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SA respiratory > S4) Chemical Control of Breathing > Flashcards

S4) Chemical Control of Breathing Flashcards

1
Q

What is the concentration of bicarbonate in the blood?

A

22–26mmol/L

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2
Q

What is the average PaO2 in the blood?

A

9.3–13.3kPa

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3
Q

What is the average PaCO2 in the blood?

A

4.7–6.0kPa

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4
Q

Identify 2 functions of the respiratory system

A
  • Maintain oxygen and carbon dioxide partial pressure gradients to optimise transfer
  • Regulate pH of ECF
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5
Q

What is hypoxia?

A

Hypoxia is a fall in pO2

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6
Q

What is hypercapnia?

A

Hypercapnia is a rise in pCO2

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7
Q

What is hypocapnia?

A

Hypocapnia is a fall in pCO2

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8
Q

What is hyperventilation?

A

Hyperventilation is when ventilation increases without change in metabolism

  • respiratory alkalosis
  • if HCO3 secereted
  • compensated respiratory alkalosis (fully or compensated depends if pH Is back to normal)
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9
Q

What is hypoventilation?

A

Hypoventilation is when ventilation decreases without change in metabolism

  • respiratory acidosis
  • increase Hco3
  • compensated respiratory acidosis
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10
Q

How is hypocapnia caused?

A
  • pO2 changes without a change in pCO2
  • correction of pO2 will cause pCO2 to drop
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11
Q

Why does a control system need to avoid marked hypoxia?

A
  • Oxygen-Haemoglobin dissociation curve is flat from approx. 8kPa
  • Hence, pO2 can fall considerably before saturation is markedly effected
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12
Q

What equation is used to make calculations in the carbonic acid-bicarbonate buffer system?

A

pH = pK + log [HCO3-] / [H2CO3-]

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13
Q

Demonstrate the effect of pCO2 on plasma pH if bicarbonate concentration doesn’t change

A

If [HCO3-] remains unchanged:

  • pCO2 increase = pH falls (notably)
  • pCO2 decrease = pH rises (notably)
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14
Q

What happens if the pH rises above 7.6?

A

Free calcium concentration drops which leads to tetany

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15
Q

What happens if the pH falls below 7.0 ?

A

Enzymes become denatured

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16
Q

In two steps, explain how respiratory acidosis occurs

A

⇒ Hypoventilation leads to an increase in pCO2

⇒ Hypercapnia leads to a fall in plasma pH

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17
Q

In two steps, explain how respiratory alkalosis occurs

A

⇒ Hyperventilation leads to a decrease in pCO2

⇒ Hypocapnia leads to a rise in plasma pH

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18
Q

How do the kidneys compensate for respiratory acidosis?

A

Respiratory acidosis is compensated by the kidneys increasing [HCO3-]

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19
Q

How do the kidneys compensate for respiratory alkalosis?

A

Respiratory alkalosis is compensated by the kidneys decreasing [HCO3-]

20
Q

How long does it take the kidney to compensate for pH changes?

A

2-3 days

21
Q

In two steps, explain how metabolic acidosis occurs

A

⇒ Tissues produce acid & this reacts with HCO3-

⇒ The fall in [HCO3-] leads to a fall in pH

22
Q

How can metabolic acidosis be compensated for?

A

Change ventilation:

  • Increased ventilation lowers pCO2
  • pH is restored to normal
23
Q

In two steps, explain how metabolic alkalosis occurs

A

⇒ If plasma [HCO3-] rises e.g. after vomiting

⇒ Plasma pH rises

24
Q

How can metabolic alkalosis be compensated for?

A

Compensated to a degree by decreasing ventilation

25
Q

State 3 principles which allow compensation for pH changes to happen?

A
  • Plasma pH depends on the ratio of [HCO3-] to pCO2 and not their absolute values
  • Respiratory driven changes in pH compensated by the kidney
  • Metabolic changes in pH compensated by breathing
26
Q

Compare and contrast the pH, pCO2 and HCO3- in the following conditions:

  • Metabolic acidosis
  • Respiratory acidosis
  • Metabolic alkalosis
  • Respiratory alkalosis
A
27
Q

Where are the peripheral chemoreceptors located?

