Control of Breathing
Describe the control of breathing
Ventilation is controlled by a feedback loop involving:
- Integration and control centres
Inputs to the respiratory centre comes from a number of sensors:
Chemoreceptors act synergistically. Chemoreceptors are divided into:
- Other effects
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
(Carotid bodies only)
- 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
- 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.
- Increased sensitivity
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
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 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
Increased core temperature stimulates respiration.
- Limbic system
- 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 VT to maintain MV.
- CICM February/April 2015
- CICM March/May 2009
- Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
- Brandis K. The Physiology Viva: Questions & Answers. 2003.