Kokko and Rector Model
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The kokko and rector model is a theory to explain how a gradient is generated in the inner medulla.
It was previously thought that the gradient was generated by the counter current mechanism (is is the case in the outer medulla). But it has been proven that this cannot be the case in the inner medulla, since there are no salt pumps, and the cell membrane is too permeable to salt.
The kokko and rector model suggests that in the inner medulla the driving force for salt reabsorption is urea accumulation.