Diffusion Flashcards
What are the 5 things that diffusion across the blood-gas barrier is determined by?
- pressure difference = the driving force
- surface area of the barrier
- 1/thickness of barrier
- 1/molecular wight of gas ^0.5
- solubility of gas in the barrier
Are gases that are equally soluble in blood and the barrier perfusion or diffusion-limited?
perfusion
What are some examples of gases that are perfusion limited?
N2O O2, CO2
What is an example of a gas that is diffusion-limited and why is it diffusion limited?
carbon monoxide
red blood cells have such an incredibly strong affinity for it that perfusion is essentially taken out as a variable for it - RBCs will ALWAYS pick it up regardless of the perfusion rate.
When can O2 be considered diffusion-limited?
only in pulmonary disease where the barrier thickness has increased - as it does with any sort of pulmonary infiltrate.
Describe N2O’s reaction with blood.
It doesn’t react with the RBCs - it dissolves in the serum
hte partial pressure of N2O will build as the blood goes through the alveolar capillaries, but the process will saturate 10% of the way thorugh the cpaillary
This means you don’t get any more movement of N2O after saturation and the process is thus limited by blood flow, not by the alveolar-blood barrier
Describe the reaction of O2 with blood? When does it reach equlibrium?
It gets picked up by the RBCs, unlike N2O
It reaches equlibrium about 1/3 of the way thorugh the pulmonary capillaries
it’s perfusion limited, so if you increased blood flow, you increase O2 crossing the membrane (exercise!)
WHy is CO2 perfusion limited?
it is super soluble so it crosses the alveolar wall relatively easy and it’s not a factor
What is the pO2 of blood entering the capillary (venous)?
40 mmHg
Alveolar pO2 is usually what?
100 mmHg
How long does it take to saturate the blood with O2 form the alveoli?
about .25 seconds
What could cause a pO2 of blood less than 100 mmHg during exertion?
a RBC is usually in a pulmonary cappillary for .75 seconds, but during exertion that can be decreased to .25 seconds
usually the RBC can still reach O2 equlibrium in that 0.25 seconds, but anything that increases the thickness of the barrier (infiltrate) will lead to lower pO2
Why do you get low pO2 when at high altitudes?
high altitudes have lower pO2 in the air being breathed in, so the driving force for diffusion is lower and less gas will diffuse into the blood!
How do you measure diffusion capacity and why?
have the patient breath in .3% carbon monoxide because it’s not perfusion-limited! Only diffusion-limited!
What’s the diffusion capacity equation?
Volume CO / (Pressure 1 - Pressure 2)
