counter current multiplier mechanism

This mechanism involves the movement of NaCl and H2O across the tubules into the interstitiums for reabsorption into the blood stream. it basically concentrates urine too!

lets start from stage 1 where we can assume this is the first fluid ever moving through this tubule

as we can see everything is iso-osmotic.

but we know that in the thick ascending loop of henley there is a transporter known as NKCC2 which ALWAYS transports 200 NaCl into the interstitium from the tubules and so this causes iso-osmolality to be gone. as we see below.

Now, at this stage we can see there is a gradient forming and the interstitium is now hyperosmotic and so when we go bafk to the descending loop of henley, water wants to exit the tubule to reestablish the gradient. and so passive absorption of H2o out of the descending limb of henle takes place.

this process continues happening as time goes on but there will always be a difference of 200 in the interstitium and the ascending limbs.

the key take home is that water is moving out of the thin descending loop of henle by passive diffusion due to the fact that the interstitial fluid is hyperosmotic, it is hyperosmotic becaus ethe ascending limbs of henle are always maintaining pushing NaCl absorption creating the hyperosmolality which causes water absorption which then causes the tubular fluid to become more concentrated as the dilution of NaCl goes down, then its pushed out at the ascending limbs again.

the tubule is very hyperosmotic with all of the NaCl concentrations but by the time you reach the DCT ut is hypoosmotic again. the tubular fluid is always changing osmolalities but the key is that the medullary interstitium (bit in middle) is becoming very hyperosmotic.

also key to note that this mechanism has not concentrated urine itself but rather has created the conditions necessary for urine concentration to occur