We are currently being taught "Homeostasis through excretory system" in college. After seeing the structure of a nephron, it is not quite evident as to why should the Loop of Henle be the thinnest.

Only sense that I could make out of it is that- "Thin cross sectional area implies higher fluid velocity". Since, water and ions are majorly reabsorbed in the proximal convoluted tube, the loop of Henle is thin to increase the flow of fluid because there isn't much left to reabsorb and hence it won't require as much reabsorption time as the PCT. This decreases the filtration time and makes the process faster. I am not quite sure if this makes sense.

Can someone help?


there is a bit more to it actually,

  1. The Henle -loop only occurs in birds and mammals, giving them the ability to make the urine hyperosmotic in comparison to their blood, other vertebrates can't do this.

  2. how does the henle loop achieve this further concentration of urea(= more resorption of H2O)?:

-> by using the physical principle of countercurrent exchange

Beware: the explanation follows the opposite direction as the flow of the liquid!

  • A potential is build up in the bigger upgoing part Pars recta by active Na+& Cl- resoption , the water cant go out because the membrane is impermeable here.

  • this leads to build up of osmotic pressure (positive osm. potential of 1200 mOsmol/kg)

  • The Henle -loop loops back up as a thin part(!) the Pars ascendens

  • the downgoing part right after the PCT (pars descendens) uses the potential of the corticomedullary osmosis gradient to let water (resorption) out using passive osmosis while taking urea in (no ATP needed).

enter image description here

CONCLUSION: So why is the left and bottom part of this henle loop thinner?! to resrobe H2O using passive osmosis ! (also, if the lowest bottom part would be thick, it would not loop so nice, therefore hindering the coutercurrence effect)


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