A
  • Carotid bodies
  • Aortic bodies

they detect changes in pCo2 and P02

impulse is carried via glossopharyngeal nerves and vagus nerves

28
Q

Explain how peripheral chemoreceptors are sensitive to large changes in pO2

A

Stimulates:

  • Increased breathing
  • Changes in heart rate
  • Changes in blood flow distribution (increasing flow to brain & kidneys)
29
Q

Compare and contrast peripheral and central chemoreceptors in terms of pCO2 sensitivity

A
  • 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
30
Q

What do the central chemoreceptors do?

A

Detect changes in arterial pCO2:

  • Small rises in pCO2 increase ventilation
  • Small falls in pCO2 decrease ventilation
31
Q

Ilustrate the negative feedback control of breathing by central chemoreceptors

A
32
Q

Central chemoreceptors respond to changes in the pH of cerebro-spinal fluid (CSF).

Explain the pH control of the CSF

A

CSF separated from blood by the blood-brain barrier:

  • CSF [HCO3-] controlled by choroid plexus cells
  • CSF pCO2 determined by arterial pCO2
33
Q

What determines the pH of CSF?

A

CSF pH is determined by ratio of [HCO3-] to pCO2

34
Q

Why is [HCO3-] constant in the short term?

A

Blood Brain Barrier is impermeable to HCO3-

35
Q

What is the signficance of this fixed [bicarbonate]?

A
  • Falls in pCO2 lead to rises in CSF pH (decreases ventilation)
  • Rises in pCO2 lead to falls in CSF pH (increases ventilation)
36
Q

What is the role of [HCO3-] in pH of CSF?

A
  • CSF [HCO3-] determines which pCO2 is seen as ‘normal’ CSF pH (‘sets’ the control system)
  • It can be ‘reset’ by changing CSF [HCO3-] (choroid plexus cells)
37
Q

Describe the events that lead to persistent hypoxia

A
  • Hypoxia detected by peripheral chemoreceptors which increases ventilation
  • But pCO2 falls further which decreases ventilation
  • So, CSF composition compensates for the altered pCO2 and choroid plexus cells selectively add H+ / HCO3- into CSF
  • Central chemoreceptors “accept” pCO2 as normal
38
Q

Describe the events that lead to persistent hypercapnia

A
  • Hypoxia and hypercapnia lead to respiratory acidosis
  • Decreased pH of CSF cause both chemoreceptors to stimulate breathing
  • Acidic pH is undesirable for neurons so choroid plexus adjusts pH of CSF by adding HCO3-
  • Central chemoreceptors “accept” the high pCO2 as normal
39
Q

which parts of the brain are responsible for the respiratory rate

A
  • dorsal neurones → medulla
  • ventral neurones → ventral lateral side of medulla
  • pneumotaxic centre of neurones → dorsally on pons (sends inhibitory pulses to to slow down inspiration)
40
Q

how do stretch receptors prevent over inflation of lungs

A
  • stretch receptors are located in bronchi and bronchioles
  • they transmit info via vagus nerve to brainstem
  • prevent over inflation
41
Q

which group of neurones set the basic respiratory rate

A

dorsal neurones on the medulla

42
Q

which group of neurones set the basic respiratory rate

A

dorsal neurones on the medulla

43
Q

what are baroreceptors and where are they located

A

they detect arteriole BP

found in carotid sinus and aortic arch

44
Q

what controls body response to oxygen

A
  • carotid and aortic bodies
  • stimulated by decrease in oxygen
  • only respond to significant falls
  • they increase tidal volume and rate of respiration
  • direct more blood to the brain and they kidneys and increase heart rate
45
Q

what is controls the body response to co2

A

aortic and carotid bodies only respond quickly and only to large changes in PcCo2

46
Q

what sense the bodies change In Ph

A

carotid bodies

47
Q

what is the order we follow to check the ABG

A
  1. pH
  2. pCo2
  3. bicarb levels

SA respiratory (23 decks)

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