Session 4 - Carbon dioxide in blood + chemical control of breathing Flashcards

1
Q

How much Carbon dioxide is in arterial blood compared to oxygen? Why?

A

21 mmol/L
Almost 2.5x as much Co2. (oxygen = 8.9mmol/L)
Co2 is more soluble than oxygen.

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2
Q

Why is control of Co2 important in arterial blood?

A

Control of Co2 is important as concentration of Co2 determines pH.

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3
Q

How does some CO2 react with water in plasma?

A

CO2 + H20 > H2CO3 > H+ + HCO3-

Forms carbonic acid (H2CO3) which quickly dissociated, forming protons and bicarbonate ions.
Reversible!

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4
Q

Which factors affect the pH of plasma?

A

1) How much CO2 reacts to form H+
2) How much CO2 is dissolved in it
3) The concentration of hydrogen carbonate

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5
Q

What affects how much CO2 forms H+?

A
  • More dissolved CO2, pushed reaction to the right, more protons.
  • Less dissolved CO2, pushes reaction the left, less protons.
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6
Q

How does dissolved CO2 change the pH? What is the main control of this?

A
  • pCO2 Rises - plasma pH FALLS.
  • pCO2 falls - plasma pH RISES.
  • pCO2 of alveoli is the determining factor, as the amount of CO2 dissolved is directly dependent of partial pressure of CO2.
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7
Q

Roughly how much hydrogen carbonate is present in the blood? Which cation is associated with it?

A

Around 25mmol/L of HCO3-
Main cation is Na+, not H+
Therefore this high bicarbonate conc. is not from CO2 in plasma.

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8
Q

Why does most of the dissolved CO2 not react in the plasma?

A

Due to the high concentration of bicarbonate ions.

Means the equilibrium is pushed to the left, where most CO2 does not react.

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9
Q

Why is the pH of blood slightly alkaline?

A

Due to the dissolved bicarbonate ions.

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10
Q

Which equation can be used to calculate the pH of blood from CO2 and bicarbonate concentration?

A

Henderson Hasselbalch.
pH = pK + Log ([HCO3-]/(pCO2 x 0.23))

pK is a constant = 6.1 at 37C
(0.23 is CO2 solubility constant)

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11
Q

Which main factors then are responsible for the pH of arterial blood? What controls these factors?

A

Ratio of [HCO3-] and pCO2.
Breathing rate changes pCO2.
Kidneys vary excretion of HCO3-

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12
Q

What determines the pCO2 of arterial blood?

A

The rate of ventilation in the lungs.

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13
Q

How is hydrogen carbonate produced in the red blood cells?

A

Carbonic Anhydrase
Reaction is forward, as H+ binds to Haemoglobin, and bicarbonate is removed by the chloride bicarbonate exchanger.
(1 bicarbonate out, 1 chloride in)

This allows bicarbonate to be made more readily in the RBC.
Creates plasma level of 25mmol/L of bicarbonate.

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14
Q

What determines the amount of bicarbonate that erythrocytes produce?

A

The amount of H+ that binds to negative sites in haemoglobin.

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15
Q

What is the relationship between plasma [HCO3-] and pCO2?

A

[HCO3-] doesnt change much with pCO2.

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16
Q

How is the amount of HCO3 in the blood controlled?

A

Kidneys vary the amount by varying secretion.

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17
Q

How does hydrogen carbonate buffer extra acid with only minimal changes in pH?

A

Acids (e.g. lactic, keto, sulphuric) react with bicarbonate forming CO2.
Therefore [HCO3-] goes down.
The extra CO2 is removed by breathing.

18
Q

Why is pCO2 slightly higher in venous blood?

A

From metabolically active tissues, which produce CO2.

19
Q

How does oxygen binding to Hb affect proton buffering?

A
  • If more O2 binds Hb = R-state, less H+ ions bind. (at lungs)
  • If less O2 binds Hb = T-state, more H+ ions bind (at tissues)
20
Q

Why are the proton binding properties of Hb important for CO2 transport in venous system?

A

At low O2 binding in tissues, T state of Hb allows MORE H+ to bind.
This allows MORE HCO3- to be produced.
This means there is MORE CO2 present in plasma in venous blood.
(in both dissolved and reacted form)

21
Q

Why is there little change in pH in venous blood when more cO2 is dissolved?

A

More HCO3- produced (as more H+ bind to Hb in R state).
[dissolved CO2] increases a little.
Both increase together, so little change in pH, as pH uses pk + log(HCO3- / pCO2 x 0.23)

22
Q

How is extra CO2, stored as HCO3 in venous blood, removed at the lungs?

