Formation of Concentrated Urine (Counter-current Mechanism)

Mammalian kidneys can produce urine roughly four times more concentrated than the original glomerular filtrate, and this depends entirely on a counter-current arrangement set up by the long loops of Henle of the juxtamedullary nephrons, together with the collecting ducts and vasa recta that run alongside them. The counter-current multiplier works because filtrate flows down the descending limb and back up the ascending limb in opposite directions while the ascending limb actively pumps out NaCl: this steadily builds an osmotic gradient in the surrounding medullary tissue that rises from about 300 mOsm near the cortex to about 1200 mOsm deep in the inner medulla. As fluid later passes down the collecting duct, which runs right through this increasingly salty medulla and is made water-permeable by ADH-controlled aquaporins, water is drawn out of it by osmosis, concentrating the urine before it reaches the renal pelvis. The vasa recta - the capillaries that shadow the long loops of Henle - protect this hard-won gradient rather than washing it away, acting as a counter-current exchanger: blood flowing down into the medulla passively picks up solutes and los …