6. Chemical Control Of Breathing

Question 1

What is hypercapnia?

Answer 1

A rise in pCO2.

Question 2

What is hypocapnia?

Answer 2

Fall in pCO2.

Question 3

What is hypoxia?

Answer 3

Fall in pO2.

Question 4

What is hyperventilation? What affect does this have on pO2 and pCO2?

Answer 4

An increase in ventilation without a change in metabolism.

pO2 rises and pCO2 falls.

Question 5

What is hypoventilation? What affect does this have on pO2 and pCO2?

Answer 5

A decrease in ventilation without a change in metabolism.

pO2 falls and pCO2 rises.

Question 6

What happens to pO2 and pCO2 in exercise?

Answer 6

PO2 drops and pCO2 rises.

Breathing restores both.

Question 7

What happens if pO2 decreases without a change in pCO2, and then pO2 is corrected?

Answer 7

pCO2 drops on correction of pO2, leading to hypocapnia.

Question 8

What happens if pH falls below 7.0?

Answer 8

Enzymes become denatured.

Question 9

What happens if pH rises above 7.6?

Answer 9

Free calcium concentration drops leading to tetany.

Question 10

What is tetany?

Answer 10

Intermittent muscle spasms causes by a deficiency of calcium.

Question 11

What causes respiratory acidosis?

Answer 11

Hypercapnia as a result of hypoventilation leads to an increase in pCO2 and a fall in plasma pH.

Question 12

What causes respiratory alkalosis?

Answer 12

Hypocapnia as a result of hyperventilation leads to a decrease in pCO2 and an increase in plasma pH.

Question 13

How do the kidneys compensate for respiratory acidosis?

Answer 13

By increasing [HCO3-] (excrete less).

Question 14

How do the kidneys compensate for respiratory alkalosis?

Answer 14

By decreasing [HCO3-] (excrete more).

Question 15

How long does it take for the kidneys to compensate for respiratory acidosis and alkalosis?

Answer 15

2-3 days.

Question 16

What happens in metabolic acidosis?

Answer 16

Tissues produce acids, which reacts with HCO3-. The fall in [HCO3-] leads to a fall in pH.

Question 17

How can a change in ventilation rate compensate for respiratory acidosis?

Answer 17

Increased ventilation lowers pCO2 (to restore [HCO3-] to [CO2] ratio as [HCO3-] has fallen), and restores the pH towards normal.

Question 18

What happens in metabolic alkalosis?

Answer 18

Plasma [HCO3-] rises, eg after vomiting. So plasma pH rises.

Question 19

Can metabolic acidosis be compensated for by ventilation?

Answer 19

Yes, to a degree by decreasing ventilation.

Question 20

Is the control of pO2 or pCO2 more critical when controlling breathing?

Answer 20

pCO2 as affects pH.

pO2 just needs to stay above 8kPa.

Question 21

What sensors located in the CNS and periphery feed information back to the respiratory control centre, so that ventilation can be adjusted where necessary?

Answer 21

Central chemoreceptors.
Peripheral chemoreceptors.
Pulmonary receptors.
Joint and muscle receptors.

Question 22

Where are peripheral chemoreceptors located?

Answer 22

In the carotid and aortic bodies.

Question 23

What do peripheral chemoreceptors detect? What does this stimulate?

Answer 23

Large falls in pO2, increase breathing, changes in heart rate, increase blood flow to brain and kidneys.
Detect changes in pCO2, but are relatively insensitive.

Question 24

Where are central chemoreceptors found?

Answer 24

In the medulla of the brain.

Question 25

What do central chemoreceptors detect? What does this stimulate?

Answer 25

Changes in arterial pCO2.
Small rises in pCO2 increase ventilation.
Small falls in pCO2 decrease ventilation.

Question 26

What forms the basis of the negative feedback control of breathing?

Answer 26

Central chemoreceptors detecting changes in arterial pCO2.

PCO2 rises, detected by central chemoreceptors, stimulates breathing, blow off CO2, pCO2 returns to normal.

Question 27

How do central chemoreceptors detect changes in arterial pCO2?

Answer 27

Respond to changes in the pH of the cerebro-spinal fluid. The CSF is separated from the blood by the blood-brain barrier. CSF [HCO3-] is controlled by choroid plexus cells and is fixed short term. CSF pCO2 is determined by arterial pCO2.
pH is determined by the ratio of [HCO3-] to pCO2. So falls in pCO2 lead to rises in CSF pH (decrease ventilation). Rises in pCO2 lead to falls in CSF pH (increase ventilation).
Persisting changed in pH are corrected by the choroid plexus cells which change [HCO3-].

Question 28

What happens in type 1 respiratory failure, persisting hypoxia (eg where pneumonia or fluid collecting in lungs)?

Answer 28

Hypoxia (low O2) detected by peripheral chemoreceptors, ventilation increased.
But pCO2 falls further, so then decrease ventilation, but then not enough O2.
CSF composition compensates for the altered pCO2, and the choroid plexus cells selectively add H+ or HCO3- into the CSF.
Causes the central chemoreceptors to accept the pCO2 as normal, so decreased pH.

Question 29

What happens in type 2 respiratory failure, persisting hypercapnia (eg in drug overdose, COPD)?

Answer 29

Reduced gas exchange, so hypoxia (fall in O2) and hypercapnia (high CO2).
Leads to fall in pH (respiratory acidosis).
pH of CSF decreases, and peripheral and central chemoreceptors stimulate breathing, but hypoxia and hypercapnia cannot be corrected.
Acidic pH is undesirable for neurons, and to the choroid plexus needs to adjust the pH of CSF. Adds HCO3-, and the central chemoreceptors accept the high pCO2 as normal.