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10^1000000 threads about this but haven't found the clarity I'm looking for...(already actually took the MCAT but thought this would be a good place to post)
Q=DeltaP/R
Flow is maintained by the pressure gradient (blood flows from high P to low P)
As the R increases, the pressure gradient increases (that is, there is a large drop in pressure when going from arteries > arterioles) - otherwise, flow would not be maintained. I understand this.
Vasoconstriction of the afferent arterioles causes a decrease in Q(flow) via an increased R. Here is where my confusion lies. Why does an increase in resistance cause an increased pressure drop (like in the transition from artery > arterioles) and a constant Q, but in vasoconstriction it causes a decrease in Q? My understanding is that vasoconstriction would lower P downstream (i.e. the capillaries) and an increase in P upstream (arteries) - essentially causing the same increase in pressure gradient. If the pressure gradient increases, shouldn't Q be maintained?
In terms of Q=deltaP(pie)(r^4)/8nl, I see that decreasing r by a factor of 2 would decrease Q by a factor of 16 (because R increases by a factor of 16). Couldn't the deltaP just increase by a factor of 16 and maintain Q constant? If we were to rearrange the equation to solve for deltaP:
deltaP=Q(8nl)/(pie)(r^4), a decrease would cause an increase in deltaP.
I understand that a change in r has a much greater effect on Q than a change in deltaP (because it's raised to the 4th) but when looking at the equation I'm having trouble understanding why the above statement doesn't happen? It seems as if the R causes both Q and deltaP to change (delta P increases, Q decreases), but it's effect on Q is much larger than it's effect on deltaP.
Is it just because vasoconstriction is local, and therefore, the majority of blood is redirected to other unconstricted vessels? Therefore, both deltaP and Q change, but Q to a larger extent because of all this blood flowing elsewhere? And this lack of flow into the capillaries is what is actually causing the decrease in capillary pressure, rather than the resistance affecting the pressure directly as it does in the artery > arteriole transition?
Q=DeltaP/R
Flow is maintained by the pressure gradient (blood flows from high P to low P)
As the R increases, the pressure gradient increases (that is, there is a large drop in pressure when going from arteries > arterioles) - otherwise, flow would not be maintained. I understand this.
Vasoconstriction of the afferent arterioles causes a decrease in Q(flow) via an increased R. Here is where my confusion lies. Why does an increase in resistance cause an increased pressure drop (like in the transition from artery > arterioles) and a constant Q, but in vasoconstriction it causes a decrease in Q? My understanding is that vasoconstriction would lower P downstream (i.e. the capillaries) and an increase in P upstream (arteries) - essentially causing the same increase in pressure gradient. If the pressure gradient increases, shouldn't Q be maintained?
In terms of Q=deltaP(pie)(r^4)/8nl, I see that decreasing r by a factor of 2 would decrease Q by a factor of 16 (because R increases by a factor of 16). Couldn't the deltaP just increase by a factor of 16 and maintain Q constant? If we were to rearrange the equation to solve for deltaP:
deltaP=Q(8nl)/(pie)(r^4), a decrease would cause an increase in deltaP.
I understand that a change in r has a much greater effect on Q than a change in deltaP (because it's raised to the 4th) but when looking at the equation I'm having trouble understanding why the above statement doesn't happen? It seems as if the R causes both Q and deltaP to change (delta P increases, Q decreases), but it's effect on Q is much larger than it's effect on deltaP.
Is it just because vasoconstriction is local, and therefore, the majority of blood is redirected to other unconstricted vessels? Therefore, both deltaP and Q change, but Q to a larger extent because of all this blood flowing elsewhere? And this lack of flow into the capillaries is what is actually causing the decrease in capillary pressure, rather than the resistance affecting the pressure directly as it does in the artery > arteriole transition?