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

Hint: Urea naturally has some yellow pigments which are known as urobilin or urochrome. The concentration of urine can be analysed by seeing the colour of the urine specifically darker or lighter.

Complete answer:
 The normal concentration of urea is 2.6 to 6.5mM in human blood and can be 50-fold more concentrated with the higher variation in human urine.
During the glomerular filtration, when plasma passes through the glomerulus, all of its parts or constituents are passed down into the Bowman’s capsule except the plasma protein.
When passing through the renal tubules, a lot of substances and water is reabsorbed by making the filtrate concentrated. This is the point which generally returns in an increase in the concentration of urea as it is not reabsorbed by the body as water and glucose are. Some substances like potassium ions and hydrogen ions are also secreted but not urine. Very little amount of urea is reabsorbed to maintain the mechanism of counter current.
Movement of a large amount of urea across cell membrane is made possible by urea transporter proteins. Urea allows the kidney to create the hyperosmotic urine (higher concentration of ions), which implies that the urine has more concentration of urea than the plasma of the body.

Note: On a general basis, a high blood urea nitrogen level means kidneys are not working properly. But elevated blood urea nitrogen can also be due to the followings:
Urinary tract obstruction, Congestive heart failure or recent heart attack and Gastrointestinal bleeding.