Both a wasteful product and a useful factor in controlling water balance
Small (MW=60d)
Highly polar
--> Does not permeate lipid membrane
Urea is freely filtered
NB:
In proximal tubules, half is reabsorbed
Urea concentration increases as water is reabsorbed
--> Passive diffusion out of lumen
* Via paracellular route
NB:
In loop of Henle (thin regions), about the same amount reabsorbed is secreted
Medullary interstitial urea concentration is high
* Increases from outer to inner medulla
* Peak concentration depends on hydration status and ADH level
Paracellular route not available because tight junctions in loop of Henle are not permeable
--> Facilitated diffusion into lumen
* Via urea uniporters
* In the thin regions only
From the thick ascending limb to medullary collecting ducts
--> Luminal membrane urea permeability is ZERO
NB:
In medullary collecting duct, about half is reabsorbed again
As more water is reabsorbed, luminal urea concentration rises even more
* Can "easily reach 50 times greater than" plasma level
--> Diffusion out of lumen into medulla interstitium (via urea uniporter in inner medulla [AV6:p93])
* Urea uniporters can be constitutive, or stimulated by ADH
--> Provides the source of urea that diffuse into the lumen at the loop of Henle
NB: