Chemical Control Ventilation (M2)

Question 1

LO’s

Answer 1

1. Describe the characteristics of the ventilatory responses to oxygen, carbon dioxide and acid
2. Describe what happens to ventilation when mixed oxygen, carbon dioxide and acid stimuli are given
3. Describe the location of the peripheral and central chemoreceptors and understand their roles in the control of ventilation
4. Explain why it is dangerous to give oxygen to some patients who are in respiratory failure
5. Describe the relative importance of oxygen and carbon dioxide in ventilatory control in healthy subjects at rest

Question 2

What is the main driving force of ventilation?

Answer 2

CO2

Question 3

How do changes in alveolar pO2 affect ventilation?

Answer 3

• Normal pO2 is 13.7kPa
• The body doesn’t respond to oxygen levels effectively, only responds when pO2 falls to 8kPa
• Pulmonary ventilation rises significantly after pO2 falls to 8kPa

Question 4

How do changes in alveolar pCO2 affect ventilation?

Answer 4

• For every 1kPa rise in pCO2 results in a 15L rise in pulmonary ventilation.
• Alveolar pCO2 drive ventilation, beacuse there is a steep rise in pulmonary ventilation rate for a small change in pCO2.
• However, once pCO2 reaches 15kPa, which is very high, ventilation doesn’t continue to increase, rather it depresses, and the patient is unconscious.

Question 5

What is the lungs’ response to acidosis?

Answer 5

• Not important in ventilation, as need a significant change in arterial pH for it to affect ventilation.
• No stimulation of ventilation until arterial pH is reduced by 0.1pH unit.
• Fall in arterial pH of 0.4 pH unit required for 2- to 3- fold increase in ventilation.

Question 6

What happens to ventilation when there is high pCO2 and low PO2?

Answer 6

Potentiation happens: because there is hypoxia and hypercapnia, the affect on ventilation is exaggerated more than, if there was high CO2 or low O2 alone.

Question 7

What senses changes in the O2 and CO2 levels?

Answer 7

• Peripheral Chemoreceptors: detect chemical changes, in periphery.
• Mainly carotid bodies: has the highest blood supply in the body relative to it’s size. Found on your corotid.
• Aortic bodies: found on the aorta.

Question 8

Describe Type 1 Glomus Cells in Carotid Bodies.

Answer 8

• Respond to hypoxia, changes in pCO2 and fall in pH.
• The T1 Glomus cells in carotid bodies are most sensitive to low oxygen levels.
• They are weakly sensitive to high CO2 or low pH.
• Secrete neurotransmitters to activate afferent nerve terminals from cranial nerve IX (glossopharyngeal)

Question 9

Describe central chemoreceptors?

Answer 9

• Cappilaries surrounding medulla oblongata in brain.
• CO2 leaves capillary, goes into cerebro-spinal fluid surrounding the medulla oblongata.
• CO2 racts with water to form H2CO3
• Hydrogen carbonate dissociates into HCO3- and H+.
• The central chemoreceptors in the medulla detect the H+ and fire.
• In the cerebro-spinal fluid there are no proteins to buffer the H+ so they can be detected by the chemoreceptors, and thus reaction can happen.

Question 10

What happens to COPD patients’ response to chronically high COPD levels?

Answer 10

• If have too much H+ in the CSF, the choroid plexus increases the amount of bicarbonate to buffer excess CO2.
• So CSF pH may be close to normal in COPD patients.
• This means patients with COPD are desensitised to CO2.
• Control of ventilation lies with peripheral chemoreceptors detecting hypoxia.
• So hypoxia is the stimulation for COPD patients to ventilate.
• This is why we don’t give COPD patients 100% oxygen.