So if we think then about what changes the amount of filtration,
we have a couple of different variables to talk about.
One is whether the afferent arteriole could change in size?
So if you get a vasoconstriction
or a decrease in the luminal diameter of the afferent arteriole,
it means that you are going to have less pressure
within the glomerular capillary.
So what does that look like in terms of a picture?
If you decrease the fluid traveling past the afferent arteriole,
what’s going to happen is you’ll have a decrease in the pressure
of the capillaries in Bowman’s space.
So you get a decrease in [P sub GC].
That decrease is [P sub UF],
and what that is the pressure of the ultrafiltrate.
It also means you get a decrease in the glomerular filtration rate.
You get a decrease in the amount of flow through the renal tubule,
and finally you get a decrease in the amount of renal blood flow.
So afferent arteriole vasoconstriction can be summarized by decreasing all of our parameters
– the plasma and the pressure of the ultrafiltrate,
glomerular filtration rate,
renal tubule flow,
and renal blood flow.
Now what happens if the opposite occurs -
if we dilate the afferent arteriole?
So, again, dilation is simply making that tubule bigger,
and therefore more flow can go through it.
If more flow goes through the afferent arteriole,
that’s going to increase the pressure of the ultrafiltrate,
that’s going to increase the pressure within Bowman’s capillary.
It also increases glomerular filtration rate,
and increases renal tubule blood flow.
So afferent arteriole vasodilation increases all four parameters.
Now let’s talk about the efferent arteriole.
This one gives people a little bit more of a headache,
so let’s go through it a little bit slower.
Afferent arteriole constriction decrease all of our variables.
Afferent arteriole vasodilation increase all of our variables.
So what happens if you constrict the efferent arteriole?
So you’re decreasing flow after the fact
after it travels through the glomerular capillary.
What does that do?
Well, it causes there to be an increase
in the pressure of the glomerular capillaries.
Because if you decrease or cause a constriction,
you decrease the flow that goes through.
But in front of that constriction,
what happens is pressure builds up.
It’s kind of like damming up a river.
You’re going to get an increase in the fluid in front of the constriction
that increases the pressure within the glomerular capillary,
increases glomerular filtration rate.
And what that does then is increase renal tubule flow.
The downside is, it does decrease renal blood flow –
an efferent arteriole vasodilation.
So now, what this does is, it opens up the outlet vessel a little bit larger.
What this does is decrease the amount of pressure of ultrafiltrate.
It decreases the glomerular filtration rate.
It decreases renal tubule flow,
but it increases renal blood flow.
Efferent arteriole changes –
if we have to think about it –
what would happen if you change the size of the blood vessels
as the blood is leaving the glomerular capillary?
Will it have any build-up effects?
And then on the afferent arteriole side, it’s simply –
are you increasing or decreasing the diameter of what’s entering the glomerular capillaries?