Counter current system is found in two limbs of loop of Henle. The ascending limb transports Na+ and some urea into surrounding medullary tissue. It is impermeable to water. So it makes urine dilute or hypotonic. The walls of descending limbs are permeable of water and salt, so Na+ and water from surrounding fluid are absorbed. The contents or urine become hypertonic.
ADH (Antidiuretic Hormone) : ADH is secreted when the osmotic pressure of blood increases due to excess loss of water from the body. The hormone renders the walls of distal convoluted tubule, collecting tubule (CT) and collecting duct of a nephron permeable to water and increases the reabsorption of water into the surrounding tissues and particular capillaries.
The urine becomes hyperosmotic and less in volume and water is conserved in the body.