Counter Current Mechanism:

Henle’s loop and Vasa recta plays on important role in this mechanism.

Vasa recta: Vasa recta is a series of straight capillaries in the medulla. They are parallel to the loop of Henle.

• Glomerular fluid in Henle’s loop flows towards renal medulla in descending limb and toward Cortex in ascending limb.  
• Also, blood in Vasa recta flow towards medulla in descending limb and towards Cortex in ascending limb.
• These 2 countercurrent systems help in concentrating the urine. These both make the hypertonic urine (urine more concentrated then blood). It conserves water of body.
• Henle’s loop makes the re-absorption of water more and vasa recta passes urea to fluid to make more water re-absorption.