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Semester 3 - respiratory > 5. Oxygen in the Blood > Flashcards

5. Oxygen in the Blood Flashcards

1
Q

How do we calculate the amount of oxygen dissolved in blood?

A

[O2]dissolved = solubility factor x pO2

pO2 = 13.3kPa, O2 solubility = 0.01mmol/L/kPa
13.3 x 0.01 = 0.13mmol/L

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2
Q

Why is a chemical reaction (HbO2) required for transport of oxygen?

A

Oxygen is not very soluble in water, at partial pressure of 13.3 kPa (normal pO2 in arterial blood), not enough oxygen us dissolved in the plasma to meet demand

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3
Q

What is myoglobin?

A

Red pigment found in muscles which contains haem

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4
Q

What conditions must the reaction of oxygen binding fulfil?

A

The reaction needs to be reversible
• Oxygen must be able to associate with the carrier at the lungs
• Dissociate from carrier at tissues to supply them with oxygen

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5
Q

Why do dissociation curves use %saturation instead of amount of oxygen bound (mmol)?

A

Amount of oxygen bounds depends on the amount of pigment present. Percentage saturation is independent of pigment concentration

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6
Q

What are the subunits of haemoglobin?

A

Tetramer - 2 alpha and 2 beta subunits

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7
Q

How many oxygens bind to a single haemoglobin?

A

4, one to each ham group in each subunit

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8
Q

What are the 2 states of haemoglobin?

A

T (tense) state and R (relaxed) state

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9
Q

What is the T state of haemoglobin?

A

Hb has low affinity for oxygen– Difficult for oxygen to bind

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10
Q

What is the R state of Hb?

A

Hb has high affinity for oxygen– Easier for oxygen to bind

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11
Q

What determines whether haemoglobin

is in the T state or the R State?

A

Several factors, most important:

Partial Pressure of Oxygen which determines oxygen binding for haemoglobin

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12
Q

When is Hb in tense and relaxed states?

A

when pO2 low it is in tense state, when pO2 is high it is in relaxed state

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13
Q

What is cooperative binding of Hb?

A
  • As each O2 binds Hb becomes more relaxed and binding of the next O2 molecule is easier – in R state Hb higher oxygen affinity
  • The opposite is also true – if an oxygen molecule leaves haemoglobin – this will make the Hb more tense making it more likely that another oxygen molecule will dissociate
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14
Q

What shape does cooperative binding in Hb produce in its dissociation curve?

A

Sigmoidal curve

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15
Q

Describe the oxygen dissociation curve

A
  • Initially the relationship between pO2 and binding is shallow
  • But as some O2 binds it facilitates further binding
  • Curve steepens as pO2 rises
  • Then flattens as saturation is reached
  • This gives a sigmoidal curve
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16
Q

What can we work out from oxygen dissociation curve and how?

A

• We can see how much O2 will be bound or given up
when moving from one partial pressure to another
• Work out difference in percentage saturations between two pO2 values
• Work out effects of changed conditions on how easily haemglobin binds or releases oxygen

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17
Q

At what special pressure is Hb 100% and 50% saturated?

A

9-10kPa, 3.5-4kPa

  • Haemoglobin is almost 100% saturated over a fairly wide range of oxygen partial pressure so we have a wide safety margin for oxygen levels
  • Once past safety margin though saturation drops dramatically and delivery of oxygen to tissues compromised – tissue hypoxia
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18
Q

What is the normal concentration of Hb and therefore what is normal concentration of oxygen content?

A

• Alveolar pO2 ≈ 13.3 kPa therefore Hb leaving the lungs well saturated

– normal Hb ≈ 2.2mmol/L, therefore x4 = 8.8mmol/L

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19
Q

How is pO2 and oxygen content affected in anaemia where lungs are functioning fine?

A

– though pO2 will be normal, and Hb saturation with oxygen 100%
– O2 CONTENT will be lower as amount of Hb in blood lower

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20
Q

Why is the amount of CO2 dissolved in blood much higher than O2

A

Co2 much more soluble that O2

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21
Q

What does tissue pO2 depend on and what is the typical value?

A

Depends on how metabolically active the tissue is. Typically 5kPa.

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22
Q

How much oxygen is given up at the tissues?

A

Hb saturation drops to 65%,

0.35x8.8=3mmol/L

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23
Q

What is the state of haemoglobin at tissues?

A
  • At this low saturation haemoglobin is tense and doesn’t want to bind oxygen
  • Wants to give it to the tissues!
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24
Q

What is the haemoglobin saturation in venous blood and what is the effect of it?

A

• Mixed venous blood – mixture of blood returning
from various tissues
• Over half the oxygen is still bound – PaO2 ~ 6 kPa
• Could the tissues remove more?
– Yes they can!
• The lower the tissue pO2, the more O2 will dissociate from Hb
– This will lower saturation of venous blood
• SPARE CAPACITY

25
Q

What pO2 in capillaries is the lowest it can be?

A

Cannot be less than 3kPa

26
Q

Why must capillary pO2 not fall too low?

A

• Capillary - tissue partial pressure gradient pO2 must

be high enough to drive diffusion of O2 to cells

27
Q

What does the extent that capillary pO2 can fall without compromising diffusion of oxygen to cells depend on?

