I'm learning physiology and I have a hard time figuring out how the cortico-papillary gradient is created.

Most explanations go as follow:
1) Sodium is pumped out the ascending branch into the interstitium until a 200 mOsm/L gradient is established between the tubular fluid and the interstitium.
2) Water passively flows from the descending branch into the interstitium to equilibrate osmolarity between the tubular fluid and the interstitum.
3) The tubular fluid flows, moving everything towards the ascending branch.
1 bis) Sodium is again pumped out until the 200 mOsm/L gradient is established Reapeating this steps over and over again creates the vertical gradient.

This is illustrated here Creation of the cortico-papillary osmotic gradient

Ok, so far, so good; here comes my question :
In step 2, how comes the final concentrations in the interstitium and the tubular fluid in the descending branch are equal to the initial concentration in the interstitium?
I would expect the final concentration to be the mean of the two. In the examples above, starting from 400 mOsm/L in the interstitium and 300 mOsm/L in the descending branch, I would expect 350 mOsm/L at the end, not 400 mOsm/L.

How is that possible? Is this because at every moment, when water flows into the interstitium diluting its content, the ascending branch keeps pumping sodium to maintain the 200 mOsm/L gradient?

Thanks a million to whoever will answer this question!


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