Question

The concentration of urea in urine is always much higher than the concentration in plasma. Does this mean that urea is secreted?

Answer 1

As plasma passes through glomerulus during glomerular filtration, all of its constituents (including urine) except plasma protein are passed into Bowman's capsule. While passing through renal tubules, water and a lot of other substances are reabsorbed making filtrate concentrated. This also gradually results in an increase in the concentration of urea as it is not reabsorbed like water or glucose. Some substances like K+, H+ are also secreted (not urine). Only a small amount of urea is reabsorbed to maintain the countercurrent mechanism.

Hence, the concentration of urea in urine is always much higher than the concentration in plasma.