Chemical Control Of Breathing

Question 1

What is normal pCO2?

Answer 1

4.7-6kPa

Question 2

What is normal pO2

Answer 2

9.3-13.3kPa

Question 3

What is normal bicarbonate?

Answer 3

22-26mmol/L

Question 4

What is normal pH?

Answer 4

7.35-7.45

Question 5

What are the function of the respiratory system?

Answer 5

Maintain oxygen and carbon dioxide partial pressure gradients to optimise transfer.

Regulate pH of extracellular fluid

Question 6

What is hypercapnia?

Answer 6

Rise in pCO2

Question 7

What is hypocapnia?

Answer 7

Fall in pCO2

Question 8

What is a fall in pO2?

Answer 8

Hypoxia

Question 9

How should the partial pressure be controlled?

Answer 9

In exercise:

• pO2 drops and pCO2 rises
• Breathing more will restore both

Question 10

What is hyperventilation?

Answer 10

Ventilation increases without a change in metabolism.

Question 11

What is hypoventilation?

Answer 11

Ventilation decrease without a change in metabolism.

Question 12

How could hypocapnia occur?

Answer 12

If pO2 changes without a change in pCO2 correction of pO2 will cause pCO2 to drop which will lead to hypocapnia.

Question 13

Why is is good that the Hb dissociation curve goes flat at the top?

Answer 13

Means pO2 can drop quite a lot before saturation is badly effected.

Question 14

What is the carbonic acid-barcobonate system?

Answer 14

This is the major buffer system in blood.

It is highly effective because the amount of dissolved CO2 is controlled by respiration.

HCO3- is regulated by the kidneys.

Question 15

What is the effect of pCO2 on plasma pH?

Answer 15

If [HCO3-] remains unchanged:

If pCO2 increases they pH falls.

If pCO2 decreases then pH rises.

Small changes in pCO2 lead to large changes in pH.

Question 16

What happens if plasma pH goes out of range?

Answer 16

Below 7 = enzymes denatured

Above 7.6 = free Ca drops leading to tetany

Question 17

What is respiratory acidosis?

Answer 17

Hypercapnia leads to fall in plasma pH

Question 18

What is respiratory alkalosis?

Answer 18

Hypocapnia leads to a rise in plasma pH

Question 19

How is respiratory acidosis / alkalosis compensated?

Answer 19

It is compensated by the kidneys by adjusting [bicarbonate]. (Acidosis - increase HCO3-. Alkalosis - decdecrease HCO3-)

This works because pH is controlled by the ratio of CO2:HCO3- rather than the absolute values.

BUT, this takes 2-3 days.

Question 20

How is metabolic acid compensated for?

Answer 20

Acid reacts with HCO3- which leads to fall in [HCO3-] and a decrease in pH.

Compensate by increasing ventilation which lowers pCO2 to restore pH.

Question 21

How does metabolic alkalosis occur and how is this compensated?

Answer 21

If plasma [HC3-] rises (e.g. vomiting), plasma pH rises.

Compensated by decreasing ventilation.

Question 22

What are the respiratory control pathways?

Answer 22

Sensors: Central chemoreceptors (H+), Peripheral chemoreceptors (oxygen, carbon dioxide, H+), pulmonary receptors (stretch), Joint and muscle receptors (stretch, tension)

Effectors: Diaphragm, Inspiration (external intercostals, accessory muscles), Expiration (internal intercostals, abdominal muscles)

Question 23

Where are the peripheral chemoreceptors?

Answer 23

In carotid and aortic bodies

Question 24

What do the peripheral chemoreceptors do?

Answer 24

Large falls in pO2 stimulate: (only if really low- like alarm)

• Increase breathing
• Changes heart rate
• Changes in blood flow distribution i.e. increasing flow to brain and kidneys.

Question 25

What do central chemoreceptors do?

Answer 25

They are located in the medulla of the brain and are much more sensitive to change of pCO2.

They detect changes in arterial CO2 (this works because CO2 is allowed to cross the blood brain barrier).

Smal rises in pCO2 increase ventilation (and small falls decrease).

This is the basis of negative feedback control of breathing.

Question 26

Describe the feedback loop of breathing by pCO2.

Answer 26

Increase in pCO2
Detected by central chemoreceptors
Blow of CO2 as breath more
PCO2 return to normal.

Question 27

What do the central chemoreceptors respond to?

Answer 27

Respond to changes in the pH of CSF.
CSF separated from blood by the blood-brain barrier.
CSF [CO3-] controlled by choroid plexus cells (as arterial can’t cross blood brain barrier).
CSF pCO2 determined by arterial pCO2 (as can cross blood brain barrier).

Question 28

How is CSF pH determined?

Answer 28

Determined by ratio of [HCO3-] to pCO2.

[HCO3-] is fixed in the short term - as blood brain barrier is impermeable to HCO3-

This means that falls in pCO2 lead to rises in CSF pH and rises in pCO2 led to falls in CSF pH.

BUT, persistent changes in pH corrected by choroid plexus cells which change [HCO3-]

Question 29

How do central chemoreceptors work?

Answer 29

Elevated pCO2 drives CO2 into CSF across the blood brain barrier.
CSF [HCO3-] initially constant.
So CSF pH falls.

Fall in CSF pH is detected by central chemoreceptors which drives increased ventilation.

This increased ventilation lowers the pCO2 and restores CSF pH.

Question 30

What does the choroid plexus do?

Answer 30

CSF [HCO3-] determine which pC)2 is associated with ‘normal’ CSF pH.
CSF [HCO3-] therefrom sets the control system to a particular pCO2.
It can be reset by changing CSF [HCO3-]

Question 31

How can persistent hypoxia occur?

Answer 31

Hypoxia detected by peripheral chemoreceptors which increases ventilation.
BUT, pCO2 will fall further and decrease ventilation.
So, CSF composition compensates for the altered pCO2.
Choroid plexus cells selectively add H+ or HCO3- into CSF
They the central chemoreceptors “accept” this is normal.

Question 32

What happens in persistent hypercapnia?

Answer 32

Hypoxia and hypercapnia leads to respiratory acidosis
This means there is a decrease in pH of CSF.
So, peripheral and central chemoreceptors stimulate breathing but the acidic pH is undesirable for neurones.
Therefore, the choroid plexus needs to adjust pH of CSF by adding [HCO3-].
The central chemoreceptors then ‘accept’ the high pCO2 as normal