Explain why :
(b) Mammalian can eliminate hypotonic urine and hypertonic urine according to body needs.
Elimination of hypotonic or hypertonic urine is under the control of ADH, Aldosterone hormone and counter current mechanism. 

ADH (Aniti diuretic hormone) - is released when the there is loss of water, it facilitates water reabsorption from latter parts of the tubule, thereby producing hypertonic urine. An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release to complete the feedback. 

Aldosterone - also causes reabsorption of water from the distal part of the tubule thus producing hypertonic urine. When the level of the aldosterone is low, water absorption is less and hence hypotonic urine is produced. 

The counter current mechanism helps in the maintenace of the intestinal gradient. The intestinal gradient helps in easy passage of water from the collecting tubules thereby concentrating the urine.