As a part of the urea recycling process, in order for the concentration gradient to be maintained, it is important that urea makes up about approximately one-half of the solutes of the interstitial fluid concentration gradient. Urea is detached from the fluid-tube in the gathering duct by urea uniporters; it spreads back in the fluid-tube in the thin part of the rising limb. Urea is cycled among the tubule collectors and loop in the nephron.