Loop of Henle question

[139871]

I just had a quick question about the loop of henle. The descending thin limb is permeable only to water so the osmolarity of the filtrate increases here because water leaves the loop. The ascending thin limb is permeable to solutes so solutes leave the filtrate at this point, which dilutes the filtrate. I am unsure as to why you would concentrate the filtrate only to dilute it again. Wouldn't it make more sense for the ATL to allow ions into the filtrate so the urine is extra concentrated? I may not be understanding something properly here so just let me know what you guys think! 🙂

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[biggiesmallsiz]

i came here to post this

 

[Chemist0157]

Maybe only certain solutes can exit the loop? Ya know, dissolved particles that the body can use...I'm not real sure.

 

[0Complications]

I just had a quick question about the loop of henle. The descending thin limb is permeable only to water so the osmolarity of the filtrate increases here because water leaves the loop. The ascending thin limb is permeable to solutes so solutes leave the filtrate at this point, which dilutes the filtrate. I am unsure as to why you would concentrate the filtrate only to dilute it again. Wouldn't it make more sense for the ATL to allow ions into the filtrate so the urine is extra concentrated? I may not be understanding something properly here so just let me know what you guys think! 🙂

The reason for the concentration gradients is to allow a system of H2O flow out of the descending loop and to allow the filtrate and additional water to be taken back up in the vasa recta (counter current system). This way your body can conserve water and only excretes excess ions (also helps to balance pH).

The idea of making the filtrate more concentrated really helps to "extract" as much water as possible and allow it to be picked up by the vasa recta. If the gradient were the same then the countercurrent system wouldn't function, because it would reach an equilibrium state and we would all excrete equivalent amounts of water from what is contained in the extracellular layers and vasa recta. This would be very bad, for a number of reasons, but imagine how much fluid you would lose per minute/hour/day if this were the case.

Also, really only Na+ is permeable through the thin ascending limb of Henle. The Na/K/Cl cotransporters along with the Na-H Exchanger are all in the Thick Ascending loop, which is impermeable to water.

 

[mrmandrake]

Wouldn't it make more sense for the ATL to allow ions into the filtrate so the urine is extra concentrated?

The role of the kidney isn't to make urine "extra concentrated". It controls how much urine needs to be concentrated in response to blood pressure and other things.

So if we need to retain water, ADH will make the distal nephron permeable to water and water will leak into the interstitum because its really salty out there due to the Na+ that leaked out in the ascending loop. Urine will be concentrated.

If we don't need to retain water, it will just empty out into the bladder and urine will be more dilute. Let me know if I'm wrong but thats how I see it.

 

[DragonWell]

^this is pretty much on the right track. you need to have a very hypertonic medullary interstitium in order to produce a concentrated urine via ADH as mentioned above, and a lot of where that Na is coming from is from the ions that were pumped out of the loop. That's why if you have a long loop ie desert rat, you can make your interstitium really salty and reabsorb almost all your water. Humans max out at around 1200 mOsm.

In addition to blocking transporters, one of the ways diuretics work is to reduce the hypertonicity of the interstitium, causing inability to concentrate urine.

 

[Instatewaiter]

So if we need to retain water, ADH will make the distal nephron permeable to water and water will leak into the interstitum because its really salty out there due to the Na+ that leaked out in the ascending loop. Urine will be concentrated.

The tubular concentrations, even prior to the distal tubule/collecting ducts (where ADH acts) change based on whether the kidney is trying to dilute or concentrate urine so your assertion is close but not quite there.

In addition to blocking transporters, one of the ways diuretics work is to reduce the hypertonicity of the interstitium, causing inability to concentrate urine.

Not all diuretics work this way. Really only Loop diuretics do. Thiazides (more distal), CA inhibitors (more proximal) and K sparing (aldo inhibitors) do not.


So to summarize at the level that the MCAT will probably ask you:
The thick ascending limb is impermeable to water allowing the area outside of the tubules (interstitium) to become concentrated and the area inside the tubule to become dilute. The important transporter here is Na-K-2Cl not the NHE (NHE is mainly impt in the proximal tubule for reabsorption of HCO3).

So the Na-K-2Cl pump acts in part to create this concentration gradient and the gradient is made larger by urea recycling.

When we need to concentrate urine, ADH causes translocation of water pores (aquaporins) to the membrane allowing water to flow down the solute gradient. This makes urine concentrated.

This doesn't happen when we are trying to dilute urine.

 

[OB1]

In plain English, the reason why water leaves the decending loop in the first place is because of the difference in osmotic pressures, and guess where all the solutes come from to create that pressure gradient? That's right, the ascending loop.

If the tubules dumped all the ions out the bladder, there wont be anything left for the kidney to create that higher osmotic pressure in order to pull the water from the decending loop.

 

[139871]

Thanks for all the responses, extremely helpful!

 

[bozz]

So is the ascending part necessary to concentrate the interstitium with salt, creating a concentration gradient.. so that WATER can be absorbed in the descending part?

 

[DragonWell]

So is the ascending part necessary to concentrate the interstitium with salt, creating a concentration gradient.. so that WATER can be absorbed in the descending part?

Yes, but...while it's true that ions pumped out of the ascending limb will pull H2O out of the descending limb and thus help to set up the countercurrent system, the point of this and the take home message you want to remember is that the hypertonic interstitium helps to absorb water in the distal tubules and collecting ducts. The distal tubules/collecting ducts are where the final concentration of urine is determined, so they are where the money is at...

Once you have the hypertonic medullary interstitum, you can control urine concentration by adding or subtracting water channels in the principal cells in the late distal tubules and collecting ducts.

As mentioned above:
Lots of ADH->lots of water channels->lots of water reabsorption->concentrated urine

Little ADH ->few water channels->little water reabsorption->dilute urine

 

[DrMattOglesby]

...

So if we need to retain water, ADH will make the distal nephron permeable to water and ...

Anti-Diuretic Hormone works on the Collecting duct by triggering vesicles of aquaporins to migrate and fuse to the plasma membrane. these aquaporins will then allow water to flow back into the body (physiologically).

just wanted to add onto it is all =]

 

[mdc1027]

I am unsure as to why you would concentrate the filtrate only to dilute it again.

Keep in mind one function of the kidney is to remove foreign chemicals and metabolic wastes/byproducts (ie urea). So by going through the loop of henle, you may be creating the same concentration of ions as you had prior to going through, but you have a much greater concentration of wastes.