Diffusion - Trachte Flashcards
What five things determine the diffusion across the blood-gas barrier?
- Solubility of gas
- Pressure difference
- Surface area of barrier
- Thickness of barrier
- Molecular weight of gas
What is the driving force of diffusion?
Pressure difference or gradient.
Gases move from higher → lower pressure.
(not concentration gradient)
Which way does O2 move in a normal pressure gradient in the alveoli & blood?
From alveoli → to blood
(100 mmHg → 40 mmHg)
Which way does CO2 move in a normal pressure gradient in the alveoli & blood?
From blood → to alveoli
(45 mmHg → 40 mmHg)
What is the relationship between diffusion and surface area of blood-gas barrier?
Diffusion is PROPORTIONAL to surface area.
Normal surface area = 50-100 m2
Can you think of anything that would reduce the surface area of the blood-gas barrier?
COPD
(degrades alveoli)
What is the relationship between diffusion and thickness of blood-gas barrier?
Diffusion is INVERSELY proportional to the blood-gas barrier.
Normal thickness: <1 micrometer
Can you think of anything that would increase the thickness of the blood-gas barrier?
Any kind of infection or disease process that increases H2O (CHF).
-maybe cystic fibrosis
How does solubility effect diffusion?
- 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
How does molecular weight effect diffusion?
Bigger things don’t diffuse as well
What is a diffusion limited process?
- 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
- carbon monoxide = diffusion limited gas
What is a perfusion limited process?
- Process in which the only limitation of diffusion is the blood flow (perfusion)
- N2O = perfusion limited gas
What happens to a perfusion limited gas when blood flow is slow?
- 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
Do diffusion limited gases equilibrate (reach equilibrium) across the blood-gas barrier?
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!
What are the diffusion limited gases that we need to know?
-
CO
- Unequal solubility in blood and the blood-gas barrier
- O2 in some disease states
- not usually diffusion limited