osmolarity in regards to kidney

[TypeSH07]

So I'm kind of confused with the terms osmolarity and osmotic pressure when it refers to the kidney. For example, when it says the descending loop of henle is impermeable to salt, so filtrate osmolarity goes up. Isn't the filtrate inside the loop, which is permeable to water at the descending loop? Does that mean the water will go out of the loop but the salts stay in? I understand osmolarity on its own but for some reason when applied to the kidney it is confusing the crap out of me.

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[BloodySurgeon]

Yes. The descending tubule is permeable to water and not solutes which creates a greater osmolarity. This serves a greater purpose when you look at urea in the collecting ducts.

 

[TypeSH07]

Yes. The descending tubule is permeable to water and not solutes which creates a greater osmolarity. This serves a greater purpose when you look at urea in the collecting ducts.

So according to that diagram, how does the collecting duct increase concentration of filtrate if both NaCl and H20 are going out? Wouldn't there it have to be impermeable to the ions?? Thanks.

 

[BloodySurgeon]

So according to that diagram, how does the collecting duct increase concentration of filtrate if both NaCl and H20 are going out? Wouldn't there it have to be impermeable to the ions?? Thanks.

You have to think about the surrounding fluid vs inside the tubules. The water permeability at the descending limb increases the osmolarity at the collecting duct which needs to be isotonic to the surrounding fluid so there will be a collection of things going on. I don't have time to explain but if you look at the diagraph and see how the cortex osmolarity is lower than the medulla and where solutes leave you could figure it out. You probably don't need to know this... just remember where things leave and don't leave and you'll be fine.

 

[DrMattOglesby]

So according to that diagram, how does the collecting duct increase concentration of filtrate if both NaCl and H20 are going out? Wouldn't there it have to be impermeable to the ions?? Thanks.

the descending limb is permeable to water.
ascending limb to NaCl.

first consider the ascending limb for the sake of understanding;
the ascending limb will use primary active transport to drive salt into the peritubular fluid around the loop of Henle. This will eventually decrease the concentration of solutes in the tubes as it ascends to the DCT (where you find an mOsm of ~100).

Now, consider the Descending Limb;
we just said it is permeable to water. we also said NaCl is present in the peritubular fluid--AKA extracellular fluid--in higher than normal concentrations (due to the primary active transport mechanism of the Ascending limb).
NaCl is osmotically active, that is to say--it sucks water toward it (think: "solutes suck")
As the Descending limb goes further into the medulla, it continues to lose water content and the filtrate becomes more concentrated as a result (approaches ~1200 mOsm).

 

[Kaustikos]

You just need to remember that the concentration gradient increases as you go down the kidney and the impermeablity of solutes causes water to flow out of the kidney tubules and thus concentrating the urine.

That's why there are impermeabilities (relatively) in the descending loop.