Countercurrent Multiplier Mechanism

[orangeman25]

Can someone explain how this plays a role in the kidneys?

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[sakabato93]

The countercurrent multiplier creates and osmotic gradient due to sodium ion pumps in the ascending limb. The blood that passes the ascending limb then has high osmolarity due to the increased sodium ions secreted into the blood. That blood then flows past the descending limb which is permeable to water but not ions, which allows for the water to flow into the blood which has higher osmolarity than the filtrate due to the activity of the sodium pumps.

Wikipedia does a pretty good job explaining: http://en.wikipedia.org/wiki/Countercurrent_multiplication