Why have birds and mammals evolved countercurrent mechanism to excrete hypertonic urine ? What are the components of this countercurrent mechanism ? How it works ?

Explanation

Higher vertebrates (birds and mammals), being terrestrial animals, have to conserve water. Therefore, they have developed countercurrent mechanism to excrete hypertonic urine from their bodies.
Loop of Henle and capillary loop (vasa recta) play important role in countercurrent mechanism.