A
  • Hb picks up O2 at lungs, goes into R-State.
  • Causes extra H+ it took up to be given up.
  • H+ reacts with HCO3- producing CO2.
  • CO2 is breathed out.
23
Q

What are carbamino compounds?

A

Co2 bound directly to amine groups on Hb.
More binds at tissues due to >pCO2.
Given up at lungs.

24
Q

So, which three forms is CO2 transported in?

A

1) Dissolved CO2
2) As hydrogen carbonate
3) As carbamino compounds

25
Q

How much CO2 is in arterial, and venous blood. What does this show?

A

Arterial = 21.5 mmol/L
Venous = 23.3 mmol/L
So only 1.8mmol/L is transported.
The rest is part of the pH buffering system.

26
Q

What is hyper/hypo-capnia?

A

Rise / Fall in pCO2.

27
Q

What is hypoxia?

A

Fall in pO2.

28
Q

In exercise what happens to pCO2 and pO2?

A

pO2 drop.
pCO2 increase.
Increased ventilation rate rectifies this.

29
Q

What physiological changes happen in hyperventilation? How is it corrected?

A
  • Increase in ventilation without metabolic change.
    p02 increase
    pCO2 decrease.
    Must decrease ventilation (paper bag?)
30
Q

What happens in hypoventilation? How is it fixed?

A

Decreased ventilation with no metabolic change.
pCO2 Rises.
pO2 falls.
Ventilation must be increased. (prevent acidosis)

31
Q

What may correction of pO2 changes, without pCO2 change, cause?

A

E.g Hypoxia.
Less oxygen available, but CO2 remains level.
Breathing increases/ more oxygen available, so more CO2 lost.
Can cause hypocapnia.

32
Q

What problems can increases and decreases in pH outside the normal range cause?

A

pH < 7.0 enzymes are denatured

pH > 7.6 free calcium concentration drops leading to tetany.

33
Q

How is respiratory alkalosis and acidosis compensated for?

A

Kidneys will change the secretion of bicarbonate, over 2-3 days, to adjust pH.

34
Q

What is metabolic acidosis, how is it corrected?

A

When acid produced by tissues reacts with bicarbonate, leading to lower [HCO3-] and fall in pH.

Increase in ventilation lowers pCO2, restores pH to normal balance (restore ratio of pCO2 to bicarb)

35
Q

What is metabolic alkalosis? How is it corrected?

A
  • If plasma [HCO3-] rises (e.g. after vomiting)
  • Plasma pH rises.
  • metabolic alkalosis.
  • Can be compensated for to a degree by decreasing ventilation.
36
Q

How is blood pO2 measured and fedback to the control centres? What happens?

A

Carotid and aortic bodies have peripheral chemoreceptors.
- Large fall in O2 stimulates:
>Increase breathing
>Changes in heart rate
>Changes in blood flow distribution (e.g. more to brain, kidneys)

37
Q

How are changes in pCO2 detected and rectified in the body?

A
  • Central chemoreceptors in the medulla of the brain are much more sensitive to pCO2.
  • Detect changes in arterial pCO2
  • Small rise pCO2 > ventilation
  • small fall pCO2 < ventilation
    Basis of negative feedback of breathing.
38
Q

Why does HCO3- and H+ not affect the central chemoreceptors in the brain which detect CO2?

A

Blood brain barrier impermeable to these ions, prevent them affecting the ECF or CSF.

39
Q

Which ion is responsible for stimulating the chemoreceptors in the brain, how is it made?

A

Protons.
CO2 diffuses across Blood brain barrier.
Carbonic anhydrase catalyses it to H+ and HCO3-.
H+ stimulates the receptors (showing decreased pH)

40
Q

What determines the bicarbonate concentration in the cerebrospinal fluid?

A

The Choroid plexus cells.
Are responsible for setting ‘baseline’ concentration.
Over time if persistent change in pH, bicarbonate concenrtration of CSF can be corrected by these cells.

41
Q

What may happen the pH control system with persisting hypoxia?

A

Hypoxia detected by peripheral chemoreceptors.
Ventilation increased.
But pCO2 will FALL further.

SO:

  • CSF composition compensates.
  • Choroid plexus cells selectively add H+ or HCO3- into CSF, and chemoreceptors ‘accept’ new pCO2 as normal.
42
Q

How would persistent hypercapnia affect the CSF and choroid plexus?

A

Constant low pH, too acidic for neurones.
choroid plexus cells add HCO3- into CSF.
Receptors accept the new high pCO2 level.