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RESP > Lecture 5: Oxygen in the blood > Flashcards

Lecture 5: Oxygen in the blood Flashcards

1
Q

Whats the solubility coefficient of oxygen?

A

0.01 mmol/L/kPa (at 37 degrees)

Not very soluble in water

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2
Q

How much oxygen is dissolved in arterial blood?

A

0.13 mmol/L
At PaO2 of 13.3 kPa
(13.3 x 0.01)

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3
Q

Why can’t we rely on dissolved oxygen only?

A

Cardiac output would have to be really high, as we need 12 mmol of oxygen per minute so we would need to pump 92 L/min around the body

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4
Q

What is the average cardiac output for a healthy person?

A

4.5-5 L

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5
Q

Why does oxygen binding need to be reversible?

A
  • 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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6
Q

What is the structure of Hb?

A

Tetramer, which binds 4 oxygen molecules

  • haem molecule on each chain
  • haem molecule has iron at its centre
  • molecular oxygen combines reversibly with iron
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7
Q

What are the 2 states of Hb?

A 
T state (tense): low affinity for oxygen, difficult for oxygen to bind
R state (relaxed): high affinity for oxygen, easy for oxygen to bind
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8
Q

What determines whether Hb is in the T or R state?

A

Partial pressure of oxygen

  • when pO2 is low, Hb is tense
  • when pO2 is high, Hb is relaxed
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9
Q

What is positive coorperativity?

A

As each oxygen binds, Hb becomes more relaxed and binding of the next oxygen molecule is easier
(opposite is true: oxygen leaving Hb makes it more tense and therefore more likely that another oxygen will dissociate)

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10
Q

Does the oxygen dissociation curve tell us how much Hb a person has?

A

It does not tell us about the amount of Hb a person has

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11
Q

Does saturation of Hb change in anaemia?

A

No, they are fully saturated, there is just less Hb.

-but O2 content will be lower so not enough oxygen present in the blood

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12
Q

Describe the oxygen dissociation graph:

A 
Y axis:
-Hb saturation %
-oxygen conc mL/100mL
X axis:
-kPa (pO2)

Sigmoidal curve:

  • initially, there is a shallow relationship with b/w the pO2 and binding. It is very hard for oxygen to bind to the Hb
  • as some oxygen binds, this facilitates further binding (steep part of curve)
  • curve steepens as pO2 rises
  • curve flattens as saturation is reached
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13
Q

What information can we get from an oxygen dissociation curve?

A
  • see how much oxygen will be bound or given up when moving from one partial pressure to another
  • work out the difference in % saturations between 2 pO2 values
  • p50: PP of oxygen where Hb is 50% saturated
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14
Q

Give some important partial pressures of oxygen on the Hb dissociation curve:

A

-Hb saturated above 9-10 kPa
-virtually unsaturated below 1 kPa
-half saturated at 3.5-4 kPa
(saturation changes greatly over a narrow range)

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15
Q

What are the different zones on the Hb dissociation curve?

A

Zone A: wide range of safe partial pressure of oxygen
Zone B: even small drops in partial pressure of oxygen are dangerous- tissue hypoxia as delivery to tissues is compromised

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16
Q

How do we define respiratory failure in terms of oxygen?

A

When the arterial partial pressure of oxygen falls below 8 kPa

17
Q

What is the average Hb concentration for oxygen?

A

2.2 kPa
Each Hb carries 4 molecules
=8.8 mmol/L (average)

18
Q

What does tissue pO2 depend on?

A

How metabolically active the tissue is

  • usually 5 kPa
  • Hb saturation drops to 65% (given off 35% of oxygen)
  • at this low saturation, Hb is tense and doesn’t want to bind oxygen
19
Q

What is spare capacity of Hb?

A

In mixed venous blood over half the oxygen is still bound: 6kPa
(in case we find ourselves in a situation where we need to extract more oxygen)

20
Q

How low can tissue pO2 get?

A

-capillaries supplying the tissues can’t fall below 3kPa
-need a partial pressure gradient
-higher the capillary density, the lower the pO2 can fall as it doesn’t have far to diffuse
(very metabolically active tissues have a higher capillary density/high altitudes will have a higher capilarry density)

21
Q

What is the Bohr effect?

