2 Chemical control of ventilation

Question 1

What is the profile of the blood entering and leaving the alveoli?

Answer 1

• Deoxygenated blood coming in

- Oxygenated blood leaving

Question 2

What is the value of PO2 of air entering the alveoli?

Answer 2

21.2kPa

Question 3

What is the value of PO2 in the alveoli?

Answer 3

13.3kPa

Question 4

What is the value of pO2 in arterial (oxygenated) blood?

Answer 4

13.3kPa

Question 5

What is the pCO2 of venous blood?

Answer 5

6kPa

Question 6

What is the pCO2 of the alveoli?

Answer 6

5.3kPa

Question 7

What is the pCO2 of arterial blood?

Answer 7

5.3kPa

Question 8

Why is a partial pressure gradient between PO2 in alveoli and pO2 in venous blood needed?

Answer 8

To encourage the movement of oxygen in alveoli into the capillary and oxygenate the blood

Question 9

Why is pCO2 important?

Answer 9

It balances the acid/base ratio

Question 10

Why is the difference in partial pressure of pCO2 between venous blood and alveoli important?

Answer 10

Allows CO2 to move from venous blod to alveoli

Question 11

What are chemoreceptors?

Answer 11

They monitor and regulate chemical control of ventilation

• they detect changes in pCO2, pO2, and [H+]
• more [H+] is acidic, low [H+] is basic (in terms of pH)
• Acid base balance is the mechanism the body uses to balance the pH

Question 12

What are the 2 forms of chemoreceptors (and give some features)?

Answer 12

Central chemoreceptors (CCRs)

• found in the CNS in the brain, on the medulla
• It is sensitive to changes in [H+] and pCO2

Peripheral chemoreceptors (PCRs)

• found within the aortic arch and carotid arteries
• it is sensitive to changes in arterial pO2 and pH

Question 13

Describe the action of central chemoreceptors (CCRs found on the medulla)

Answer 13

• In the brain, there are blood vessels, and the blood-brain barrier (BBB)
• In BBB, it is impermeable to H+ ions and HCO3-, but it is permeable to CO2
• Due to increased pCO2, CO2 will diffuse into ECF and CSF, a chemical reaction takes place to produce H+ ions

(CO2 + H2O ⇌ H2CO3 ⇌ HCO3- + H+)

• The H+ dissociates away
• If chemoreceptors in ECF detect a rise in H+ ions, due to increased pCO2 levels, they will send a message to respiratory muscles to increase ventilation
• So, detection of ACIDITY by chemoreceptors promotes HYPERVENTILATION
• If there is low CO2 content, the opposite occurs, due to low H+ ions in ECF and CSF, and less breathing occurs, to keep the H+ levels the same (and so, pCO2 is kept level)

Question 14

Describe the actions of peripheral chemoreceptors (PCRs)

Answer 14

• Found in the heart in aortic and carotid arteries
• They detect changes (mainly decreases) in pO2 and pH (in the arteries)
• e.g. due to exercise, altitude
• A message is sent to the respiratory centre in the brain > another message sent to the respiratory system to increase ventilation

Question 15

What happens if there is decreased Arterial O2?

through PCR detection

Answer 15

Hyperventilation

- stimulated when arterial pO2 falls below 13.3kPa

Question 16

What happens if there is increased pCO2 (PCR detection)?

Answer 16

Not as important as CCR response

Question 17

What happens when there is a fall in pH (PCR detection)?

Answer 17

It is detected by the carotid and not the aortic bodies

Question 18

Describe the events in respiratory-focused ventilation and pH imbalance in hypoventilation

Answer 18

in Hypoventilation, no CO2 is expired (effectively)
- so, increases in pCO2, which then increases [H+], leading to
REAPIRATORY ACIDOSIS

The body uses a compensatory mechanism to fix this (renally, in the kidney)

• increase in [H+] excretion
• increase in [HCO3-] reabsoprtion into blood, which will act as buffer to help alleviate this increase in [H+]

Question 19

Describe the events in respiratory-focused ventilation and pH imbalance in hyperventilation

Answer 19

In Hyperventilation, too much CO2 is expired
- This decreases pCO2, which then decreases [H+], leading to RESPIRATORY ALKALOSIS

The body uses a compensatory mechanism to fix this (renally, in the kidney)

• Increase in [H+] reabsorption
• Increase in [HCO3-] excretion into blood, which will act as a buffer to help alleviate this decrease in [H+]

Question 20

Describe the events in metabolic-focused ventilation and pH imbalance, in uncontrolled diabetes

Answer 20

Uncontrolled diabetes - leads to the formation of ketone bodies

• Leads to METABOLIC (KETO)ACIDOSIS
• The compensatory mechanism means the bicarbonate ions are used up to help buffer this
• So, there is a decrease in the ability of the kidney to excrete H+ and reabsorb HCO3-

Compensatory mechanism of respiratory
- Increase in ventilation = decrease in pCO2, so [H+] reduced

Question 21

Describe the events in metabolic-focused ventilation and pH imbalance, in vomiting

Answer 21

In vomiting - there is a loss of gastric juices (acid)

• Leads to METABOLIC ALKALOSIS
• Need to return acid/base balance to normal

The compensatory mechanism is respiratory:
- Decrease in ventilation = decrease in pCO2 and promote the reaction between CO2 and H2O to increase [H+]

Question 22

What is the priority of responses in terms of a ventilation stimulus

Answer 22

Order of important (most to least)

• pCO2
• pH
• pO2

Deviation from the norm in CO2 is the body’s priority and this is linked to pH, hence pH is 2nd and pO2 is 3rd

Question 23

Describe the control of pCO2

Answer 23

• Central chemoreceptors are the most sensitive to pCO2 changes > levels held to within 0.3kPa
• Peripheral chemoreceptors will detect rapid changes in pCO2 but are comparatively insensitive > levels are held to within 1.3kPa

Why control pCO2?
- to avoid acid/base problems

Question 24

Describe the control of pH

Answer 24

• Decrease in pH = increase in ventilation

- This is influenced by pCO2 levels

Question 25

Describe the control of pO2

Answer 25

• Peripheral chemoreceptors detect changes in pO2
• pO2 levels have a wider control margin but PCRs are stimulated when pO2 levels drop below 13.3kPa

Why control pO2?
- To avoid hypoxia