A

Depends on the capillary density, higher capillary density means reduced diffusion distance so oxygen doesn’t have far to diffuse.

28
Q

Which tissues have high capillary density?

A

• Very metabolically active tissue will have a higher
capillary density (eg heart muscle)
• People living in high altitudes will also have higher
capillary density

29
Q

What is the Bohr shift?

A

Decrease in pH (more acidic) shifts the dissociation curve to the right.

30
Q

What is the effect of shifting the dissociation curve to the right?

A

↓pH promotes T state of Hb - Hb

has lower affinity for O2 so more oxygen dissociates for a given pO2

31
Q

What is pH like in most metabolically active tissues and what is the effect of this?

A

Most have lower pH’s, so extra o2 is given up

32
Q

What effect does temperature have on dissociation curve?

A

Increase temperature shift curve to the right, reducing affinity– more oxygen released

33
Q

What is temperature like in most metabolically active tissues and what is the effect of this?

A

Metabolically active tissues have higher temperature so extra o2 is given up

34
Q

What is the maximum unloading of oxygen and in which conditions would it occur?

A
  • Maximum unloading occurs in tissues where pO2 can fall to low level
  • Also in conditions where increased metabolic activity results in more acidic environment and higher temperature
  • In tissues with high capillary density where partial pressure gradient of oxygen can be very small
  • Under these conditions about 70% bound oxygen can be given up
35
Q

How much oxygen is extracted on average at rest and what about the rest?

A
  • On average ≈ at rest 30% oxygen extracted from blood
  • This is a reserve – for when we need extra oxygen to be given up – exercise, metabolic stress (sepsis, burns) – without having to increase Cardiac Output to excess and strain heart
36
Q

When is 2,3-bisphosphoglycerate released

A

2,3-DPG is an intermediate of RBC glycolysis normally

rapidly consumed but in hypoxia RBC production of 2,3 DPG increases

37
Q

What effects does increase 2,3-bisphosphoglycerate have on haemoglobin?

A

Reduces affinity for oxygen - facilitates O2 unloading

in tissues

38
Q

When would concentration of 2,3-BPG increase?

A

In anaemia or at high altitudes, allows more oxygen to be given up

39
Q

When would concentration of 2,3-BPG decrease?

A

2,3-DPG levels drop in stored blood due to refrigeration

40
Q

What is the effect of carbon monoxide on Hb?

A

• Carbon monoxide 200 times affinity for haemoglobin as oxygen
– Reduced oxygen transport and total oxygen content
• Carbon monoxide increases affinity of UNaffected subunits for oxygen
– Leftward shift in oxy-haemoglobin dissociation curve,
– Reduced oxygen release peripheral tissue

41
Q

What level is HbCO fatal?

A

Fatal if >50%

42
Q

Why are children at increased risk of carbon monoxide poisoning?

A

because they breathe faster

43
Q

does carbon monoxide poisoning affect PaO2?

A

Does not decrease PaO2

44
Q

symptoms of carbon monoxide poisoning

A
  • Headache
  • Nausea
  • Vomiting
  • Slurred speech
  • Confusion
  • Initially may not have many respiratory symptoms
45
Q

Define hypoxia

A

low oxygen levels in body or tissues

46
Q

Define hypoxaemia

A

Low pO2 in arterial blood - not oxygen bound to haemoglobin

47
Q

What is cyanosis?

A

Bluish colouration of the skin due to unstaturated Hb, (oxyhaemoglobin is more red than deoxy)

48
Q

Where does peripheral cyanosis occur and what is it due to?

A

Hands or feet, usually due to poor circulation

49
Q

Where does central cyanosis occur and what is it due to?

A

Mouth, tongue, lips, mucous membranes, due to poorly saturated blood in systemic circulation

50
Q

WHat is the effect of low PAO2?

A

If PAO2 (alveoli) levels are low, not all the Hb will be saturated – both hypoxia and hypoxaemia

51
Q

if Hb levels are low what is the O2 level?

A

not enough O2 will be present in the blood but Hb is saturated

52
Q

what conditions can result from tissues using O2 faster that it is delivered?

A

– Peripheral arterial disease
– Raynaud’s
– Congestive heart failure with low Cardiac Output

53
Q

describe PaO2 and O2 saturation levels in Hypoxia secondary to anaemia

A

– PaO2 will be normal, O2 saturation will be normal

54
Q

What is pulse oximetry?

A

Non-invasive method of detecting levels of Hb saturation

55
Q

How does pulse oximetry work?

A

Detects difference in absorption of light between oxygenated and deoxygenated Hb (deoxygenated Hb absorbs more red light than oxygenated, it is less red)

56
Q

What does pulse oximetry not show?

A

Does say how much Hb is present

57
Q

How does pulse oximetry only measure Hb in arteries?

A

Detects pulsatile blood flow, ignores Hb levels in tissues and non-pulsatile venous blood

58
Q

Why can a Regular two wave length pulse oximeter not be used to diagnose carbon monoxide poisoning?

A

can’t detect carboxyhaemoglobin (CO-Hb) – will report normal saturation with CO-Hb

59
Q

Can an ABG used to diagnose carbon monoxide poisoning?

A

no - does not decrease PaO2

Semester 3 - respiratory (20 decks)

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