Effect of blood pressure on glomerular filtration

[reising1]

I know that increased blood pressure increases glomerular filtration rate in the kidney. But I'm not sure why. The way I envision it is that the blood passes by and some of it leaks into bowman's capsule. It's like a river with a small outlet on the side for the water to go to. That's how I picture the system working.

So if blood pressure is high, and blood is traveling fast, wouldn't it have less of a chance to go into the outlet and hence enter bowman's capsule? Wouldn't less be absorbed?

Now clearly, I know I'm wrong. But I'm just wondering if someone could help me visualize this.

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[Patassa]

I know that increased blood pressure increases glomerular filtration rate in the adrenal cortex. But I'm not sure why. The way I envision it is that the blood passes by and some of it leaks into bowman's capsule. It's like a river with a small outlet on the side for the water to go to. That's how I picture the system working.

So if blood pressure is high, and blood is traveling fast, wouldn't it have less of a chance to go into the outlet and hence enter bowman's capsule? Wouldn't less be absorbed?

Now clearly, I know I'm wrong. But I'm just wondering if someone could help me visualize this.

Adrenal cortex??

Is the high pressure the result of a blockage? If that's the case keep in mind the higher pressure is being generated behind the blockage because the blood is backing up and putting back pressure on the artery. To make up for the decrease in cross section area around the blockage, the blood has to raise pressure to maintain flow rate. So the average velocity may not have changed much.

So think of putting water in something that's semi-permiable, I don't know, maybe a sock. It will hold the water momentarily but will slowly leak out. Now what if you squeezed the sock aka increasing the pressure of the fluid inside? It will leak through the fabric faster, basically the higher pressure is a force reservoir to increase the rate of H2O diffusion.

 

[reising1]

Sorry, I meant kidney, not adrenal cortex, lol.

 

[reising1]

Adrenal cortex??

Is the high pressure the result of a blockage? If that's the case keep in mind the higher pressure is being generated behind the blockage because the blood is backing up and putting back pressure on the artery. To make up for the decrease in cross section area around the blockage, the blood has to raise pressure to maintain flow rate. So the average velocity may not have changed much.

So think of putting water in something that's semi-permiable, I don't know, maybe a sock. It will hold the water momentarily but will slowly leak out. Now what if you squeezed the sock aka increasing the pressure of the fluid inside? It will leak through the fabric faster, basically the higher pressure is a force reservoir to increase the rate of H2O diffusion.

So you are using this sock analogy. But is this really how blood enters the kidney? Is it really a depot of blood that leaks into the kidney? I thought it was more like a stream of blood and the kidney lies on the outskirt and the blood can diffuse into the kidney as the blood goes by. Think of a river where the sides of the river are made out of permeable membranes. Isn't that more like what the kidney is? So if it is, then that means the blood moving faster would mean less blood has a chance to escape through the membrane.

 

[mik30102]

Are you referring to a general blood pressure increase in the body effecting the kidneys, or the balance between the constriction/dilation of the efferent and afferent arterioles?

 

[Patassa]

So you are using this sock analogy. But is this really how blood enters the kidney? Is it really a depot of blood that leaks into the kidney? I thought it was more like a stream of blood and the kidney lies on the outskirt and the blood can diffuse into the kidney as the blood goes by. Think of a river where the sides of the river are made out of permeable membranes. Isn't that more like what the kidney is? So if it is, then that means the blood moving faster would mean less blood has a chance to escape through the membrane.

Well I assumed you were referring to the Glomerulus, which is a big ball of capillaries that are wrapped around by Bowman's capsule. In which case I don't know why the sock analogy breaks down because I was merely trying to correlate what an increase in pressure can do for filtration rates. But here, from my physiology text if you don't find the analogy useful.

"Assuming that all other factors stay constant, as the glomerular capillary blood pressure goes up, the net filtration pressure increases and the Glomerular Filtration Rate increases correspondingly."
they reiterate later
"Because arterial blood pressure is the force that drives blood into the glomerulus, the glomerular capillary blood pres-sure and, accordingly, the GFR would increase in direct propor- tion to an increase in arterial pressure if everything else re-mained constant "

It's equally correct that higher blood pressure sans blockage will result in higher flow rates which would also increase filtration rate. They are one in the same.

 

[Patassa]

Think of a river where the sides of the river are made out of permeable membranes.

This analogy doesn't really work because your "river" aka artery in this case, forces blood into the "sides" aka afferent arteriole to capillaries in the glomerulus, the permeable membrane is on the sides of the "sides" not on the side of the "river."

 

[Patassa]

"River", "sides", and "membrane." or a sock, whatever works.

 

[reising1]

"River", "sides", and "membrane." or a sock, whatever works.

I suppose this picture says it best. So if the "sock" replaced the capillaries, then it's clear.

Thanks!

 

[reising1]

Are you referring to a general blood pressure increase in the body effecting the kidneys, or the balance between the constriction/dilation of the efferent and afferent arterioles?

I was referring to an increase in blood pressure in general. Normally the MCAT doesn't explain any more detailed than that, as I noticed. What is the difference between the two things you mentioned?

 

[Patassa]

I suppose this picture says it best. So if the "sock" replaced the capillaries, then it's clear.

Thanks!

Something that might help is to remember that pressure is always acting normal to the surface, so instead of thinking High P = high velocity therefore less time to go off into the side branches and exchange, think High P = greater force to go off into the side branches, and once in side branches, more force to diffuse out of the system.

 

[reising1]

How about urine volume? Does more blood pressure always equate to higher urine volume? I would suppose not, because the reabsorption depends on other factors, as does blood volume.

 

[Patassa]

How about urine volume? Does more blood pressure always equate to higher urine volume? I would suppose not, because the reabsorption depends on other factors, as does blood volume.

It's usually the other way around, more concentrated urine = less volume of urine because water diffused out which then raises blood pressure. There are too many variables here because of homeostasis to try and make hard and fast rules I think. Any question you get should set it up in a way that there is only one clear answer.

 

[mik30102]

I was referring to an increase in blood pressure in general. Normally the MCAT doesn't explain any more detailed than that, as I noticed. What is the difference between the two things you mentioned?

Perhaps that is why you are having some trouble then. The blood vessels that pass by the glomerulus are not like one long continuous tube. As shown in the picture above there as a afferent arteriole section and a efferent arteriole section.What happens physiologically when the kidney wants to regulate GFR is the efferent section constricts more then the afferent section which causes blood to build up in the glomerulus. Since there is more fluid the pressure here increases causing a gradient to push more fluid into bohmans capsule. As far as your other question more fluid entering bohmans capsule without a change of reabsorption would lead to higher fluid velocity in the nephron and more urine production but it would be hard to make the assumption of no change in absorption without more info