Afferent and Efferent Arterioles

00:01 So if we think then about what changes the amount of filtration, we have a couple of different variables to talk about.

00:09 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.

00:27 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.

00:44 So you get a decrease in [P sub GC].

00:49 That decrease is [P sub UF], and what that is the pressure of the ultrafiltrate.

00:55 It also means you get a decrease in the glomerular filtration rate.

01:00 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.

01:10 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.

01:26 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.

01:38 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.

01:52 It also increases glomerular filtration rate, and increases renal tubule blood flow.

01:59 So afferent arteriole vasodilation increases all four parameters.

02:06 Now let’s talk about the efferent arteriole.

02:09 This one gives people a little bit more of a headache, so let’s go through it a little bit slower.

02:14 Afferent arteriole constriction decrease all of our variables.

02:19 Afferent arteriole vasodilation increase all of our variables.

02:24 So what happens if you constrict the efferent arteriole? So you’re decreasing flow after the fact after it travels through the glomerular capillary.

02:37 What does that do? Well, it causes there to be an increase in the pressure of the glomerular capillaries.

02:46 Because if you decrease or cause a constriction, you decrease the flow that goes through.

02:55 But in front of that constriction, what happens is pressure builds up.

03:01 It’s kind of like damming up a river.

03:03 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.

03:16 And what that does then is increase renal tubule flow.

03:22 The downside is, it does decrease renal blood flow – an efferent arteriole vasodilation.

03:31 So now, what this does is, it opens up the outlet vessel a little bit larger.

03:39 What this does is decrease the amount of pressure of ultrafiltrate.

03:46 It decreases the glomerular filtration rate.

03:49 It decreases renal tubule flow, but it increases renal blood flow.

03:56 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?