Diffusion - Trachte Flashcards

1
Q

What five things determine the diffusion across the blood-gas barrier?

A
  1. Solubility of gas
  2. Pressure difference
  3. Surface area of barrier
  4. Thickness of barrier
  5. Molecular weight of gas
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2
Q

What is the driving force of diffusion?

A

Pressure difference or gradient.

Gases move from higher → lower pressure.

(not concentration gradient)

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3
Q

Which way does O2 move in a normal pressure gradient in the alveoli & blood?

A

From alveoli → to blood

(100 mmHg → 40 mmHg)

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4
Q

Which way does CO2 move in a normal pressure gradient in the alveoli & blood?

A

From blood → to alveoli

(45 mmHg → 40 mmHg)

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5
Q

What is the relationship between diffusion and surface area of blood-gas barrier?

A

Diffusion is PROPORTIONAL to surface area.

Normal surface area = 50-100 m2

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6
Q

Can you think of anything that would reduce the surface area of the blood-gas barrier?

A

COPD

(degrades alveoli)

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7
Q

What is the relationship between diffusion and thickness of blood-gas barrier?

A

Diffusion is INVERSELY proportional to the blood-gas barrier.

Normal thickness: <1 micrometer

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8
Q

Can you think of anything that would increase the thickness of the blood-gas barrier?

A

Any kind of infection or disease process that increases H2O (CHF).

-maybe cystic fibrosis

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9
Q

How does solubility effect diffusion?

A
  • More soluble things diffuse better
    • CO2 diffuses (20x) better than O2
    • He is not soluble at all
      • that is why this works for the dilution test → it does not get into the blood
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10
Q

How does molecular weight effect diffusion?

A

Bigger things don’t diffuse as well

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11
Q

What is a diffusion limited process?

A
  • Process in which the diffusion across the blood-alveolar membrane is the only constraint on gas transfer.
    • carbon monoxide = diffusion limited gas
      • the only barrier is the membrane
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12
Q

What is a perfusion limited process?

A
  • Process in which the only limitation of diffusion is the blood flow (perfusion)
    • N2O = perfusion limited gas
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13
Q

What happens to a perfusion limited gas when blood flow is slow?

A
  • Partial pressure of gas builds
  • Blood becomes saturated with gas
    • no movement (diffusion stops) => reach equilibrium
    • process limited by blood flow!
      • not alveolar-blood barrier
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14
Q

Do diffusion limited gases equilibrate (reach equilibrium) across the blood-gas barrier?

A

NO

The red blood cell affinity is so great → that there is only little rise of partial pressure in the capillary.

Diffusion limited gases = don’t equilibrate!

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15
Q

What are the diffusion limited gases that we need to know?

A
  • CO
    • Unequal solubility in blood and the blood-gas barrier
  • O2 in some disease states
    • not usually diffusion limited
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16
Q

What are the perfusion limited gases that we need to know?

A
  • N2O
  • O2
  • CO2
  • everything else . . .
    • gases with similar solubilities in blood and the blood-gas barrier

***Equally soluble in blood and the blood-gas barrier!

17
Q

What is diffusion capacity defined as?

A

The volume of CO transferred divided by the partial pressure difference.

Diffusion capacity = VCO/(P1-P2)

18
Q

Why do we use carbon monoxide in the measurement of diffusion capacity?

A

It is NOT perfusion limited.

19
Q

How do you measure the resistance of the blood-gas barrier (barrier resistance)?

A

Partial pressure difference divided by the volume of gas transferred.

(P1-P2)/Vgas

(Similar to Resistance = Pressure/Flow, rearranged from P=RxF)