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Excretory Products and Their Elimination - Mechanism of concentration of the filtrate (counter current mechanism)

Grade 11CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The counter current mechanism is the process used by the kidneys to concentrate urine, primarily occurring in the Henle's loop and vasa recta.

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Counter current flow refers to the flow of filtrate in the two limbs of Henle's loop in opposite directions and the flow of blood through the two limbs of vasa recta in opposite directions.

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The proximity between the Henle's loop and vasa recta, as well as the counter current in them, helps in maintaining an increasing osmolarity towards the inner medullary interstitium.

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The gradient of osmolarity ranges from 300 mOsmolL−1300 \text{ mOsmolL}^{-1} in the cortex to about 1200 mOsmolL−11200 \text{ mOsmolL}^{-1} in the inner medulla.

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This gradient is mainly caused by NaClNaCl and urea. NaClNaCl is transported by the ascending limb of Henle's loop and is exchanged with the descending limb of vasa recta.

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The ascending portion of vasa recta returns NaClNaCl to the medullary interstitium. Similarly, small amounts 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.

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The presence of such interstitial gradient helps in an easy passage of water from the collecting tubule, thereby concentrating the filtrate (urine).

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Human kidneys can produce urine nearly 44 times concentrated than the initial filtrate formed.

📐Formulae

Concentration of Urine≈4×Concentration of Initial Filtrate\text{Concentration of Urine} \approx 4 \times \text{Concentration of Initial Filtrate}

Osmolarity Range=300 mOsmolL−1 (Cortex)→1200 mOsmolL−1 (Inner Medulla)\text{Osmolarity Range} = 300 \text{ mOsmolL}^{-1} \text{ (Cortex)} \rightarrow 1200 \text{ mOsmolL}^{-1} \text{ (Inner Medulla)}

Osmolarity=mOsmoles of soluteLitre of solvent\text{Osmolarity} = \frac{\text{mOsmoles of solute}}{\text{Litre of solvent}}

💡Examples

Problem 1:

If the initial osmolarity of the glomerular filtrate is 300 mOsmolL−1300 \text{ mOsmolL}^{-1}, calculate the expected osmolarity of the concentrated urine excreted by a healthy human kidney under the influence of the counter current mechanism.

Solution:

1200 mOsmolL−11200 \text{ mOsmolL}^{-1}

Explanation:

The counter current mechanism in human kidneys is capable of concentrating the initial filtrate by approximately 44 times. Therefore, the calculation is: 300 mOsmolL−1×4=1200 mOsmolL−1300 \text{ mOsmolL}^{-1} \times 4 = 1200 \text{ mOsmolL}^{-1}. This matches the osmolarity of the inner medullary interstitium.

Problem 2:

Identify the primary solutes responsible for maintaining the medullary interstitial gradient and their direction of movement in the vasa recta.

Solution:

NaCl and UreaNaCl \text{ and Urea}

Explanation:

NaClNaCl is transported out of the ascending limb of Henle's loop and enters the descending limb of the vasa recta. It is then released back into the interstitium by the ascending limb of the vasa recta. Urea diffuses into the ascending limb of Henle's loop and is recycled back to the medulla from the collecting duct.