Diffusion - Trachte

Question 1

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

Answer 1

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

Question 2

What is the driving force of diffusion?

Answer 2

Pressure difference or gradient.

Gases move from higher → lower pressure.

(not concentration gradient)

Question 3

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

Answer 3

From alveoli → to blood

(100 mmHg → 40 mmHg)

Question 4

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

Answer 4

From blood → to alveoli

(45 mmHg → 40 mmHg)

Question 5

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

Answer 5

Diffusion is PROPORTIONAL to surface area.

Normal surface area = 50-100 m²

Question 6

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

Answer 6

COPD

(degrades alveoli)

Question 7

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

Answer 7

Diffusion is INVERSELY proportional to the blood-gas barrier.

Normal thickness: <1 micrometer

Question 8

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

Answer 8

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

-maybe cystic fibrosis

Question 9

How does solubility effect diffusion?

Answer 9

• 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

Question 10

How does molecular weight effect diffusion?

Answer 10

Bigger things don’t diffuse as well

Question 11

What is a diffusion limited process?

Answer 11

• 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

Question 12

What is a perfusion limited process?

Answer 12

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

Question 13

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

Answer 13

• 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

Question 14

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

Answer 14

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!

Question 15

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

Answer 15

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

Question 16

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

Answer 16

• 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!

Question 17

What is diffusion capacity defined as?

Answer 17

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

Diffusion capacity = V_CO/(P1-P2)

Question 18

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

Answer 18

It is NOT perfusion limited.

Question 19

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

Answer 19

Partial pressure difference divided by the volume of gas transferred.

(P1-P2)/V_gas

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