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A Handbook of Biology

3. TÜBÜLÅR SËÇRËTÏØÑ

Cells of PCT & DCT maintain ionic (Na⁺, K⁺ balance) and acid base balance

(pH) of body fluids by selective secretion of H⁺, K⁺ & NH3 into the filtrate and

absorption of HCO^-

3 ^{from it.}

Collecting duct maintain pH and ionic balance of blood by selective secretion

of H⁺ and K⁺ ions.

MËÇHÅÑÏSM ØF THË ÇØÑÇËÑTRÅTÏØÑ ØF THË FÏLTRÅTË:

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7

3

5

2

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6

The flow of filtrate in the 2 limbs of Henle’s loop and the flow of blood

through the 2 limbs of vasa recta are in opposite directions (i.e. in a

counter current pattern).

NaCl is transported by ascending limb of Henle’s loop that is exchanged

with descending limb of vasa recta. NaCl is returned to interstitium by

ascending limb of vasa recta.

This transport of substances facilitated by Henle’s loop & vasa recta

is called counter current mechanism. It maintains a concentration

gradient (interstitial gradient) in medullary interstitium . It enables

easy passage of water from collecting tubule thereby concentrating

the filtrate (urine).

Henle’s loop & vasa recta help to concentrate the urine.

Human kidneys produce urine four times concentrated than the initial

filtrate formed.

Due to the counter current and proximity between the Henle’s loop &

vasa recta, osmolarity increases from cortex (300 mOsmolL^–1) to the

inner medullary interstitium (1200 mOsmolL^–1). This gradient is mainly

caused by NaCl and urea.

Similarly, small amount of urea enters the thin segment of the

ascending limb of Henle’s loop which transported back to the

interstitium by the collecting tubule.