Now, these particular transporters also do one other thing besides move water -
they can move urea.
Urea can move across the apical membrane
and then across the basolateral membrane.
But these UT-A1 receptors are sensitive to arginine vasopressin.
So once arginine vasopressin binds,
it causes these protein phosphorylation cascade,
-which up regulates the UT-A1s to allow urea to enter across the apical membrane,
and then the UT receptors on the basolateral membrane are now regulated
but then they allow urea to travel out the [outside] into the interstitial space,
and eventually picked up by the blood.
The urea that's left in the renal tubule is then excreted out of the body.
Now urea is an interesting molecule because it does a few thing for us.
It adds to the osmolality that occurs between the cortex of the kidney to the medullary region.
A lot of the urea is reabsorbed in the proximal tubule - about 50%.
And this occurs as it is reabsorbed into peritubular capillaries.
However, urea doesn't always stay put.
It doesn't just be reabsorbed once and then doesn't return.
It has a lot of transporters and is fairly soluble,
andt has that molecule hard to kind of hold on to as we see here.
Urea then can enter the tubule around the hairpin loop,
or around the thick, thin ascending limb.
It travels up the thick ascending limb, the cortical collecting duct,
the cortical tubule and the inner medullary tubule,
and then can be recycled.
So this whole process is regulated at the cortical tubule level by
arginine vasopressin or antidiuretic hormone.
Thus, only about 40% of the urea is actually excreted.
The other 60% just cycles around in this particular process.
That is one of the reasons why we measure urea in the blood as BUN (Blood, Urea and Nitrogen).
But you also think of urine as urea,
because you are excreting about 40% of it.
This recycling process seems ineffecient, but it does have a function.
Urea helps establish this corticopapillary osmolality gradient,
So urea participates in the osmotic gradient.
Why do you need an osmotic gradient?
If you didn't have a large osmotic gradient,
you would not be able to reabsorb water in the collecting ducts.