longer loop of henle/steeper gradient

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echoyjeff222

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Why is it that a steeper loop of henle causes a more hyperosmotic urine? I know that there would be more time for Na+ to be pumped out of the ascending limb, which means an overall saltier medulla ... however, even though H2O is lost on the descending, isn't Na+ lost in the ascending which kind of cancels out? It ends up pretty dilute once it passes the ascending ... or is the net effect still "more concentrated?" thanks.

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I dont remember fully as I took Physio 3 years ago, but if i remember correctly the ascending limb is a semi permeable membrane that doesnt allow Na to escape...only water to be brought in if need be.
 
I dont remember fully as I took Physio 3 years ago, but if i remember correctly the ascending limb is a semi permeable membrane that doesnt allow Na to escape...only water to be brought in if need be.

Echo has it right on this one. It is the descending loop where water is absorbed and the ascending loop were ions are taken up. This is because it runs in a counter current exchange with the vasa recta. The blood in the vasa recta absorbs the ions from the ascending loop and then travels to the descending loop to absorb the water (because of the inward osmotic gradient generated from the ions). We still appreciate your input though Exacto so keep it up!

Now as for OPs question. I am unsure really, because I do know that the ions Na+ and Cl- are concentrated at the lower parts of the loop of hence (due to gravity). However, I would assume the effects would cancels due to equal absorptions of both ions and waters throughout the loop. I would really like to know the answer though.
 
From what I understand from my physio class, the proximal convoluted tubule is responsible for the majority of solute and water reabsorption. Sodium and potassium trickle into the interstitial fluid through concentration gradients, subsequently creating an electrical gradient that attracts chloride across. Water also passes through facilitated diffusion through water channels. PCT only equilibrates the filtrate with the outer medulla.

The descending limb of the Loop of Henle is responsible for additional water reabsorption which solely depends on osmotic pressure. So as you run deeper into the medulla, the osmolarity of the interstitial fluid increases and pulls water out of the filtrate. Since this is also a passive process, the descending limb of the Loop of Henle also only equilibrates the filtrate with the medulla.

The ascending limb of the Loop of Henle is impermeable to water, but can transport sodium into the medulla to establish the osmotic gradient. This is how it equilibrates with the medulla as it comes closer and closer to the cortex where the osmolarity is at its lowest (the same as blood plasma). So yes the filtrate loses lots of water and solutes in the PCT and also the Loop of Henle, but it's important to note that the ascending limb is important for the establishment of the osmotic gradient.

If there is a longer Loop of Henle, then there should be a larger osmotic gradient from the inner medulla to the cortex. The collecting duct must also equilibrate with the osmotic gradient as it travels toward the renal pelvis. It does this through water channels that follow the osmotic gradient. So with a larger osmotic gradient, the filtrate will become very concentrated by the time it reaches the calyces.

The ascending limb of the Loop of Henle is important for all of the passive processes that take place in the PCT, descending limb, DCT, and collecting duct because it establishes the medullary osmotic gradient. Moreover, specifically for this question, if you have a longer Loop of Henle, it makes sense to have a larger osmotic gradient. Why have all this extra tissue running toward the bottom of the Loop of Henle if it can't be used for water reabsorption? You would just be running the filtrate an extra distance with no function.
 
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