Control of Ventilation

Question 1

why must ventilation be controlled?

Answer 1

• Ventilation must be controlled to regulate the blood gases.
• are wide fluctuations in VO2 + VCO2, but PaO2 and PaCO2 don’t change under normal circumstances
• This is because ventilation is changing in response to metabolic changes.

Question 2

respiratory rhythm

Answer 2

• The respiratory rhythm is generated in the respiratory centre of the medulla oblongata brain stem
• The activity of the respiratory centre is influenced by inputs from the Pons (sits above the medulla oblongata = has more high level function/ more advanced) and the Vagus (is a nerve = brings info back from the lungs + airways)

Basic rhythmic activity of respiratory muscles that is modified by:

• voluntary cortical control e.g. holding your breath
• involuntary control eg. swallowing, coughing, sneezing etc.

Question 3

CHEMICAL CONTROL OF VENTILATION

Answer 3

• If PCO2 increases, our pH will fall (blood/ tissues will become more acidic = impacts enzyme function)
• Arterial PCO2 (PaCO2) is only allowed to change by + 1 Torr before respiration is altered:
• If PaCO2 increases, ventilation increases
• If PaCO2 decreases, ventilation decreases
• The increase in ventilation caused by an increase in PaCO2 is called the hypercapnic drive
• PaO2 is allowed to fall much more severely before ventilation is stimulated
• PaO2 must fall from about 100 Torr to 50-60 Torr before ventilation is increased.
• The increase in ventilation caused by a decrease in PaO2 is called the hypoxic drive
• Normal respiration relies on the hypercapnic drive. If the hypercapnic drive is suppressed, the hypoxic drive may be insufficient to maintain breathing.

Question 4

chemoreceptors

Answer 4

• Requires a system to measure blood gas values - uses chemoreceptors.
• Chemoreceptors = structures we use to measure the amount of O2 + CO2 in the blood

Question 5

PERIPHERAL CHEMORECEPTORS

Answer 5

• Carotid bodies = measure amount of O2 + CO2 in the blood thats going to the brain
• Aortic bodies = measure amount of O2 + CO2 in the blood thats going to other parts of the body
• Most important in response to changes in PO2

Question 6

CENTRAL CHEMORECEPTOR FUNCTION

Answer 6

• Most important in response to changes in PCO2
• Located on surface of medulla
• 85% of PCO2 response is mediated centrally.
• CO2 which enters the CSF (cerebral spinal fluid) enters carbonic anhydrase reaction: H+ + HCO3 ←→ H20 + CO2
• A decrease in pH of CSF stimulates ventilation
• PCO2 response may be lost due to hypercapnia, eg in COPD patients
• With prolonged hypercapnia, pH will slowly return to normal due to diffusion of HCO3 into CSF

Question 7

flow chart info = need to look at image

Answer 7

Cerebral coretx = send info of voluntary control

• Afferents: chemoreceptors, cerebral cortex, pons etc
• Efferents: diaphragm, intercostal muscles
  = responsible for changing lung volume