Counter current transport mechanism

[SamarEsawy]

Written in my book " Counter current system : a system in which the inflow runs parallel, counter to and in close proximity to the outflow for some distance"
why should they lie in close proximity to each other?
and when applying this to a nephron , do the two loops of nephron lie very near ? what is the significance of such thing?

another thing I've been thinking of is a comparison between the counter current multiplier system in fish ( or temperature exchange ) and that of kidney , I can't actually relate both and I think they are related somehow..
In fish , the blood leaves full of oxygen , while water leaves with little or no oxygen , while in kidneys , the fluid entering the descending limb and fluid coming out of ascending limb are near in osmolarity ( yes I know the fluid leaving the ascending limb is less concentrated but actually there's not drastic differences between entering and leaving only 200 mosm)

thanks

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[dyspareunia]

Written in my book " Counter current system : a system in which the inflow runs parallel, counter to and in close proximity to the outflow for some distance"
why should they lie in close proximity to each other?
and when applying this to a nephron , do the two loops of nephron lie very near ? what is the significance of such thing?

another thing I've been thinking of is a comparison between the counter current multiplier system in fish ( or temperature exchange ) and that of kidney , I can't actually relate both and I think they are related somehow..
In fish , the blood leaves full of oxygen , while water leaves with little or no oxygen , while in kidneys , the fluid entering the descending limb and fluid coming out of ascending limb are near in osmolarity ( yes I know the fluid leaving the ascending limb is less concentrated but actually there's not drastic differences between entering and leaving only 200 mosm)

thanks

200mOsm is a lot. I don't know (and don't care) about what happens in fish, but if the descending/ascending loops aren't nearby then the gradient isn't very useful. The farther apart your loops the greater the concentration gradient needs to be (diffusion).

 

[SamarEsawy]

200mOsm is a lot. I don't know (and don't care) about what happens in fish, but if the descending/ascending loops aren't nearby then the gradient isn't very useful. The farther apart your loops the greater the concentration gradient needs to be (diffusion).

I still can't imagine , do you mean than if the two tubes where far apart then the osmolarity of the fluid in the last part of the descending loop will change greatly as the fluid goes in the long intersegment ( if I suppose they are far apart) and goes into the ascending loop?sorry can you explain it in another way?

 

[dyspareunia]

I still can't imagine , do you mean than if the two tubes where far apart then the osmolarity of the fluid in the last part of the descending loop will change greatly as the fluid goes in the long intersegment ( if I suppose they are far apart) and goes into the ascending loop?sorry can you explain it in another way?

The whole point is to create a concentration gradient. If you suck all the water out of the descending limb and the ascending limb is really far away, then your concentration gradient will not be as significant as if the two limbs are side by side. It's just basic chemistry.

 

[SamarEsawy]

The whole point is to create a concentration gradient. If you suck all the water out of the descending limb and the ascending limb is really far away, then your concentration gradient will not be as significant as if the two limbs are side by side. It's just basic chemistry.

ok but why would the concentration gradient be lost on its way to the ascending limb if the two limbs were far away?
thats the way I imagine far and near lims may be there's somekind of misunderstanding

 

[CherryRedDracul]

ok but why would the concentration gradient be lost on its way to the ascending limb if the two limbs were far away?
thats the way I imagine far and near lims may be there's somekind of misunderstanding

The descending limb is permeable to water and impermeable to solutes. The interstitium of the medulla is solute-rich and has a high osmolarity. Thus, when filtrate is going down the descending limb, it loses water to the insterstitium, becoming more concentrated. But what happens to the interstitium? It gains water without a change in the amount of solute so its osmolarity will decrease. If filtrate continued to pass through the descending limb without the interstitial osmolarity getting fixed, eventually the interstitium would be dilute to reabsorb any water.

That's when the thick ascending limb steps in and reabsorbs solutes out of the filtrate. That does two things: 1. it dilutes the filtrate by taking salts out and 2. it takes those salts and puts it in the interstitium to restore its osmolarity. Now the interstitium can continue absorbing water through the descending limb and the cycle repeats.

The reason why the limbs are in close proximity is that its just more efficient. The thick ascending limb's job of restoring the interstitium becomes a lot more difficult if the interstitium its trying to fix is too far away. Changes in osmolarity aren't necessarily instant. That's how I like to think about it and I may have left out a few details, but that's why there are nephrologists and then there's the rest of us.

 

[SamarEsawy]

The descending limb is permeable to water and impermeable to solutes. The interstitium of the medulla is solute-rich and has a high osmolarity. Thus, when filtrate is going down the descending limb, it loses water to the insterstitium, becoming more concentrated. But what happens to the interstitium? It gains water without a change in the amount of solute so its osmolarity will decrease. If filtrate continued to pass through the descending limb without the interstitial osmolarity getting fixed, eventually the interstitium would be dilute to reabsorb any water.

That's when the thick ascending limb steps in and reabsorbs solutes out of the filtrate. That does two things: 1. it dilutes the filtrate by taking salts out and 2. it takes those salts and puts it in the interstitium to restore its osmolarity. Now the interstitium can continue absorbing water through the descending limb and the cycle repeats.

The reason why the limbs are in close proximity is that its just more efficient. The thick ascending limb's job of restoring the interstitium becomes a lot more difficult if the interstitium its trying to fix is too far away. Changes in osmolarity aren't necessarily instant. That's how I like to think about it and I may have left out a few details, but that's why there are nephrologists and then there's the rest of us.

Good Imagination
Vasa Recta helps preserve interstitium osmolarity as well