A

-acidic conditions shift dissociation curve to the right, so it has less affinity for oxygen: promotes the T state (metabolically active tissues have a low pH)

22
Q

How does the temperature effect the oxygen dissociation curve?

A

High temp shift curve to the right, so there is less affinity for oxygen
-as metabolically active tissues have high temperatures

23
Q

What is the most metabolically active tissue?

A

The heart

-it removes the most oxygen from the blood

24
Q

Where does maximum unloading of oxygen occur?`

A

-in tissues where pO2 can fall to a low level
-conditions where increased metabolic activity results in a more acidic environment and higher temp
-in tissues with a high capillary density where partial pressure gradient of oxygen can be very small
Here up to 70% of bound oxygen can be given up (on average 30% of oxygen is extracted from blood)

25
Q

What is 2,3-BPG and its effect on the dissociation curve?

A

Metabolically active substance, which is an intermediate of RBC glycolysis normally rapidly consumed

  • in hypoxia, RBC production of 2,3-BPG increases
  • this shifts the curve to the right so there will be more unloading of oxygen which is helpful in hypoxic conditions
26
Q

In who is 2,3-BPG increased?

A

-people with anaemia
-people who live in a high altitude
(2,3-BPG are decreased slightly in blood bank blood so in someone critically ill you want to get them as fresh blood as possibleso the 2,3-BPG isn’t as decreased)

27
Q

How does carbon monoxide affect oxygen delivery to tissues?

A
  • reacts with Hb to form COHb
  • CO has 200x affinity for Hb than oxygen, causing reduced oxygen transport and total oxygen content
  • CO increases affinity of unaffected subunits for oxygen so they hold onto oxygen: shifts curve to the left, leading to reduced oxygen release to peripheral tissue
28
Q

When is carbon monoxide levels fatal?

A

If HbCO is >50%

-children are at increased riskas they breath faster so get more CO in

29
Q

What are the symptoms of carbon monoxide poisoning?

A

-headache
-nausea
-vomiting
-slurred speech
-confusion
Initially may not have many resp symptoms as PaO2 is normal

30
Q

What is the difference between hypoxia and hypoaemia?

A

Hypoxaemia: low PaO2 in arterial blood (reflects the dissolved oxygen, not bound to Hb)
Hypoxia: low oxygen levels in body or tissues

31
Q

If PAO2 levels are low what do you experience?

A

If pO2 levels in the alveoli are low, not all the Hb will be saturated so you get both hypoxia and hypoxaemia

32
Q

How does shock cause peripheral hypoxia?

A

Reduce blood flow via peripheral vasoconstriction

33
Q

Give some examples where oxygen is being used up faster that it is being delivered?

A
  • peripheral artery disease (arteries are narrowed)
  • raynaud’s
  • congestive heart failure with low cardiac output
34
Q

What is cyanosis?

A

Blue colouration due to unsaturated Hb

  • as deoxygenated Hb is less red than oxygenated Hb
  • can be peripheral due to poor circulation
  • can be central cyanosis due to poorly saturated blood in the systemic circulation (mouth/tongue/lips/mucous membranes)
35
Q

Why is cyanosis difficult to detect?

A

-hard to see in poor lighting
-harder to see if your skin colour is darker (look at fingernails)
-wearing shoes
Therefore you need to be looking for it

36
Q

What is pulse oximetry?

A

Detects levels of Hb saturation (non-invasive)

  • detect difference in absorption of light between oxygenated and deoxygenated Hb (more blue)
  • only detects pulsatile arterial blood levels (not in tissues/non-pulsatile venous blood)
37
Q

What is the disadvantages of pulse oximetry?

A
  • regular 2 wave pulse oximeter can’t detect COHb, so will report normal saturation
  • can’t give information about Hb levels but it gives the saturation
38
Q

What other tests can you do as well as a pulse oximetry?

A

ABG (arterial blood gas)
-painful
-does not detect CO
-some ABG’s calculate pO2 from dissociation curve
Detects oxygen levels, electrolytes and acid/base balance

RESP (18 decks)

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