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Proximal convoluted tubule makes no sense???

The filtrate reaching the end of the loop of henle at the end of the proximal convoluted tubule is isotonic( at the same concentration) with the tissue fluid surrounding the tubule and isotonic with the blood

Why????

Reply 1

sigma9012

The goal is to at the end make the urine more concentrated than the blood depending on osmotic levels of the blood.

Reply 2

username2396569

Original post by APersonYo

Lol, you've opened up a whole can of worms here. Basically, the tissue fluid in the nephron has a concentration gradient, with there being a high solute concentration at the bottom of the loop of Henle and a low solute conc at the top of the loop. This is formed by the "counter current system", whereby in the ascending limb of the loop of Henle, ions are pumped into the tissue fluid, which thus draws water out of the descending limb. The urine in the descending limb is now more concentrated. As new dilute urine enters the descending limb, it also begins to equilibriate with the tissue fluid, and because the ascending limb is pumping out the same amount of ions each time, you eventually get a concentration gradient forming. It's hard to explain without diagrams, but just appreciate that the gradient exists. Anyway this gradient is important because it allows us a huge amount of control over how dilute our urine is. If you want concentrated urine, then you simply put a load of aquaporin channels in the collecting duct cells and the urine will equilibriate with the high concentration of solute at the bottom of the nephron. Anyway, the reason everything is isotonic is because it needs to be to maintain this concentration gradient in the tissue fluid. If the blood was hypotonic for example, then it would flush away the concentration gradient and wouldn't allow us to tighly control our urine concentration.

I hope some of that made sense haha, it's a tricky topic