Why does the thickness of the Loop of Henle have to be less than the convoluted proximal tube and the distal convoluted tube?
there is a bit more to it actually,

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.

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


-&gt; 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+&amp; 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).



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)