Where is the respiratory centre located in the brain?
What types of neurons are located there?
What are their roles?
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
Describe the respiratory feedback cycle
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)
What influences the respiratory centres?
Which are the principal regulators of respiration?
Arterial blood gases: measured by central and peripheral chemoreceptors
- Measure arterial pO2, pCO2 and pH
Hering-Breuer reflex: stretch receptors in the lung
- Less important reflex but comes into play when lungs are overstretched, i.e in deep inspiration.
Involuntary control, non rhythmic control:
- E.g. cough reflex
Where are central chemoreceptors?
Describe how they regulate respiration
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.
Describe the Hering-Breuer reflex
- Stretch receptors in airways detect overstretching of lungs
- Communicate afferent information to respiratory centre in medulla which inhibits inspiratory neurones
- Efferent information sent to respiratory effector muscles to cease inspiration permitting exhalation.
How do blood gases regulate respiration?
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
Hypercapnia: rises in paCO2 (arterial partial pressure of CO2)
Hypocapnia: falls in paCO2
Hypoxia: falls in paO2
What is the effect of hyperventilation and hypoventilation on O2 and CO2 levels if there is no change in O2 and CO2 usage?
- Fall in paCO2
- Rise in paO2
- Rise in paCO2
- Fall in paO2
What effect will an increase in perfusion have on alveolar CO2?
Increase in pACO2
At what oxygen saturation does ventilation quickly increase?
Where are peripheral chemoreceptors located?
How do they regulate respiration?
Primarily detect paO2 but also detect paCO2, 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 paO2 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.
How are central chemoreceptors moderated?
Response of central chemoreceptors to changes in pCO2 are regulated by levels of O2:
- If paO2 is low when increased paCO2 is high, response will be rapid.
- If paO2 is high when paCO2 is high, response will be slower and will require a higher level of CO2 to increase ventilation.
How do central chemoreceptors control respiratory compensation?
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.
What is the base excess on an ABG?
What base excess indicates metabolic acidosis and alkalosis?
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
What are 5 causes of metabolic acidosis?
Lactic acidosis (sepsis/ischaemia)
Acute renal failure
Excessive loss of HCO3- (e.g. raised plasma chloride)
Poisons (e.g. aspirin)