Explain the mechanism of formation of concentrated urine in mammals.

Mammals have the ability to produce a concentrated urine. The Henle’s loop and vasa recta play an important role in the same.

The transport of sodium chloride in the ascending limb of Henle's loop results in an osmolal gradient. This is passive in the thin region of the loop while active in the thick region. The creation of a hypertonic medulla is facilitated by the renal tubules and vasa recta in the following manner:
There is an active absorption of urea that occurs from the collecting tubule.

Passive absorption of water without NaCl takes place in the descending limb of the loop of Henle, which helps to concentrate NaCl as it enters the ascending limb.

In the vasa recta, absorption of water into the blood takes place and return of the absorbed solute to the interstitium follows. This is called the countercurrent exchange mechanism.

Human kidneys can produce urine nearly four times concentrated than the initial filtrate which is initially formed.