Lecture 18: Control of breathing

Question 1

Control of breathing is controlled by what 3 mechanisms?

Answer 1

1. Central control = Brainstem
   - sets patterns and rhythms of breathing
   - coordinates sensors and effectors to maintain respiratory homeostasis
2. Sensors - CNS or PNS gather info either chemical or physical - respiratory centre receives neural and chemical inputs form central & peripheral receptors
3. Effectors - respiratory muscles which adjust ventilation

Question 2

Why do you need control of breathing?

How do you keep the levels of PaO2 and PaCO2 constant?

Answer 2

You need to maintain normal levels of PaO2 and PaCO2 for metabolic and biochemical stability

• O2 uptake and CO2 production is quite variable throughout the day but LEVELS of PaO2 and PaCO2 are kept within close limits by tight control of ventilation
• Levels remain constant due to tight control of ventilation

Question 3

What are central chemoreceptors CCR?
Where are they located?
What do they respond to?

Answer 3

• Located in the medulla
  CCR respond to pH change - they are sensitive to the PCO2 —> pH —-> increased H+
• CO2 diffuses out of the cerebral capillaries which changes pH of ECF and CSF
  CENTRAL = CO2
  CCR are bathed in brain ECF though which CO2 easily diffuses from blood vessels the cerebral spinal fluid. The CO2 reduces pH, thus stimulating the chemoreceptors

Question 4

What are the peripheral chemoreceptors PCR?
Where are they located?
What do they respond to?

Answer 4

peripheral = O2

• located in the carotid and aortic bodies
• respond to decreased arterial PO2
• Limited response to PCO2 changes

Question 5

Ventilatory response to CO2:

• what is the most important stimulus to ventilation?
• Where does most of the stimulus come from?
• How is the ventilatory response reduced?
• What effect does ventilatory response have on breathing?

Answer 5

• PaCO2 is the most important stimulus to ventilation
• Most of the stimulus comes from the CCR as CO2 is the main factor that effects ventilation. PCR also contributes and its response is faster
• Response to CO2 is reduced by sleep, increasing age, genetic factors, training and drugs
• increasing PaCO2 creates a powerful drive to breath
  = breathing is driven by the accumulation of CO2

CO2 drives ventilation

Question 6

What is the ventilatory response to hypoxia? (low O2)

Answer 6

There is negligible control during normal conditions but when PO2 is less than 60mmHg you stimulate hypoxia control of ventilation

• Hypoxic control becomes important at high altitude, and in long term hypoxemia caused by chronic lung disease
• Peripheral receptors are involved

Question 7

You produce about 13000 mmol CO2 per day. You need to be continuously exhaling CO2 to match its production rate. How is CO2 removed?

Answer 7

CO2 is removed through ventilation if you do not ventilate properly you get:
Hypercapnia = when you accumulate CO2 - pH problems

Question 8

What is hypoventilation?

What is hyperventilation?

Answer 8

Hypoventilation: CO2 is in excess - excess H+ which is called “respiratory acidosis” = equation shifts right

Hyperventilation: CO2 is removed, not enough - decreased H+ “ respiratory alkalosis” = equation shifts left

Question 9

What is compensation?

Answer 9

It occurs when there is an integrated response - the lungs and kidneys work together to compensate for pH abnormalities

e. g kidneys compensate during ‘respiratory alkalosis’ by excreting more H+ and by synthesising more HCO3
e. g lungs compensate by exhaling CO2 which reduces H+ level during ‘respiratory acidosis’