What is the function of countercurrent exhanger in kidney?

[sameeralord]

Hello everyone,

I understand that countercurrent multiplier makes the area around medulla hypertonic, so water can go out of collecting duct. When I checked the function of countercurrent exchanger it was to kep this concentration gradient intact. I looked at the solutes movement in countercurrent exchanger but I don't understand how it keeps this intact or why it it nessecary at all. I understand countercurrent multiplier, and isn't that enough. Could anyone please help. Thanks

 

[Andy Resnick]

I agree, it's a confusing concept. The purpose is to regulate the body salt and water content (homeostasis). The mechanism is the 'countercurrent multiplier". Here's the best analogy I can think of:

Imagine you are standing on ice (barefoot). Never mind why. Your blood travels down your leg, through your foot, and up your leg. If the artery and vein were far apart, your warm blood would cool considerably as it went through your foot, and the ice-cold blood would travel up and have to get warmed, which costs you a lot of energy.

Instead, let your artery and vein be very close together. (I don't know if they are in real life, the original analogy uses a bird, because birds hang out on the ice for long periods of time)

Now, as the warm blood descends, it gives up some heat to the venous blood going back up. This way, instead of the heat going out of your feet and into the ice, the heat is used (the countercurrent) to warm the cooled blood. And your feet stay at the same temperature either way!

That's the principle of the countercurrent multiplier, but with osmolarity rather than heat.

 

[nobahar]

Imagine you are standing on ice (barefoot). Never mind why.

That made me laugh!

I also covered this recently. I found it difficult to see how the gradient could be maintained between the loop of henle and the interstitial fluid in a passive process (as I was led to believe it is), but I think active transport is involved in the ascending limb, and that urea also features in the process. Without these, I always picture the gradient being reduced until it is in equilibrium (even if the fluid is moving).

Can you clarify what you mean by countercurrent exchanger and countercurrent multiplier?

 

[Andy Resnick]

Silbernagel's "Color Atlas of Physiology" refers to the flow of water in the vasa recta as the 'countercurrent exchange'. This is accomplished by the passive diffusion of water from the decending vasa recta to the ascending vasa recta in the osmostic gradient of the medulla. This allows normal blood supply to the medulla without altering the local hypertonicity of the medulla. The term 'countercurrent' refers to the fact that the direction of flow in each arm is opposite to the other.

The 'countercurrent multiplier' refers specifically to the loop of Henle. The descending limb is water permeable, salt impermeable, while the ascending limb is water impermeable, salt permeable- the pumping of salt out of the ascending limb requires energy- and amplifies the slight osmotic gradient that exists between neighboring points of the ascending and descending limb to a large gradient along the total length of the limb. The longer the loop (the deeper into the medulla), the steeper the gradient.

Urea does play a role in maintaining the gradient- I'm not exactly sure how, but it seems to be an osmotic buffer, like albumin in the blood (AFAIK).

Silbernagel's book is an excellent resource, btw.

 

[nobahar]

Thanks Andy.

The descending limb is water permeable, salt impermeable, while the ascending limb is water impermeable, salt permeable- the pumping of salt out of the ascending limb requires energy- and amplifies the slight osmotic gradient that exists between neighboring points of the ascending and descending limb to a large gradient along the total length of the limb.

I presume it is necessary to maintain the gradient as well as amplify it?

Urea does play a role in maintaining the gradient- I'm not exactly sure how, but it seems to be an osmotic buffer, like albumin in the blood (AFAIK)

Ohhhhh, I always imagined it just to increase the concentration gradient for the passage of water out of the loop of henle. I have a Biology textbook, but its not that clear, internet sources are even worse...

 

[Andy Resnick]

I presume it is necessary to maintain the gradient as well as amplify it?

Good question- the osmotic gradient in the medulla can get washed out by drinking lots of water (for example), and I don't know the mechanism that re-establishes the gradient.

http://www.springerlink.com/content/k286l27221455854/