question on kidney and medullary interstitium

[goldngrl1611]

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Hey guys-
Quick question for you all.
So I know the one of the many functions of kidney anatomy is to increase the osmolarity of the medullary interstitium, so there is a driving force for H2O in the descending limb and in the collecting duct if ADH acts. So, when H2O is picked up from the descending limb, does it go right into the peritubullar capillaries? Otherwise, wouldn't the medulla become diluted and therefore defeat its purpose? If it is picked up in the peritubullar capillaries, does this mean that when ions flow out of the ascending limb, do they just go right into the interstitium, instead of being picked up by the capillaries? This would keep the medulla concentrated, and would prevent ions that were excreted from re-entering into the blood. I just didn't know if it worked that way or not. Thanks!

 

[fizzgig]

regular osm is 300, and fluid coming out of the ascending limb is ~100 i thought. so yes you dump water in the descending as you go, but you must be pumping out relatively more salt on the ascending limb to get a final hypotonic solution.

 

[Rollo]

Sodium and other ions that are actively pumped out by thick ascending limb do enter the capillaries (specifically vasa recta in juxtomedullary nephrons) however because of the U-shape of the vasa recta, the solute are able to leave the vasa recta and enter the medullary interstitium.

Vasa recta does NOT form the medullary hyperosmolarity, they prevent it from dissipating. How are the solutes able to both enter and leave the vasa recta? Concentration gradients.

Picture the vasa recta similarly to the loop of henle for illustration purposes. Along the "descending" portion of vasa recta, solute concentration in the lumen of the vasa recta is relatively lowerthan the solute concentration outside in the medullary interstitium. Therefore, solutes will travel down their concentration gradient INTO the vasa recta lumen.

In contrast, along the "ascending" portion of vasa recta, solute concentration in the lumen of the vasa recta is relatively higher than the solute concentration outside in the medullary interstitium. Therefore, solutes will travel down their concentration gradient OUT of the vasa recta.

Also, keep in mind that urea makes a significant contribution to medullary hyperosmolarity because urea is passively reabsorbed by the medullary interstitium from the inner medullary collecting duct when ADH levels are high. ADH activates one of the urea transporters in the inner medullary collecting ducts thereby allowing facilitated diffusion of urea into the interstitium.

Hope this helps.

 

[fizzgig]

good point - this is how i understand it too. the tubular system is there to do the concentrating. the vasa recta is there to bring nutrients and to not screw up that gradient.

 

[goldngrl1611]

that explanation rocked! thank you!