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Explain the mechanism of formation of concentrated urine in mammals.

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The Henle's loop and vasa recta have a significant tole to play when It comes to the ability of mammals to produce urine that is concentrated.

The Counter Current Mechanism
There is an opposite flow of the filtrate in the Henle's loop's both limbs, which results in a counter-current eventually. This same effect is also recognised in the flow of current through the vasa recta's both limbs. The maintenance of increasing osmolarity towards the inner medullary interstitium is achieved as a result of the nearness of vasa recta and Henle's loop and also the presence of counter-current in their limbs. The level of osmolarity increases from the cortex (300 mOsmol/L) to the inner medulla (1200 mOsmol/L).

The Role of Sodium hydrochloride (NaCl) and Urea in this process
The significance of NaCl and Urea is to cause osmotic gradient. The Henle's loop and its ascending limb are responsible for the transportation of NaCl which is then exchanged with the Vasa recta's descending limb. Post this exchange, through the ascending limb of vasa recta and collecting tubule, it is sent back to the interstitium.

The Effect of Osmotic Gradient
The easy exit of water from the collecting tubule is facilitated by the osmotic gradient, which is present in the interstitium, and this effect leads to the urine being concentrated. Hence, human urine is four times concentrated than the one out of the initial filtrate.

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