Perfusion Limited vs. Diffusion Limited

[ACE Stimulator]

Could someone please explain this concept in details. Thank you.

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[kdburton]

Could someone please explain this concept in details. Thank you.

Diffusion limited is exemplified by CO2. There is a large gradient in the lungs because the concentration of CO2 in the alveoli is pretty much 0 and you have CO2 in your mixed venous blood because of metabolism. Exchange of CO2 is therefore diffusion limited because in the absense of diffusion problems it should equilibrate very quickly. If youre running a marathon you're going to blow off all the CO2 and you're probably going to actually become hypocapneic because your resp rate is increased.

Perfusion limited is exemplified by O2. There is a smaller graident in the lungs for O2 than CO2 because you're breathing in O2 (PO2 in alveoli is 100 if I remember correctly) and the PO2 of mixed venous blood is also relatively high even after perfusing your tissues. The amount of O2 exchange in the lungs is therefore less dependent on a large gradient, and more dependent on how much time it has to equilibrate with the blood that is passing through the alveolar capillaries on the other side of the membrane. If youre running a marathin your cardiac output is high and your blood will pass the alvelo faster than O2 can equilibrate so you're going to have lower arterial PO2 than normal

 

[future_dr_house]

To understand diffusion vs perfusion limited you have to know the equation for diffusion (Fick's). J = A*D(P1-P2)/x

where a = cross sectional area, D = diffusion coefficient (CO2 diffuses 20x more than O2, CO diffuses 200x more than O2; D is the diffusion constant and is proportional to solubility and inversely proportional to MW), P1-P2 is the difference in partial pressures (explained by KDBurton), and x is the thickness (can increase in ARDs, CHF, or pulmonary fibrosis, etc)

The easiest way to think about it is Nitrous Oxide versus Carbon Monoxide.
Nitrous oxide does not bind hemoglobin at all and is totally dependent on it's solubility in the blood. It reaches it's peak concentration in the blood very quickly (diffuses quickly until it reaches saturation) and thus, because it finishes it's diffusion quickly, it is NOT diffusion limited, it is PERFUSION limited. This is explained in Ficks by the P1-P2 term (P1 quickly equals P2, and diffusion quickly stops)

CO on the other hand binds Hb avidly and is removed from the blood, meaning the partial pressure of CO in the blood is almost always 0. This means that P1-P2 is almost never 0, meaning that diffusion will never reach 0. This means that it is almost entirely depends on diffusion and is DIFFUSION limited.

Oxygen is somewhere in between.
It both binds Hb (duh) and is soluble in the blood. There are 2 situations, low cardiac output, and high cardiac output. At low cardiac output, there is more time for blood to have contact with the alveoli. Since there is more time, more diffusion takes place and the blood will saturate with oxygen. This sounds like nitrous oxide right? P1-P2 eventually becomes 0, no more diffusion occurs, it's not diffusion limited...therefore it's PERFUSION limited at low cardiac output.

At strenous exercise, cardiac output increases, and now the blood is shooting through the capillary bed quickly. The oxygen does not have as much time to saturate the blood...thus it's dependent on diffusion (P1-P2 never reaches 0) and is DIFFUSION limited.

 

[iblunk]

I'm going to disagree with kdBurton here. Equilibrium isn't reached in diffusion limited. If equilibrium is reached, then it's a perfusion limited process. Also, alveolar Pco2 should be around 40 mm Hg, not 0. It's 0 in the air, but as CO2 leaves your blood and goes into the alveoli, it increases it there.

 

[DOMan79]

Perfusion is based on bloodflow to that area. It can only be increased by increasing the bloodflow.

Diffusion has to do with the barrier. You cannot increase diffusion no matter how much you increased blood flow if there is a barrier issue (ie, fibrosis). It can only be increased by the time

 

[orthodoc23]

CO2 is considered perfusion limited because it has a high gradient (its concentration in the air and blood is many magnitudes of times different) between the blood and alveoli and therefore how much CO2 is exchanged really is limited by how how fast the blood is circulated through the lungs - - FAST TRUCK

O2 is considered diffusion limited because it has a lower gradient (its concentration in the air and blood is not many magnitudes of times different) and thus it is limited by the rate at which it diffuses through the respiratory membrane.--BIG TRUCK