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scilearner

What is the function of countercurrent exhanger in kidney?

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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 :)

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Well, I would not describe its purpose "so that water can go out the collecting duct". The primary function is to concentrate the solutes into the urine stream using reverse osmosis across a membrane. It is countercurrent to increase the efficiency of solute removal and thus dramatically reduce the volume of water (the solvent) required in the process. Countercurrent flow keeps the concentration gradient between the two streams as consistent as possible or as you say "intact". It does so by ensuring that the membrane exposed to the most concentrated urine stream is also in contact with the most concentrated transport stream and the most dilute next to the most dilute etc.

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To frustrate medical students.

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scilearner, I'll give it a try and others can correct me if I get this wrong.

 

Here goes:

 

The point of the countercurrent multiplier is to concentrate salts (sodium and potassium ions) in the tissue fluid between the two parts of the Loop of Henle. The 'countercurrent' part seems to come from the downward flow of water and dissolved substances (solutes) in the descending Loop of Henle and the upward flow of solutes in the Ascending (upwards) Loop of Henle.

 

The 'multiplier' bit seems to be from the fact that the movement of fluid increases the concentration of sodium and chloride ions at the bottom of the Loop of Henle and also the vasa recta (capillaries wrapped around the Loop of Henle).

 

You need to know about osmosis. Osmosis is the movement of water from a high to a low concentration across a partially permeable membrane until the water concentration is the same (equilibrium) on both sides. Osmosis does not need energy.

 

If a solution on one side of a membrane has more water than the other, it is called hypotonic. The side of a membrane with salt, sugar or other soluble substances dissolved in it is called hypertonic.

 

If you have a membrane dividing two parts of a box with pure water on the left and salt on the right side, the water will move from the hypotonic to hypertonic side until equilibrium is reached. The more salt you add to the right side of the membrane, the more osmotic pressure you have. This is measured in kiloPascals (kPa)

 

Pure water has a water potential of 0 kPa; any solute added to pure water will lower its water potential to below zero.

 

Water in osmosis flows from a more positive to a lower water potential. This is an automatic and passive process, requiring no energy as I mentioned earlier - it depends on the random movement of water molecules.

 

Do you understand this part scilearner?

Edited by jimmydasaint

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