Question

Explain the mechanism that results in the formation of a concentrated urine in an individual with hyperosmotic blood

Answer 1

This mechanism is called as Counter current mechanism.

• Mammals have the ability to produce a concentrated urine.
• The Henle's loop and vasa recta play a significant role in this.
• The flow of filtrate in the two lombs of Henle's loop is in opposite directions and thus forms a counter current.
• The flow of blood through the two limb of vasa recta is also in a counter current pattern.
• The proximity between the Henle's loop and vasa recta ,as well as the counter current in them help to maintaining an increase osmolarity towards the inner medullary interstitium , that is from 300mOsmol/L inthe cortex to about 1200mOsmol/L in the inner medulla.
• This gradient is mainly caused by NaCl and urea.
• NaCl is transported by the ascending limb of Henle's loop which is exchanged with the descending limb of vasa recta.
• NaCl is returned to the interstitium by the ascending portion of vasa recta.
• Similarly, small amount of urea enter the thin segment of the ascending limb of Henle's loop which is transported back to the interstitium by the collecting tubule.

The above described transport of substances facilitated by the special arrangement of Henle's loop and vasa recta is called as COUNTER CURRENT MECHANISM.

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