Chemical Control Of Breatheing

Question 1

Define hypercapnia, hypocapnia and hypoxi

Answer 1

Hypercapnia - rise in pCO2
Hypocapnia - fall in pCO2
Hypoxia - fall in pO2

Question 2

Define hyperventilation and hypoventilation

Answer 2

• Hyperventilation
• Ventilation increase without change in metabolism pO2 will rise, pCO2 will fall
• Hypoventilation
• Ventilation decrease without change in metabolism - pO2 will fall, pCO2 will rise

Question 3

What happens when there ar pO2 changes without a change in pO2

Answer 3

If pO2 changes without a change in pCO2, correction of pO2 (increase ventilation) will cause pCO2 to drop. Leading to hypocapnia

Question 4

What are the elects of pH disturbance

Answer 4

• Plasma pH is controlled between 7.38 – 7.46
• If pH falls below 7.0 enzymes become denatured
• If pH rises above 7.6 free calcium concentration drops leading to tetany

Question 5

How does ventilation influence pH

Answer 5

• Hypoventilation leads to an increase in pCO2
• Hypercapnia leads to a fall in plasma pH
– Respiratory acidosis
• Hyperventilation leads to a decrease in pCO2
• Hypocapnia leads to a rise in plasma pH
– Respiratory alkalosis
Respiratory bc cause if in ventilation

Question 6

How do the kidneys compensate for respiratory acidosis/alkalosis

Answer 6

• plasma pH depends on the ratio of [HCO3-] to pco2, not on their absolute values
• changes in pCO2 can be compensated by changes in [HCO3-]
• the kidney controls [HCO3-]
• respiratory acidosis is compensated by the kidneys increasing [HCO3-]
• respiratory alkalosis is compensated by the kidneys decreasing [HCO3-]
• this takes 2-3 days

Question 7

What happens when the tissue produce acid

Answer 7

• if the tissues produce acid, this reacts with HCO
• the fall in [HCO3-0 leads to a fal in pH
• metabolic acidosis
• this can be compensated by changing ventilation • increased ventilation lowers pCO • restores pH towards normal

Question 8

What happens if plasma [HCO3-] rises

Answer 8

• if plasma [HCO3-]
• plasma pH rises
• metabolic alkalosis
• can be compensated to a degree by decreasing ventilation

Question 9

Where is precise control needed

Answer 9

• Control of two partial pressures

• No need for precise control of pO2 as it stays above 8kPa
• Control of pCO2 much more critical
• Changes in ventilation can correct metabolic disturbances of pH

Question 10

Describe teh respiratory control athwart

Answer 10

See slide

Question 11

Describe the peripheral chemoreceptors

Answer 11

• Carotid and aortic bodies - large blood flow
• large falls in pO2 stimulate - very low
– increased breathing
– changes in heart rate
– Changes in blood flow distribution
• i.e. increasing flow to brain & kidneys
Getting blood at most oxygenated at highest possible flow

Question 12

Decsribe central chemoreceptors

Answer 12

• peripheral chemoreceptors will
detect changes but are relatively insensitive to pCO2
• central chemoreceptors in the medulla of the brain are much more sensitive to pCO2 - measuring in CSF not plasma -

• detect changes in arterial pCO2
• small rises in pCO2 increases ventilation
• small falls in pCO2 decrease ventilation
• the basis of negative feedback
control of breathing

Question 13

Describe feedback controlof breathing by pCO2

Answer 13

Ph o csf not ph of plasma - critical to regulation of breathing -s EE lies

Question 14

Describe central chemoreceptors physiology

Answer 14

• respond to changes in the pH of cerebro-spinal fluid (CSF)
• CSF separated from blood by the blood-brain barrier
• CSF [HCO3-] controlled by choroid plexus cells which produce CSH
• CSF pCO2 determined by arterial pCO2
See slide e

Question 15

Explain the central chemoreceptors feedback op

Answer 15

See slide

Question 16

Describe how csf ph is controlled

Answer 16

• determined by ratio of [HCO3-] to pCO2
• [HCP3-] fixed in short term as bbb impermeable tip HCO3-
• so alls in pCO2 lead to rises in CSF pH
• but persisting changes in pH corrected by choroid plexus cells which change [HCO3-]

Question 17

What happens in he CSF if pCO2 is elevate

Answer 17

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

• Fall in CSF pH detected by central
chemoreceptors • Drives increased ventilation
• Increased ventilation • Lowers pCO • and restores CSF pH
2

Question 18

What sets the control system pCO2

Answer 18

• CSF [HCO3- determines which pCO2 is associated with normal CSF pH
• CSF [HCO3-] therefore sets the control system to a particular pCO2
• It can be ‘reset’ by changing CSF [HCO3-]

Question 19

How does csf composition compensate for altered pCO2 in persisting hypoxia?

Answer 19

Persisting hypoxia
• Example
– pO2 = 8.6 kPa
– pCO2 = 3.9 kPa
– pH = 7.47
• Hypoxia detected by peripheral chemoreceptors
– Increase ventilation
• BUT pCO2 will fall further
– Decrease ventilation
• SO: CSF composition compensates for the altered pCO2
• Choroid plexus cells selectively add H+ or HCO3 into CSF
• Central chemoreceptors “accept” the pCO2 as normal

Question 20

Describe the role of the central chemoreceptors in persisting hypercapnia

Answer 20

• Hypoxia and hypercapnia
• Respiratory acidosis
• Decreased pH of CSF
• Peripheral and central chemoreceptors stimulates breathing
• But acidic pH undesirable for neurons
• Therefore choroid plexus needs to adjust pH of CSF
• Addition of HCO3- into CSF
• Central chemoreceptors “accept” the high pCO2 as normal