Urine blood flow

[NewYorkCity11]

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The question is about constriction of the efferent arteriole of the glomerulus and this impact on the Urine blood flow. The efferent arteriole is the continuation of the glomeruli (afferent-->glomerulus-->efferent). So lets imagine that this is one tube (one vessel with different parts). When we constrict the efferent arteriole, due to continuity equation the urine blood flow in this glomerulus will not change (because of increased velocity in the stenotic area), but ALL the books say that there is decreased blood flow due to increased RESISTANCE which is not appropriate here (the tube starts in the afferent arteriole and the efferent arteriole is just the continuation of it, so poiseuille's law in this context doesn't suitable, unlike if we constrict afferent arteriole, that is logical). Maybe there is another explanation of this effect? Thank you!

 

[NewYorkCity11]

I'm not sure I'm understanding what exactly your question is. If just the afferent arteriole is constricted there is increased resistance and decreased blood flow through the glomerulus. If just the efferent arteriole is constricted blood flow remains the same, vut filtration increases. Also, you keep saying 'urine blood flow'. Do you mean urine flow or blood flow?
For the bolded, did you mean efferent?

I mean blood flow through the "hole tube" (from the afferent arteriole to the efferent). Yeah, sorry, there should be ''efferent''. I know, that it intuitively "flows", but why the pressure in the glomerulus will increase? (due to what law? Because Bernoulli's Law states, that, the pressure will not change before the constriction of the part of the vessel (i.e. efferent arteriole). Again, ALL the books say that there is decreased blood flow (because of R1(afferent)+↑↑R2(efferent) in series will decrease blood flow due to poiseuille's law. But why doesn't here continuity equation work instead? Thank you.

 

[hopefulERdoc251]

I mean blood flow through the "hole tube" (from the afferent arteriole to the efferent). Yeah, sorry, there should be ''efferent''. I know, that it intuitively "flows", but why the pressure in the glomerulus will increase? (due to what law? Because Bernoulli's Law states, that, the pressure will not change before the constriction of the part of the vessel (i.e. efferent arteriole). Again, ALL the books say that there is decreased blood flow (because of R1(afferent)+↑↑R2(efferent) in series will decrease blood flow due to poiseuille's law. But why doesn't here continuity equation work instead? Thank you.

As for why the continuity equation doesn't work...I can't answer that. But...if it helps, think about it in more qualitative manners, think of the renal blood flow system as a long continuous tube with a filter that's permeable to certain things (glomerulus). Before the filter = afferent arteriole...after the filter = efferent arteriole. Say you restrict the efferent arteriole. What happens is that you have a backup of blood through the afferent arteriole and the glomerulus and since there's only a finite amount of elasticity that can accomodate an increased volume, you'll have decreased fluid flowing through that area (like a clogged pipe...before the clog, you can only have so much fluid go through the pipe). Therefore, blood flow decreases through afferent arteriole. Now you also have a high volume of fluid that's attempting to be pushed through the glomerulus, so my guess would be this increases pressure in the glomerulus.

 

[NewYorkCity11]

2) Here, for example (one tube with series resistance), why the flow will drop during stenotic areas ("resistors") if initially Flow was normal, so due to continuity equation it will only accelerate in the stenotic area but poiseuille's law states, that here will be decreased blood flow due to resistance in series (↑↑R1+↑↑R2+↑↑R3)? Why? Thanks a lot!

 

[NewYorkCity11]

As for why the continuity equation doesn't work...I can't answer that. But...if it helps, think about it in more qualitative manners, think of the renal blood flow system as a long continuous tube with a filter that's permeable to certain things (glomerulus). Before the filter = afferent arteriole...after the filter = efferent arteriole. Say you restrict the efferent arteriole. What happens is that you have a backup of blood through the afferent arteriole and the glomerulus and since there's only a finite amount of elasticity that can accomodate an increased volume, you'll have decreased fluid flowing through that area (like a clogged pipe...before the clog, you can only have so much fluid go through the pipe). Therefore, blood flow decreases through afferent arteriole. Now you also have a high volume of fluid that's attempting to be pushed through the glomerulus, so my guess would be this increases pressure in the glomerulus.

