Mammals have the ability to produce concentrated urine. The capacity of concentrating the urine depends on the length of the loop of Henle. Vasa recta also helps in concentrating the urine. The flow of filtrate in the two limbs of the loop of Henle is in the opposite direction and thus forms a counter current mechanism. The proximity between Henle's loop and vasa recta as well as counter-current in them helps in maintaining an increasing osmolarity towards the inner medullary interstitium i.e. 300mmol/liter in the cortex to 1200 mmol/liter in the medulla. PCT lies in the cortex too hence the concentration is 300mosmol/liter.