GFR is 125 mL/min but urine is only about 1.5 L a day, so where does the rest go?

8 answers & replies

• [+][–]Farhan ShaikhDropperSample· 10d ago

  Quick calc: 125 mL per minute, 24 hours in a day, so 125 × 24 = 3000 mL = 3 L. Which is nowhere near 180 L, so I think the book either means 125 mL per second or it's a misprint? Doubt the 180 L tbh.

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  • [+][–]Ishita GoyalDropperSample· 10d ago

    Farhan you forgot the minutes: 24 hours is 24 × 60 = 1440 minutes, not 24. So $125 \times 1440 = 180{,}000$ mL = 180 L/day. NCERT is right.

    And then: 1.5 L out of 180 L is under 1%, which means roughly 99% of the filtrate gets reabsorbed. Most of it happens in the PCT, which takes back nearly all the essential nutrients and (NCERT says) 70-80% of the electrolytes and water. The loop of Henle, DCT and collecting duct handle the rest, with the final concentration decided in the collecting duct by ADH.

    Ananya your question 1 answered: always convert min to day first, check the units before checking the answer.

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    • [+][–]Ananya Reddy11thSample· 10d agoAuthor

      oooh minutes. okay. thank you 😭 I was multiplying 125 by 60 only and stopping at 7.5 L. Also so the "other" 178 L is basically not lost, it just goes back in blood. Makes sense.

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    • [+][–]Ananya Reddy11thSample· 10d agoAuthor

      but is it exactly 180 minus 1.5 that gets reabsorbed? because I read something about secretion also adding stuff to the tubule

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      • [+][–]Ishita GoyalDropperSample· 10d ago

        Good catch, it's not exact subtraction. Tubular secretion adds some substances from the blood into the filtrate (H⁺, K⁺, ammonia, etc.), mostly in the PCT and DCT, which helps with maintaining the ionic and acid-base balance. So the final urine = filtered − reabsorbed + secreted. But volume-wise the reabsorbed part is huge compared to the secreted part, so "about 99% reabsorbed" is fine for NEET.

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• [+][–]Nisha Verma12thSample· 10d ago

  For question 2, the three layers act like a sieve with progressively smaller holes:

  • Endothelium of the glomerular capillaries: has small pores (fenestrations), so it holds back the blood cells but passes water and dissolved stuff.
  • Basement membrane: a thin non-living layer between the two cell layers, it acts as the main filter for large molecules.
  • Podocytes (the epithelial cells of Bowman's capsule): they have foot-like extensions arranged to leave tiny gaps called filtration slits or slit pores.

  Almost all the plasma constituents except proteins pass into the capsule. That's why it's "ultrafiltration": it's filtration under pressure through such a fine sieve that even the big proteins stay in the blood. So the filtrate is basically plasma minus proteins (and obviously no blood cells).

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• [+][–]Siddharth RaoDropperSample· 10d ago

  Ananya, the teacher circled it because the efferent arteriole is the narrower one. That's the whole trick. Blood comes in through the wider afferent arteriole and leaves through the narrower efferent arteriole, so it is like water entering a wide pipe and leaving by a narrow nozzle: pressure builds up in the glomerulus. That glomerular blood pressure is what pushes fluid through the three layers.

  And yes JGA belongs in the pink box. It's formed at the contact of the DCT and the afferent arteriole, and when GFR falls the JG cells release renin, which starts the renin-angiotensin pathway that tries to bring the GFR back up. So it's a GFR regulator, not a reabsorption thing. You can draw a little pink arrow from it to the glomerulus.

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• [+][–]Kavya NaikDropperSample· 10d ago

  One more thing for the big picture. Kidneys receive roughly 1100-1200 mL of blood per minute, which is about a fifth of the cardiac output. So the 125 mL/min that gets filtered is only a small fraction of what flows through. Most of the blood leaves through the efferent arteriole and then forms the peritubular capillaries around the tubules, and these are what take back the stuff reabsorbed from the filtrate. So the flow chart is: afferent → glomerulus → (filtrate goes to the capsule) → efferent → peritubular capillaries → back to circulation.

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