Name: Countercurrent Mechanism of Urine Formation
Uploaded: 2019-07-19
Duration: 9 min 15 s
Description: Kidneys are endowed with a remarkable capacity of conserving water by making concentrated urine through the efficient countercurrent mechanism. Come, let’s e...

Question

Explain the mechanism of the formation of concentrated urine in mammals.

Accepted Answer

Mammals have an ability to produce concentrated urine. The Henle's loop and Vasa recta play a vital role in this. Formation of concentrated urine involves three main steps.

Counter Current Mechanism: The filtrate flows in two limbs of Henle's loop in opposite direction which forms a counter current. Similarly, the flow of blood through two limbs of vasa recta also produces counter current. The proximity between the loop of Henle and vasa recta and the counter current between them helps in maintaining an increasing osmolarity in inner medullary interstitium. As a result, osmolarity increases from 300 mosmol/L in the cortex to about 1200 mosmol/L in the inner medulla.

Role of NaCl and Urea: The osmotic gradient is mainly caused by NaCl and urea. Transportation of NaCl occurs by ascending limb of Henle's loop. Exchange of NaCl occurs with the descending limb of vasa recta. After that, NaCl is returned into the interstitium by the ascending limb of vasa recta. A little amount of urea enters inside ascending limb of Henle's loop and is transported back to the interstitium by the collecting tubule.

Effect of Osmotic Gradient: The presence of osmotic gradient in the interstitium helps the water to easily exit from the collecting tubule. This results in increased concentration of urine. So, if compared with the initial filtrate, human urine is nearly four times concentrated and excreted about 1.8 to 2 liters per day.

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