Respiratory: Control of breathing

Question 1

What is hyperventilation and hypoventilation?

Answer 1

There is a mismatch:
In hyperventilation there is an increase in ventilation without a change in metabolism, leading to increased O2 and decreased CO2 - resp alkalosis
In hypoventilation there is a decrease in ventilation without a change in metabolism, leading to decreased O2 and increased CO2 - resp acidosis

Question 2

What is the effect of pCO2 on plasma pH?

Answer 2

If HCO3- is unchanged

• an increase in pCO2 will cause pH to fall
• a decrease in pCO2 will cause pH rise

There is a logarithmic relationship therefore small changes in pCO2 can cause big changes in pH

Question 3

What can cause a metabolic acidosis or metabolic alkalosis? How are they compensated?

Answer 3

Metabolic acidosis: If tissues produce acid it reacts with HCO3-, therefore HCO3- falls and pH falls.
(can be compensated forby increasing RR)

Metabolic alkalosis: HCO3- can rise after vomiting leading to a rise in pH.
(can only be partially compensated for by decreasing RR because this risks hypoxia)

Question 4

How do the central chemoreceptors detect a change in pCO2?

Answer 4

CO2 can diffuse across the BBB into the CSF then reacts with water to form HCO3- and H+ which is detected by the chemoreceptors

Question 5

What controls the levels of HCO3- in the cerebrospinal fluid?

Answer 5

The choroid plexus cells - however persistent changes to HCO3- conc which is corrected by the choroid plexus cells eventually leads to the ‘normal’ HCO3- conc being reset higher

Question 6

Is the BBB permeable to HCO3- and CO2?

Answer 6

HCO3- cannot cross

CO2 can readily diffuse across

Question 7

What changes occur (by chemoreceptors) when there is persisting hypoxia?

Answer 7

The hypoxia is detected by peripheral chemoreceptors and increases ventilation - however pCO2 will then fall causing decreased ventilation.
The CSF compensates for the high pCO2 and accepts it as normal
Therefore hypoxia becomes the driving force of ventilation