please explain the line '' some amount of urea may be retained in the kidney matrix of some of these animals to maintain a desired osmolarity"?? NEED FAST PLEASE EXPLAIN THIS LINE OF NCERT.....need fast..

Osmolarity is essential to continue existence. If you excrete all the urea, the chemical balance of the body is affected, and that can have adverse affects. So, some amount of urea is retained in the matrix of the kidney, to maintain the ionic balance of the body.