Chemical Control of Breathing

Question 1

What is HYPERCAPNIA?

Answer 1

A rise in pCO₂

Question 2

What is HYPOCAPNIA?

Answer 2

A fall in pCO₂

Question 3

What is HYPOXIA?

Answer 3

A fall in pO₂

Question 4

In exercise, how are changes in pO₂ and pCO₂ controlled?

Answer 4

In exercise, pCO₂ increases and pO₂ decreases

Both will be restored by increased breathing

Question 5

What happens in hyperventilation?

Answer 5

Hyperventilation= extra breathing

• pO₂ will rise
• pCO₂ fall

There is no change in metabolism

Question 6

What happens in hypoventilation?

Answer 6

A decrease in hypoventilation

• pCO₂ will rise
• pO₂ will fall

There is no change in metabolism

Question 7

Explain why pO₂ is able to fall considerably before it cause hypoxia

Answer 7

Oxygen binding to Hb curve is sigmoidal

meaning pO₂ can drop but Hb remains saturated to an extent

Question 8

What is the normal range of plasma pH and what happens if pH goes beyond these limits?

Answer 8

Normal plasma pH = 7.38 - 7.46

Fall below 7.0 → enzymes denature

Rise above 7.6 → concentration of [Ca²⁺] drops causing tetany

Question 9

Explain what happens in to plasma pH in hypoventilation

Answer 9

Hypoventilation increases dissolved CO₂ shifting the reaction to the right → increase in H⁺ ions causes respiratory acidosis

Question 10

Explain what happens in to plasma pH in hyperventilation

Answer 10

Decrease in pCO₂ shifts equilibrium to the left → less H⁺ causes pH to rise

Causes respiratory alkalosis

Question 11

How do the kidneys respond to respiratory acidosis and respiratory alkalosis?

Answer 11

Response to respiratory acidosis= kidneys increase [HCO₃^-] (retain)

Response to respiratory alkalosis= kidneys decrease [HCO₃^-] (excrete)

Question 12

How long does it take for the kidneys to respond to changes in plasma pH?

Answer 12

2-3 days

Question 13

Explain how metabolic acidosis occurs?

Answer 13

Tissues producing acid react with HCO₃^-

Fall in HCO₃^- causes a drop in pH→ more acidic

Question 14

Explain how metabolic alkalosis occurs?

Answer 14

Plasma [HCO₃^-] can rise after extreme vomiting → vomiting causes a rapid loss of H⁺ ions

Plasma HCO₃^- rises

Question 15

How can metabolic acidosis and metabolic alkalosis be restored?

Answer 15

Changes in breathing

Metabolic acidosis= resolved by increased breathing

Metabolic alkalosis = resolved by decreased breathing

Question 16

What do peripheral chemoreceptors detect and where are they located?

Answer 16

Sense changes in O_2, CO_2, and H⁺

Located in the carotid and aortic bodies

Question 17

Where are central chemoreceptors located? And what do they detect?

Answer 17

Located in the medulla

Detect changes in H⁺ and increased pCO₂

Question 18

Explain how peripheral chemoreceptors respond to large falls in pO₂

Answer 18

1. Fall in pO₂ detected by peripheral chemoreceptors
2. Impulses carries by glossopharangeal and vagus nerves to the respiratory centre in the brain stem
3. Increase in breathing and blood flow distribution

Question 19

Explain how central chemoreceptors respond to changes in pCO₂

Answer 19

1. Increase CO₂ causes increased H⁺ ions in CSF
2. low pH detected by central chemoreceptors in medulla of brain
3. Stimulates increased breathing to blow of CO₂
4. CO₂ levels return to normal feeding back on the central chemoreceptors to return resp rate to normal

Question 20

How are pCO₂ and [HCO₃^-] levels controlled in the CSF?

Answer 20

• pCO₂ determined by arterial pCO₂
• [HCO₃^-] controlled by choroid plexus cells (cells that line the ventricles of the brain and produce CSF) as HCO₃^- cannot cross the BBB

Question 21

What determines the pH of CSF?

Answer 21

The ratio of pCO₂ : [HCO₃^-]

Question 22

Explain how CSF pH is determines in the short vs long term

Answer 22

Short term: CSF pH determined by cahnges in pCO₂ as the [HCO₃^-] is fixed due to impermeability of BBB

Long term: Choroid plexus cells respond to changes in pH to change [HCO₃^-]

Question 23

What happens to central chemoreceptors in persisting hypoxia

Answer 23

Initially … Hypoxia detected by peripheral chemoreceptors to increase ventilation → pCO₂ falls

If hypoxia is persistant the negative feedback doesn’t occur so the choroid plexus cells selectively add H⁺ or HCO₃^- to CSF to correct pH

As CSF pH is corrected, changes in ventilation driven by altered pCO₂ dissapear and the system is set to a new normal

Question 24

What do central chemoreceptors do in response to peristing hypercapnia?

Answer 24

• Hypercapnia causes respiratory acidosis, lowering pH of CSF
• Decrease in pH stimulates peripheral chemoreceptors in stimulate breathing
• Choroid plexus cells need to adjust to low pH to keep neurones healthy so add HCO₃^-
• Central chemoreceptors accept high pCO₂ as the new normal