hi ulikedaggers, thanks alot! Could u further explain what you mean in diffusion limited , with gradient being the same?? Also, for this graph, why is the abnormal slope same throughout?
I was doing goljan audio and he says pulmonary infarction are mostly in lower lobe b/c perfusion is better here. do you get that?
thanks and god bless
The slope is the same throughout the length of the capillary because the gas exchange is diffusion-limited. This means that gas exchange is occurring along the
entire capillary. The driving force for gas exchange is the partial pressure gradient between PA and Pa. The classic example of a diffusion-limited gas is Carbon monoxide (CO).
Truthfully, I don't know if the slope is the same but thinking of it that way helps me remember that diffusion continues along the entire capillary. The point is that a diffusion-limited gas will be exchanged across the full length of the capillary.
If you take a look at the graph in my previous post you will see that at the beginning of the capillary PaCO is 0. The blue line represents PaCO and there is no CO in the blood as it enters the capillary. If there is CO in the alveolar air there will be a partial pressure gradient driving CO into the capillary. You can clearly see that the blue line in that graph has a positive slope, implying that PaCO is increasing. What you also need to notice is that PaCO increases throughout the entire length of the capillary, unlike PaN2O and
normal PaO2. This is because Hb has a very high affinity for CO and Hb happily takes CO out of the plasma and into the RBC.
Only dissolved gas contributes to partial pressure and therefore the partial pressure gradient for CO is maintained. Because there's a gradient throughout the entire capillary, PaCO continues to increase through the whole capillary.
Perfusion is better at the base of the lung than the apex of the lung (when you're standing) because of gravity. When you're laying flat, perfusion is pretty much equal everywhere in the lung. When standing, arterial pressure is the highest near the base of the lung. Increased arterial pressure means more blood flow, more open capillaries, and thus better perfusion. At the apex of the lung, arterial pressure is the lowest and capillaries can be compressed by alveolar pressure making it more difficult for blood to flow = lower perfusion.
As for goljan I haven't taken path yet, but this is my guess. Perfusion is different in different regions of the lung (as above), but ventilation is much less variable. This means that the V/Q ratio in the apex of the lung is higher than the V/Q ratio in the base of the lung. So, PaO2 is highest in the apex, and lowest in the base. Since the base has the lowest PaO2 to begin with, it seems reasonable to assume that it is the most susceptible to infarcts. But again, I haven't taken path so I have no clue.
