S4.2 Chemical Control Of Breathing

Question 1

What is the effect of hyperventilation on pO2 and pCO2?

Answer 1

Rise in pO2 and a fall in pCO2.

Can cause hypocapnia, which leads to increased pH and respiratory alkalosis.

Question 2

What is the effect of hypoventilation on pO2 and pCO2?

Answer 2

Fall in pO2 and a rise in pCO2.

Can cause hypercapnia which leads to decreased pH and respiratory acidosis.

Question 3

What are the affects of pH being too high or low?

Answer 3

If pH rises above 7.6 free calcium concentration drops leading to tetany. pH <7 causes denatured enzymes.

Question 4

How does compensation occur to correct pH in Respiratory conditions?

Answer 4

Respiratory acidosis: kidneys increase [HCO3-]

Respiratory alkalosis: kidneys decrease [HCO3-]

Question 5

How does compensation occur to correct pH in metabolic conditions?

Answer 5

Metabolic acidosis: increased breathing to reduce pCO2

Metabolic alkalosis: decreased ventilation to increase pCO2

Question 6

How is breathing controlled?

Answer 6

Respiratory control centre receives input from:
Central chemoreceptors (H+) and Peripheral chemoreceptors (O2, CO2, H+).
These then send impulses to muscles of inspiration and expiration

Question 7

What is the role of peripheral chemoreceptors?

Answer 7

Found in carotid and aortic bodies.
Large falls in pO2 stimulate increased breathing, changes in HR and increased blood flow.
Can detect CO2 but not very sensitive

Question 8

What is the role of the central chemoreceptors?

Answer 8

Found in medulla.
More sensitive to changes in pCO2.
Increase in CO2 reduces pH of CSF, this stimulates central chemoreceptors causing increased ventilation

Question 9

What is the role of the choroid plexus?

Answer 9

Controls CSF [HCO3-].

Determines which pCO2 is associated with a normal CSF pH, so ‘sets’ the control system to a particular pCO2.

Question 10

What happens if hypoxia persists?

Answer 10

Detected by peripheral chemoreceptors so ventilation increases, but pCO2 will fall further so ventilation decreases.
Choroid plexus cells selectively add H+ or HCO3- into CSF
Central chemoreceptors accept the pCO2 as normal

Question 11

What happens if hypercapnia persists?

Answer 11

CSF pH falls.
Peripheral and central chemoreceptors stimulate breathing but acidic pH is undesirable for neurones, so choroid plexus adjusts pH of CSF by adding HCO3-.
Central chemoreceptors then accept the high pCO2 as normal (they reset).
The persistent hypoxia stimulates peripheral chemoreceptors, so
Respiratory drive is now driven by hypoxia