Chemical Control of Ventilation

Question 1

What do chemoreceptors do?

Answer 1

Chemical control of ventilation

Detect changes in arterial pCO2, pO2, [H+]

Question 2

Types of chemoreceptors

Answer 2

1. Central chemoreceptors (CCRs)

2. Peripheral chemoreceptors (PCRs)

Question 3

Central chemoreceptors

Answer 3

Found on the medulla

Sensitive to changes in [H+] and pCO2

Question 4

Peripheral chemoreceptors

Answer 4

Found within the aortic arch and carotid arteries

Sensitive to changes in arterial pO2 and pH

Question 5

How do the CCRs work

Answer 5

BBB impermeable to H+ and HCO3-, permeable to CO2
increased pCO2 causes it to diffuse out of the blood vessel
CO2 + H2O -> H2CO3 -> HCO3- + H+
Increase in H+ in ECF and CSF is detected by CCRs
Hyperventilation

Question 6

How do PCRs work?

Answer 6

Decreased arterial O2 - hyperventilation (stimulated when falls below 13.3 mmHg)
Increase pCO2 and pO2 not really detected
Fall in pH - detected by carotid and not aortic bodies

Question 7

What can hypoventilation cause?

Answer 7

Respiratory acidosis

Retaining CO2 - reaction with H2O - H+ ions

Question 8

Compensation for respiratory acidosis

Answer 8

Renal
Increased H+ excretion
Increased HCO3- reabsorption (buffer and neutralisation)

Question 9

What is a consequence of hyperventilation?

Answer 9

Respiratory alkalosis

Question 10

Compensation for respiratory alkalosis

Answer 10

Renal
Increased H+ reabsorption
Increased HCO3- excretion

Question 11

What is a consequence of uncontrolled diabetes?

Answer 11

Metabolic acidosis
Utilising fats for energy, forming ketone bodies which are acidic in nature
Decreased ability of kidneys to excrete H+ and reabsorb HCO3-

Question 12

Compensation for metabolic acidosis

Answer 12

Lungs

Increased ventilation - decrease atrial pCO2

Question 13

Consequence of vomiting

Answer 13

Metabolic alkalosis

Decrease (stomach) acid

Question 14

Compensation for vomiting

Answer 14

Decreased ventilation - increase atrial pCO2

Question 15

Priority of responses (ventilation stimulus)

Answer 15

1. pCO2
2. pH
3. pO2

Question 16

Why is pCO2 the prioritised response?

Answer 16

Central chemoreceptors are the most sensitive to pCO2 changes (levels held within 0.3 kPa)
Peripheral chemoreceptors detect too, comparatively insensitive (1.3 kPa)

Avoid acid-base problems

Question 17

Why is pO2 the lowest priority of response?

Answer 17

Only peripheral chemoreceptors detect changes in pO2
Levels have a wider control margin but PCRs are stimulated when pO2 levels drop below 13.3 kPa
To avoid hypoxia

Question 18

Response to reduced pH

Answer 18

Increased ventilation

Influenced by pCO2

Question 19

Series of response to mild hypoxia

Answer 19

Reduced pO2, increased ventilation, reduced pCO2
Increased CSF pH (alkaline), increased HCO3-
Choroid plexus cells export HCO3- from CSF as a oH correction mechanism
Hypoxic drive is re-instated and ventilation increases further
Hours: breathing controlled around new lower pCO2, increased ventilation from hypoxic drive
Days: alkalinity of blood corrected by excretion of HCO3- in urine

Question 20

Example in clinic

Answer 20

Cheyne-Stokes

Period of hyperventilation then apnoea