ACEM primary - physiology > Respiratory diffusion > Flashcards

Respiratory diffusion Flashcards

West's chapter 3 (73 cards)

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1
Q

Diffusion through tissues is described by which law?

A

Fick’s law

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2
Q

What does Fick’s law state?

A

That the rate of transfer through a sheet of tissue is proportional to the tissue area and difference in gas partial pressure between the two sides and inversely proportional to the tissue thickness.

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3
Q

What is the area of the blood gas barrier of the lung?

A

50-100 square metres

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4
Q

What is the thickness of the blood gas barrier in the lung?

A

0.3 micrometres

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5
Q

As well as the properties mentioned in Fick’s law the rate of transfer of gases is also proportional to what?

A

A diffusion constant

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6
Q

The diffusion constant that determines rate of transfer is proportional to what?

A

Proportional to the solubility of the gas.

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7
Q

The diffusion constant that determines rate of transfer is inversely proportional to what?

A

The square root of the molecular weight of the gas molecule.

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8
Q

How much more quickly does CO2 diffusion when compared with oxygen?

A

CO2 diffuses 20 times quicker than oxygen.

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9
Q

Why does CO2 diffuse 20 times faster than oxygen?

A

Co2 has a higher solubility than oxygen, but a similar molecular weight.

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10
Q

How do you calculate the diffusion constant of a gas?

A

D = solubility / square root of molecular weight

D = diffusion constant

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11
Q

Give the equation for Fick’s law.

A

V^gas = (A/T) x D x (P1-P2)

V^gas = rate of diffusion
A = area
T = thickness
D = diffusion constant
P1-P2 = pressure gradient

**This is how you calculate the rate of diffusion.

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12
Q

For the following characteristics, say whether the rate of diffusion through tissues is proportional or inversely proportional:

a) surface area of tissue
b) thickness of tissue
c) partial pressure gradient
d) solubility of gas
e) square root of molecular weight of gas

A

a) proportional
b) inversely proportional
c) proportional
d) proportional
e) molecular weight of gas

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13
Q

Diffusion rate through tissues is proportional to….?

A

Surface area of tissue
Partial pressure difference
Solubility of the gas

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14
Q

Diffusion rate through tissues is proportional to….?

A

Thickness of the tissue
Square root of the molecular weight

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15
Q

How long does it take a red blood cell to move through a pulmonary capillary?

A

0.75s

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16
Q

The transfer of nitrogen from an alveolus to a red blood cell is limited by what?

A

PERFUSION limited

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17
Q

The transfer of carbon monoxide from an alveolus to a red blood cell is limited by what?

A

DIFFUSION limited

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18
Q

The transfer of oxygen from an alveolus to a red blood cell is limited by what?

A

Diffusion limited OR perfusion limited depending on conditions.

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19
Q

Why is the transfer of carbon monoxide from the alveoli into the blood said to be diffusion limited?

A

After diffusing across blood-gas barrier, CO binds tightly to Hb in RBCs.
This means that most CO is taken up by the RBC and is not left in the blood.
Therefore, the partial pressure only increases by very small amounts.
No appreciable back pressure develops = gas continues to move rapidly across the alveolar wall.

***THEREFORE, the amount of CO that enters the blood is limited by the diffusion properties of the blood-gas barrier and NOT the amount of blood available.

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20
Q

What happens to the partial pressure of nitrous oxide (N2O) during diffusion into the blood, by the time the RBC has moved about 1/10th the way along the capillary?

A

By the time the RBC has moved about 1/10th along the capillary the partial pressure in the blood will almost have reached the same level as in the alveoli.

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21
Q

Why is the transfer of nitrous oxide (N2O) from the alveoli into the blood said to be perfusion limited?

A

Nitrous oxide does not combine with haemoglobin.
Reduced avidity of the blood for nitrous oxide means that the partial pressure rises rapidly.
After the initial N2O transfer, almost no further nitrous oxide diffuses across the blood gas barrier.

***Therefore, the amount of N2O taken up by the blood depends entirely on the amount of available blood flow, NOT the properties of the blood gas barrier.

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22
Q

What does O2 to when it enters the blood?

A

Combines with Hb (less avid than CO).

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23
Q

Compare the rise in partial pressure of O2 after diffusion across the blood gas barrier with the rise in partial pressure of O2.

