Perfusion limited and diffusion limited

[ctizzle13498]

this is VERY confusing. does anyone know this really well that they could possible explain it in basic terms. thanks in advance

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[jfgavina]

for gas transport you need 3 things operating well:

1. ventilation (integrity of breathing physiology)
2. perfusion (blood flow)
3. diffusion (alveolar wall)

let's forget about ventilation, assume that it is constant. you might have 2 situations affecting transport of gas into and out of blood:

-perfusion limited is when O2 enters blood and CO2 exits along the capillary, you have normal thickness of alveolar wall and no dead zones in your lung. by this I mean that the only way to increase your gas exchanges is increasing the blood flow as the other factors remain constant

-diffusion limited let's say that you have pulmonary fibrosis, or ARDS or emphysema and your alveolar walls are thickened or destroyed. can you imagine gas going through that space? even if blood reaches well the alveolar wall it cannot be packed with O2 because the gas is not passing through the walls. you can increase ventilation and perfusion, but your bridge between this 2 is broken so you can't improve gas exchange. that's why we say it is diffusion limited

 

[ctizzle13498]

WOW! i tried rewatching DIT, didnt understand it. Tried wikipedia, didn't get it. But you just made it so simple! Thanks a lot!

 

[sdvb469]

But, for emphysema why is the Area decreased? Is it because you have destroyed the alveolar wall or is there another reason? Thanks!

for gas transport you need 3 things operating well:

1. ventilation (integrity of breathing physiology)
2. perfusion (blood flow)
3. diffusion (alveolar wall)

let's forget about ventilation, assume that it is constant. you might have 2 situations affecting transport of gas into and out of blood:

-perfusion limited is when O2 enters blood and CO2 exits along the capillary, you have normal thickness of alveolar wall and no dead zones in your lung. by this I mean that the only way to increase your gas exchanges is increasing the blood flow as the other factors remain constant

-diffusion limited let's say that you have pulmonary fibrosis, or ARDS or emphysema and your alveolar walls are thickened or destroyed. can you imagine gas going through that space? even if blood reaches well the alveolar wall it cannot be packed with O2 because the gas is not passing through the walls. you can increase ventilation and perfusion, but your bridge between this 2 is broken so you can't improve gas exchange. that's why we say it is diffusion limited

 

[Torquer]

But, for emphysema why is the Area decreased? Is it because you have destroyed the alveolar wall or is there another reason? Thanks!

no thats exactly it, the whole point of having tiny alveoli with loads of surfactant keeping them open is the increased amount of surface area (think capillaries that are tiny but offer maximum amount of exchange).

now considering that, if you have one big alve-mega-olus in the lung, the surface area to volume RATIO will be extremely reduced. same thing happens in emphysema when the septa break resulting in larger alveoli.

 

[jfgavina]

in emphysema your compliance will be increased but your area will be decreased. by area I mean zone of effective gas exchanging