreceptors are divided into:

• The carotid body
  Located at the bifurcation of the common carotid artery, and are innervated by the glossopharyngeal nerve (CN IX).
• The aortic body
  Located in the aortic arch, and innervated by the vagus (CN X).

Peripheral chemoreceptors are stimulated by:

• Low PaO₂
  Peripheral chemoreceptors are stimulated by low O₂ tension
  
• High PaCO₂
  Peripheral receptors have a rapid (~1-3s) but weaker (~20% of response) to changes in CO₂, compared to central chemoreceptors
  
• Acidaemia
  (Carotid bodies only)
• Hypotension

Central Chemoreceptors

• Central chemoreceptors are located on the ventral medulla, and are stimulated by a fall in CSF pH
  • H⁺ and HCO₃^- are ionised, and cannot cross the BBB by diffusion
  • Because of this, central chemoreceptors respond indirectly to changes in arterial PaCO₂
    • Carbon dioxide is lipid soluble and freely diffuses into CSF
    • In CSF, carbon dioxide combines with water (catalysed by carbonic anhydrase) to form H⁺ and HCO₃^-

• This gives the central chemoreceptors a number of special properties:
  • Increased sensitivity
    Increased relative to plasma due to minimal buffering (as there is less protein in CSF)
  • Respond to respiratory acidosis
    Fixed acid does not cross the blood brain barrier and so have a minimal response on CSF pH. Cerebral hypoxia increases CSF lactate, which will stimulate respiration.

Mechanism of CO₂ Retention

• Prolonged respiratory acidosis (i.e. prolonged CSF acidosis) stimulates active secretion of bicarbonate into the CSF

• When pH normalises, the stimulation of central chemoreceptors ceases

• Similarly, renal absorption of bicarbonate increases, which normalises arterial pH and reduces peripheral chemoreceptor stimulation

Mechanoreceptors

Stretch receptors in bronchial muscle are stimulated by overinflation, and stimulate the apneustic centre to reduce inspiratory volumes. This is the Hering-Breuer reflex.

Other Stimulants

Other inputs which stimulate respiration include:

• Juxtacapillary receptors (J-receptors)
  Receptors in alveolar walls, potentially stimulated by oedema and emboli.
• Irritant receptors
  Inhalation of noxious gases stimulates respiration.
• Pain receptors
• Thalamus
  Increased core temperature stimulates respiration.
• Limbic system
  Emotional responses.
• Cerebral cortex
  Conscious control of breathing.
• Muscle spindles
  Ventilatory response to exercise.

Integration and Control

The respiratory centre is located in the medulla and the pons. It consists of four groups:

• Dorsal Respiratory Group (DRG)
  Controls the diaphragm, and is so only involved with inspiration.
• Ventral Respiratory Group (VRG)
  Controls the intercostal muscles, and so is involved in inspiration and expiration.
• Apneustic Centre
  Modulates DRG function to prevent over-expansion. Loss of this area causes apneusis - long, deep breaths.
• Pneumotaxic Centre
  Also modulates the DRG, increasing RR and decreasing V_T to maintain MV.

References

1. CICM February/April 2015
2. CICM March/May 2009
3. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
4. Brandis K. The Physiology Viva: Questions & Answers. 2003.