A

The partial pressure rise of O2 is much greater than the partial pressure rise for the same number of CO molecules.

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24
Q

When a RBC enters a pulmonary capillary, why is the partial pressure of O2 already 4/10ths of the alveolar value?

A

Due to the presence of O2 in mixed venous blood.

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25
Under typical resting conditions, how far along the capillary does a red blood cell need to be before the capillary PO2 reaches that of the alveolar gas?
About 1/3rd of the way along the capillary.
26
Under normal resting conditions, diffusion of oxygen across the blood gas barrier is limited by what?
Perfusion limited
27
When might diffusion of O2 across the blood gas barrier be diffusion limited?
Under abnormal conditions such as lung diseases that cause thickening of the blood gas barrier **In these circumstances, blood PO2 might not reach the alveolar value by the end of the capillary = diffusion limitation now present as well.
28
Whether a gas is diffusion limited or not depends on what?
It's solubility in the blood gas barrier rather than it's solubility in the blood. **For CO these are very different, but for N2O they are the same.
29
What is the PO2 of a red blood cell as it enters a pulmonary capillary?
~40mmHg
30
What is the PO2 of the alveolar gas?
~100mmHg
31
In extreme exercise, what is reduced within the blood gas barrier?
Time available for oxygenation is reduced during extreme exercise as the RBCs only spend about 0.25s in the pulmonary capillaries. **This is because pulmonary blood flow is greatly increased. ***There is not normally any measurable fall in end-capillary PO2 in normal patients breathing air.
32
What might cause a reduced RBC PO2 during exercise?
A thickened blood gas barrier
33
Why can a thickened blood gas barrier cause a slow rising or reduced PO2?
Thickened blood gas barrier = O2 diffusion impeded. This causes a reduced rate of rise of the PO2 in RBCs. Therefore, RBC PO2 might not reach that of the alveolar gas before time available for oxygenation in the capillary has run out
34
Aside from extreme exercise and a thickened blood gas barrier, how else might the diffusion properties of the lung be stressed?
By lowering the alveolar PO2 **E.G by going to high altitude or by inhaling a low O2 mixture.
35
Why does lowering the alveolar PO2 cause slower diffusion across the blood gas barrier?
Because the difference in partial pressure between the alveolar PO2 and the blood PO2 is smaller and so O2 moves across the blood gas barrier more slowly.
36
Why is failure to reach the alveolar PO2 more likely if there is a lower alveolar PO2 to start with?
Because the rise in PO2 along the capillary is relatively slow.
37
How can diffusion impairment of O2 transfer in normal patients be convincingly demonstrated?
By performing severe exercise at high altitudes.
38
At rest, PO2 of the blood reaches that of he alveolar gas after how long in the capillary?
1/3rd the total time (about 0.25s)
39
During exercise, how long does each RBC spend in the pulmonary capillaries?
About 0.25s
40
Give 3 ways in which the diffusion process can be challenged?
Exercise Alveolar hypoxia Thickening of the blood-gas barrier
41
Measured diffusion capacity for carbon monoxide depends on what 3 factors?
Surface area of blood gas barrier Thickness of blood gas barrier Volume of blood in pulmonary capillaries
42
In diseased lungs, diffusion capacity of the lung (transfer factor) is affected by which 3 things?
Distribution of diffusion properties Alveolar volume Capillary blood
43
Define Fick's law.
The rate of diffusion of a gas through a sheet of tissue is proportional to the area of the sheet and the partial pressure difference across it, and inversely proportional to the thickness of the sheet.
44
Oxygen transfer is normally limited by what?
Normally perfusion limited.
45
When might oxygen transfer become diffusion limited?
Exercise Thickening of blood-gas barrier Alveolar hypoxia
46
The finite reaction rate of oxygen with haemoglobin can reduce what?
It's transfer rate into the blood (similar effect to if diffusion rate was reduced).
47
Describe the transfer limitations of carbon dioxide transfer across the blood gas barrier.
Probably NOT diffusion limited.
48
What is the gas of choice for measuring diffusion properties of the lung?
Carbon monoxide
49
Why is carbon monoxide the drug of choice for measuring diffusion properties of the lung?
Because transfer of carbon monoxide is solely diffusion limited.
