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How is breathing controlled?
NEED to maintain normal levels of PaO2 and PaCO2 for metabolic and biochemical stability (e.g pH)
BUT O2 uptake and CO2 production quite variable
DESPITE this PaO2 and PaCO2 are normal kept within close limits.
Achieved by tight control of ventilation (air in and air out).
Components of control system for respiration
- 1. Central control - in brainstem.
- - This sets pattern /rhythm of breathing, without thinking. Sets basal background level.
- - Coordinates sensors and effectors to maintain respiratory homeostasis when changes occur.
- 2. Sensors
- - We have central / peripheral - gather information
- - 2 different types - chemical(CO2) and physical (muscle stretch receptors).
The respiratory centre receives a variety of neural and chemical inputs from central and peripheral receptors.
- 3. Effectors- Respiratory muscles --> adjust ventilation
Diagram of control system
Flow diagram of control system
Can breathing be altered consciously?
Yes - the cortex can over-ride the autonomic/rhythmic centre.
What are the types of control/signal for breathing
- - CCRs = Central chemo-receptors
- - PCRs = Peripheral chemo-receptors
- - Physical receptors
- - Voluntary input descending from the cortex
- - Central control system collects information from these centres and sends innervation signals to the respiratory muscles with the goal of keeping gas levels steady.
It is uncertain how the intrinsic rhythmicity of respiration arises
The brainstem has different respiratory centres.
Neurons located in pons and medulla control rhythmic nature of inspiration and expiration.
Other parts of brainstem involved
- Medullary respiratory centre
- - Rhythm generator??
- - Dorsal area respiratory group (inspiration) (at back)
- - Ventral area respiratory group (mainly expiration) (front)
Apneustic centre / Pneumotaxic centre
Cortex can overide / modify
brainstem -> voluntary control / emotion (limbic system)
Where are the receptors involved in the control of ventilation, and what do they respond to?
- 1. Central Chemoreceptors - are in brainstem, adjacent to the respiratory control centre.
- Respond to CO2 only.
- CO2 is most sign driver of ventilation.
- Is soluble --> moves across membranes
- --> drives pH drop in ECF or CSF surrounding brainstem
- --> Chemoreceptors respond to changes in pH, not CO2.
- 2. Peripheral Chemoreceptors - are in peripheries
- Respond to CO2 and O2 levels.
In same place as baroreceptors (left and right carotid arteries, and aortic arch)
- - Detect peripheral gas levels
- - Tend to respond to changes in peripheral arterial P02 levels. Limited response to PCO2 changes
- - Act as a backup control to the CCRs CO2 control.
- - Rapidly responding
Diagram of CO2 movement.
What is the process by which the CCRs detect changes in gas partial pressure?
Where are the Peripheral Chemo Receptors located?
- - left and right carotid arteries
- - aortic arch
Where are the Central Chemo Receptors located and what do they respond to?
- - Located in medulla (brain stem)
- - Respond to CO2 levels by detecting changes in pH.
Describe the ventilatory response to CO2
- 1. PaCO2 is the most important stimulus to ventilation
- --> Normally tightly controlled (+/-3 mmHg)
2. Most of the stimulus comes from the central chemoreceptors - peripheral chemoreceptors also contribute - PCRs response is much faster but not as significant.
- 3. The ventilatory response to PCO2 is reduced by:
- - sleep
- - increasing age
- - genetic factors
- - training
- - drugs.
- 4. Limits breath holding – increasing PaCO2 creates powerful drive to breathe
- --> this is why people "breathe water" and drown.
What is the graphical relationship between Ventilation and CO2 levels?
Changes in CO2 levels cause a larger increase in ventilation levels, especially as PCO2 increases.
Curve becomes flatter with sleep, increasing age, genetic factors, training, drugs.
What is hypoxia? (Low PO 2 )
Hypoxia is low PO2
What is the ventilatory response to hypoxia (Low PO2?)
1. Only the peripheral chemoreceptors are involved
- 2. There is negligible control during normal conditions
- – need PO 2 < 60mmHg to get stimulation of ventilation by hypoxia
3. Get increased response if hypercapnic (too much carbon dioxide (CO2) in the blood)
- 4. Hypoxic control becomes important at:
- - high altitude
- - in long-term hypoxemia caused by chronic lung disease.
What is the graphical relationship between ventilation and PO2? How does this change when hypercapnic?
Where does the vagus nerve get its name from?
Vagus, from vagrant - wanders everywhere
What are the non-chemical receptors that control breathing?
Receptors located in the airways and lungs connected to respiratory centers via vagus nerve
- Lung receptors
- - Stretch - Lung distension
- - Irritants – Smoke, dust, cold air
- Other receptors
- - Nose/upper airway - change breathing if in polluted area.
- - Joint/muscle - change breathing in anticipation of movement
- - Arterial baroreceptors - driven by changes in blood pressure.
- - Pain and temperature - can increase ventilation rate.
Where does the phrenic nerve pass and what can damage to the phrenic nerve do?
- Passes down the cervical spine and controls the diaphragm.
- Exits the spinal cord at C3, 4 and 5 (keeps you alive)
- Therefore any damage to the spine above C3 will definitely damage the phrenic nerve and affect breathing.
- Signal to respiratory muscles is via the phrenic nerve, so damage to this nerve will mean that the muscles are not innervated
- --> no output to the respiratory muscles
- --> cannot breathe.
At any given point in time, what affects the control system?
- - choice/emotion
- - sleep, drugs
- - exercise - CCR affected by CO2 --> output to the respiratory muscles.
However, people change their ventilation in advance of the exercise commencing - some anticipation involved.
- As starting to exercise (before CO2 levels increase), the respiratory control system has already taken action. Ventilation can change in advance due to:
- - voluntary inputs
- - movement
--> CCRs kick in slightly later once CO2 has increased.
Overview of the control system, showing:
- controlled variable