Question 4 of 10advanced💡 UnderstandLong Answer5 marks

Give a brief account of the counter current mechanism.

Correct Answer

The counter current mechanism operates between the two limbs of the loop of Henle and those of the vasa recta. The filtrate flows in a counter current pattern within these structures, which helps in maintaining an increasing osmolarity towards the inner medullary interstitium.

As the filtrate moves down the descending limb, it gets concentrated, but it is diluted by the ascending limb. This arrangement helps in retaining electrolytes and urea within the interstitium.

The proximity between the Henle's loop and vasa recta facilitates the transport of substances. NaCl is transported by the ascending limb of Henle's loop and exchanged with the descending limb of vasa recta, then returned to the interstitium by the ascending portion of vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb and are transported back to the interstitium by the collecting tubule.

This mechanism maintains an increasing osmolarity towards the inner medullary interstitium, ranging from 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla. The DCT and collecting duct concentrate the filtrate about four times. This gradient is mainly caused by NaCl and urea, serving as an excellent mechanism for the conservation of water.

Exercise: EXERCISES | Q: 4
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Explanation

The answer explains the counter current mechanism as described in the NCERT textbook. It covers the interaction between the loop of Henle and vasa recta, the specific roles of NaCl and urea in creating the osmotic gradient, and the significance of this gradient in water conservation and urine concentration. The explanation follows the structure required for a 5-mark long answer, ensuring all key terms from the context are included.

Solution Steps

  1. Step 1: Define the site of the counter current mechanism (Loop of Henle and vasa recta).

  2. Step 2: Explain the movement of filtrate (concentration in descending limb, dilution in ascending limb).

  3. Step 3: Detail the specific transport of NaCl and urea between the limbs and interstitium.

  4. Step 4: Describe the resulting osmolarity gradient (300 to 1200 mOsmol/L).

  5. Step 5: Conclude with the function of the mechanism (water conservation and urine concentration).

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