50
Give the equation for Fick's law.
V^gas = (A/T) x D x (P1-P2) V^gas = rate of diffusion A = area T = thickness D = diffusion constant P1-P2 = pressure gradient **This is how you calculate the rate of diffusion.
51
For complex structures like the blood gas barrier, it is not possible to measure area and thickness during life, so how is the rate of diffusion equation written?
V^gas = DL x (P1-P2) V^gas = rate of diffusion per unit time DL = diffusing capacity of the lung P1-P2 = partial pressure gradient
52
Which 3 factors constitute the diffusing capacity of the lung?
Area Thickness Diffusion properties of sheet AND gas concerned
53
What is the equation for the diffusing capacity of carbon monoxide?
DL = VCO / PACO DL = diffusing capacity of CO VCO = diffusion rate of CO in ml/min PACO = alveolar partial pressure
54
The diffusing capacity of the lung for carbon monoxide is equal to what?
The volume of carbon monoxide transferred in millimetres per mmHg of alveolar partial pressure.
55
How can the diffusing capacity of the lung for carbon monoxide be determined?
The single breath method.
56
Describe what happens in the single breath method for calculating the diffusing capacity of the lung for carbon monoxide.
A single inspiration of a dilute mixture of CO is made and the rate of disappearance of CO from the alveolar gas during a 10 second breath hold is calculated. **Often done by measuring the inspired and expired concentrations of CO with an infrared analyser.
57
What is the normal value of the diffusing capacity of the lung for carbon monoxide when at rest?
25 ml/min/mmHg
58
Why does the diffusing capacity of the lung for carbon monoxide increase to 2 or 3 times the normal value with exercise?
Because of the recruitment and distension of pulmonary capillaries.
59
What happens to the diffusing capacity of the lung for carbon monoxide during exercise?
It increases by 2-3 times the normal.
60
Aside from the blood gas barrier, give 2 other reasons that there can be resistance to diffusion.
A comparably large path length from the alveolar wall to the centre of the red blood cell when compared with the alveolar wall. Resistance caused by the finite rate of reaction of O2 of CO with Hb inside the red blood cell.
61
How quickly do O2 and CO combine with haemoglobin when added to the blood?
Combination happens within about 0.2 seconds.
62
What are the 2 stages of O2 uptake at the blood gas barrier?
Stage 1 = diffusion of O2 through the blood gas barrier (including the plasma and red cell interior). Stage 2 = reaction of the O2 with the Hb **The above 2 times can be summed inn order to produce an overall diffusion resistance.
63
What is the diffusing capacity of the lung defined as?
The flow of gas per unit time divided by the partial pressure difference.
64
What is the equation for working out diffusing capacity of the lung?
DL = V^gas / (P1-P2) DL = diffusion capacity of the lung V^gas = volume of flow of gas per unit time P1-P2 = partial pressure gradient
65
What is the calculation for the resistance of the blood gas barrier?
1/DM DM = diffusion capacity of the membrane
66
Which symbol is used to describe the rate of reaction of O2 with haemoglobin?
0 (with horizontal line through middle)
67
What does the rate of reaction of O2 with haemoglobin describe?
The rate in mm per minute of O2 that combines with 1ml of blood per mmHg of partial pressure of O2.
68
What is the complete calculation for calculating diffusion resistance?
1/DL = (1/DM) + (1/0 x VC) DL = diffusing capacity of the lung DM = diffusing capacity of the membrane 0 = rate of reaction of O2 with haemoglobin VC = volume of capillary blood
69
A reduction of capillary blood volume by disease can cause what with regards to diffusing capacity of the lung?
Can cause a reduced diffusing capacity of the lung.
70
What should you do if a person has a high concentration of CO bound haemoglobin?
Get them to breathe in high concentrations of O2 so that there is more O2 to compete with CO for haemoglobin binding. **Measured diffusing capacity of CO is reduced by high concentration O2 breathing.
71
Why does the reaction rate of O2 binding with haemoglobin become a limiting factor in O2 uptake?
Because so little time is available in the pulmonary capillary.
72
The resistance to the uptake of O2 by haemoglobin attributable to reaction rate is probably about the same as what?
The resistance attributable to diffusion across the blood gas barrier.
73
ACEM primary - physiology
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