So now, let’s move on to the countercurrent multiplier.
How do you utilize having both these cortical collecting ducts,
these nephron loops, and these blood vessels all right at the same spot?
How is that going to allow for this gradient to occur?
I’m going to through the theory of the countercurrent multiplier first,
and this sets up the reason why we have the gradient.
Does it ever really occur in the body?
Well, maybe not.
But this is how the process is set up to start.
Once it’s started, you will continue to have this process go over and over again until the gradient develops,
but this is just assuming that you start off with no gradient.
But you will always have a little bit of a gradient, so it’s a little bit artificial.
But unless we go through the theory of it, you will never really know how this gets established.
So let’s start off prior to the multiplication process occurring.
Let’s assume that the thin descending limb, where the fluid is first traveling into, is 300 milliosmoles.
It’s going to travel down through the tube, through the hairpin loop,
and then back up again through the thin ascending limb.
All of them are 300 milliosmoles.
We have to have a starting place.
What happens is, as you start to enter the thick ascending limb,
there’s active transport of sodium.
So you start kicking out sodium into the interstitium.
There’s also sodium leaving via the thin ascending limb.
So in this condition, we’re going to have now movement of sodium from the ascending limb into the interstitium.
We call this a single effect.
So we’re moving some of the sodium out of the ascending limb.
What this does is concentrates the sodium in the interstitial fluid.
It doesn’t change anything in the descending limb right now.
We've only moved sodium from the ascending limb into the interstitium.
That is the single effect.
Now, once we have the single effect present, fluid is going to be continually traveling down the tubule.
So this means that new fluid enters the thin descending limb.
As that travels down, you’re going start losing some water out of that thin descending limb.
Why do you lose water?
You lose water in this case because it is water permeable,
and you have a higher solute concentration in the interstitium.
So the water always wants to travel to where the sodium is.
They are coupled.
The water always will like an area that has high salt,
so they’ll travel towards it based on osmotic forces.
So this is called the fluid displacement component.
As the fluid then travels down around the hairpin loop and starts to go back up to the thin ascending limb,
you’re now going to have another single effect that happens.
This starts to set up now a gradient where you have iso-osmotic fluid right entering the thin descending limb.
It starts to get more concentrated as it goes around the hairpin loop and starts to head back up again –
you start to have now less concentrated fluid because, again, the sodium now is being transported into the interstitial fluid.
So now, we have a condition where we have an iso-osmotic fluid starting to travel down the thin descending limb.
It gets more concentrated as it starts to go around the hairpin turn,
and then it becomes less concentrated as it travels up the thin ascending limb.
If we look at the interstitial fluid, you notice that it’s iso-osmotic all the way down to somewhat hyper-osmotic.
So after this fluid displacement occur, we’re going to have another single effect.
So remember what the single effect is,
It’s moving out sodium from the thin ascending limb passively
and the thick ascending limb actively about 200 milliosmoles.
So this sets up this process now of being hypo-osmotic fluid traveling up the thin ascending limb and the thick ascending limb.
The fluid that travels down is iso-osmotic.
The gradient allows for the filling in of this process of having high osmolality down towards the hairpin loop,
and having it less osmotic as it moves towards the cortex.
This fluid displacement and single effect repeats over and over again
every time fluid travels down the loop of Henle, and then back up again.
Over time, this creates a huge osmotic gradient
– all the way from 300 in the cortex to 1,200 milliosmoles down in the medulla.
This process will always happen every time fluid travels through.
Therefore, you never really have a time where you have an iso-osmotic condition throughout the whole nephron.
We only use this to set-up this countercurrent multiplier system.
The countercurrent multiplier in itself is not inherently difficult if you break it down into its 2 steps.
You first have a fluid displacement, and then a single effect – and then repeat, and repeat, and repeat,
and you end up with the cortical medullary osmotic gradient.