Chemical control of breathing Flashcards
How is ventilation controlled?
- Automatic centres in brainstem activate respiratory muscles rhythmically and subconsciously
Which tasks does ventilation need to accommodate?
- Maintain adequate oxygen status
- Adjust respiration for changing metabolic status/needs reflected by altered pO2, pCO2, pH
Where is the the respiratory control centre located?
Medulla, pons
What are the sensors for respiration?
- Peripheral chemoreceptors
- Central chemoreceptors
- Pulmonary mechanoreceptors
What are the effectors for respiration?
- Respiratory muscles
- Diaphragm
What are the different types of respiratory peripheral chemoreceptors?
- Carotid bodies
- Aortic bodies
Where are carotid bodies located?
- Bifurcation of common carotid arteries
- Do not confuse with carotid sinus
Where are aortic bodies located?
- Located in aortic arch of thorax
What is the major function of the peripheral chemoreceptors?
- Sense hypoxaemia
- Signal cells in medulla to increase ventilation
- Both carotid and aortic bodies primarily sensitive to decreases in arterial pO2
- Also stimulated by hypercapnia and acidosis
Describe glomus cells
- Derived from ectoderm
- Neuron-like characteristics
- Excitable cells (depolarisation generates action potentials)
What is the function of glomus cells?
- When pO2 is low, membrane depolarises
- Causes release of intracellular vesicles of neurotransmitters
- Neurotransmitter release from glomus cells triggers action potentials in carotid body nerve afferent fibres
- This signals low paO2 to medulla
What do carotid body glomus cells do?
- When PO2 decreases below ~8kPa
- Stimulates neurotransmitter and ATP release which activates afferent fibres (carotid sinus nerve)
- Signals sent to CNS and respiration stimulated
- Similar mechanism for PCO2 and H+ - alveolar ventilation increased
What effect does paO2 have on carotid body afferent nerves?
- Alters nerve impulse firing rate from carotid body
- Firing rate increases significantly after paO2 drops from 8-4 kPa
- I.e. when Hb saturation with O2 decreases rapidly
What happens when peripheral chemoreceptors sense low paO2 and/or high paCO2?
- They feed back to medulla respiratory centres to increase minute ventilation
- Leads to increase in paO2 and decrease in paCO2
- Limited response in people with lung disease
How does increasing minute ventilation compensate for acidosis?
- CO2 strongly influences blood pH
- If CO2 levels increase, H+ increases - pH falls
- If CO2 levels decrease, H+ decreases - pH rises
What is the sensory innervation for the carotid body?
- Branch of CN IX
What is the sensory innervation for the aortic body?
- Branch of CN X
What increases peripheral chemoreceptors’ sensitivity to acidosis and hypercapnia?
- Hypoxaemia
What are the first chemoreceptors to respond to hypoxaemia?
- Carotid and aortic bodies
What are central chemoreceptors?
- Anatomical collection of neuronal chemoreceptors (specialised neurones)
- Located just beneath ventral surface of medulla on brain side of blood brain barrier
- Bathed in CSF
- Few hundred microns away from brainstem respiratory centres
What is the function of the central chemoreceptors?
- Sense increases in paCO2
- Also senses decreases in arterial pH but much more slowly
- Does not detect changes in pO2
When are central chemoreceptors the primary source of feedback to respiratory centres?
- When blood gas parameters are nearly normal
What happens if pCO2 increases suddenly?
- Ventilation increases rapidly
- Augments minute ventilation
How do central chemoreceptors sense changes in pCO2 and pH?
- BBB separates central chemoreceptors in medulla from arterial blood
- BBB has low permeability to ions (H+ and HCO3-) but high permeability to small molecules (CO2)
- CO2 diffuses into CSF bathing central chemoreceptors
- CNS receives very limited HCO3- buffering capacity
- Acidosis develops
- Even small decreases in pH raise firing rate of central chemoreceptor neurones
- Ventilation increases