I can't understand how and where does it work? And I can't find the answer for so long( Always authors use these laws (continuity equation and poiseuille's law) in their own benefit. I can send you this paradoxical examples in famous books of physiology and hemodynamics. Where can I find normal explanation with some examples?( 🙁

 

[mehc012]

Continuity equation only applies if there is a fixed flow rate. You use it to compare different parts of the same tube, such as the beginning of a vessel and the stenotic region in the middle. It does not apply as readily when you're talking about a complex system with variable distribution between different, linked tubes...which is more like how the human body works.

Here, you are going to think more in terms of current and resistance, more akin to Ohm's law and the other fun stuff we learned way back when discussing electronic circuitry. Raise the resistance in one path relative to other paths, and that path will decrease the amount of current going through it (and other paths will increase). Raise the pressure after a fork point, and more flow will divert into the other channel. If you simplify the kidney into one giant glomerulus, you have 2 main fork points: artery→afferent arteriole/non-kidney flow and afferent arteriole→glomerulus/efferent arteriole. The glomerulus branch always has a high resistance since it's filtering.

Raise the resistance in the afferent arteriole, and flow in the afferent and all downstream forks will decrease; more blood will bypass this area altogether (think of it as less blood flow to your single-arteriole kidney). Thus flow to the glomerulus, or GFR, will naturally decrease. Of note, flow to the 'alternative' fork from the afferent, 'non-kidney flow' will increase.

Raise the resistance in the efferent arteriole and the flow to its alternative fork will increase. In this case, that's the glomerulus. However, since there are now high-resistance areas on both available outflow tracts from the afferent arteriole, overall resistance has increased, and you will probably see some changes to overall flow (distribution in the first fork, afferent/non-kidney) as well.


Basically, you're thinking about this as a linear system, afferent → glomerulus → efferent, but you need to be thinking of it as a FORKING system...afferent → glomerular filtration OR efferent flow

 

[WheezyBaby]

It's more than that. The kidneys are in parallel with the rest of the body. You raise the resistance of the renal vasculature by any means, you'll have decreased renal blood flow assuming characteristics of other vasculature through the body remains the same

 

[mehc012]

It's more than that. The kidneys are in parallel with the rest of the body. You raise the resistance of the renal vasculature by any means, you'll have decreased renal blood flow assuming characteristics of other vasculature through the body remains the same

Exactly! That's the 'first fork' in the oversimplified model up there.

 

[longhaul3]

Having a hard time understanding all your posts (so I also haven't read many of the replies, sorry if I am repeating others' comments) but the point that you're missing is that there are other forks in the road. For a given cardiac output there is a constant blood flow throughout the entire circulation; constricting the afferent arteriole diverts blood away from the kidney and increases flow to other organs.

Constricting the efferent arteriole also diverts fluid volume elsewhere—into the nephron, which is another fork within the renal circulation. As less fluid can pass through the efferent arteriole, it backs up in the glomerulus, increasing pressure, and so filtration increases. So even though a constant flow is passing through the afferent arteriole, varying the resistance of the efferent arteriole changes the distribution of flow.

 

[mehc012]

Having a hard time understanding all your posts (so I also haven't read many of the replies, sorry if I am repeating others' comments) but the point that you're missing is that there are other forks in the road. For a given cardiac output there is a constant blood flow throughout the entire circulation; constricting the afferent arteriole diverts blood away from the kidney and increases flow to other organs.

Constricting the efferent arteriole also diverts fluid volume elsewhere—into the nephron, which is another fork within the renal circulation. As less fluid can pass through the efferent arteriole, it backs up in the glomerulus, increasing pressure, and so filtration increases. So even though a constant flow is passing through the afferent arteriole, varying the resistance of the efferent arteriole changes the distribution of flow.

Probably wouldn't get completely constant flow in the afferent anyway, since the glomerulus is always going to be high resistance and a constricted efferent is high-resistance, but yeah, this is basically a case of trying to apply an equation to a situation more complex than what it fits.