So when we think about this loop of Henle,
what we want to start thinking about is what areas are involved in the various transport.
We talked a lot about the proximal tubule.
In fact, the proximal tubule transports somewhere around 120 liters per day of solutes and solvent.
So it’s done a lot of the work.
But if we break it down in an ion-by-ion basis,
how much movement actually occurs out of the proximal tubule?
How much occurs out of the loop of Henle?
How much occurs along the thick ascending limb,
the distal convoluted tubule,
and the collecting tubule?
So let’s take it ion by ion and talk through each of those mechanisms.
Let’s start with glucose.
Glucose is primarily absorbed in the proximal convoluted tubule.
In fact, 98% of all the glucose is reabsorbed by that mechanism.
The 2% that’s left over is transported out of the proximal straight tubule.
So you can see here, there’s going to be none or very little glucose in the urine.
This is based upon that principle that we’ve discussed earlier about the transport maximum.
But as long as you have a normal blood glucose level,
all of your glucose will be reabsorbed.
It makes a lot of sense.
It took a lot to eat that food and get that glucose,
you might as well keep it in the body and not get rid of it.
Amino acids and peptides are dealt with in a primary and in a similar manner as glucose.
About 99% of it is reabsorbed in the proximal convoluted tubule,
and the other 1% is reabsorbed in the proximal straight.
That, again, leaves none to be excreted in a normal healthy condition.
Phosphate, another one of our anions to deal with.
About 80% of it is reabsorbed in the proximal tubule.
This is a highly regulated process, and that regulation step involves parathyroid hormone.
So if you have parathyroid hormone present, you’ll block that reabsorption.
About 10% of phosphate is reabsorbed in the distal convoluted tubule,
and then about 10% is excreted.
So you notice with this particular anion,
that you will always lose some phosphate in your urine.
Urea is an interesting molecule, and about half of it is reabsorbed in the proximal tubule.
About 30% of it is reabsorbed in the thick ascending limb,
and then about 50% is reabsorbed in the collecting tubule.
I know we have some math wizards out there.
50 + 50 + 30 -- that’s over 100%, isn’t it?
So why do we have over 100%?
We must have it being secreted into the renal tubule somewhere.
Where does that occur?
Well, in the loop of Henle.
About 50% occurs in the loop of Henle in terms of secretion.
So that’s how come you can have greater than 100% because this is going through the urea cycle
and allowing for some of that urea to be both reabsorbed and secreted.
And you end up with still about 40% in the urine.
Bicarbonates as well is really important anions.
We use it for acid-base balance.
But how do we retain this important anion?
Most of it is reabsorbed in the proximal tubule – about 80%.
So most all of it is reabsorbed at that point.
About 10% is reabsorbed in the thick ascending limb.
About 6% in the distal convoluted tubule.
About 4% occurs in the collecting tubule.
And that leaves none that is going to be in the urine.
So even though it occurs in 4 different spots, the primary portion is in the proximal tubule,
and then more minor aspects are removed at different other points along the nephron.
Calcium and magnesium.
These 2 divalent cations are reabsorbed in the very similar spots.
So let’s start with calcium.
About 65% of it is reabsorbed in the proximal tubule.
About 25% in the thick ascending limb.
about 8% in the distal convoluted tubule,
and that is highly regulated by parathyroid hormone.
So if you have parathyroid hormone present, you will reabsorb that calcium.
If parathyroid hormone is absent,
you will have that calcium move past the tubule and end up being secreted.
About 1% is reabsorbed in the collecting duct,
and that leaves only 1% in the urine.
Magnesium is reabsorbed a little bit less – about 15% in the proximal tubule.
But its real spot of regulation and of reabsorption is in the thick ascending limb,
and that can be seen here as 70% is reabsorbed in that locale.
But that still leaves about 15% to deal with.
Where do those occur?
10% in the distal convoluted tubule,
and that is primarily regulated by endothelial growth factor,
and then about 5% is eliminated in the urine.
So you see, magnesium is lost to a greater degree than calcium.
Potassium is an important cation.
Potassium is a little bit dependent upon how much you intake via diet.
So sometimes, you will be able to secrete it, sometimes you reabsorbed it,
so let’s go through this complicated process.
Always, you’ll have about 80% reabsorption of potassium in the proximal tubule.
You’ll get a further 10% in the thick ascending limb.
So those are constant.
What does change though is how much is secreted in the convoluted tubule
and how much is reabsorbed in the collecting duct.
So let’s go through a couple of examples of that.
If you have a normal diet, there will be a little bit of secretion –
about 10-100% in the convoluted tubule.
That is primarily controlled by aldosterone.
Then you have 5-50% that is reabsorbed in the collecting duct.
So you’ll notice there’s high ranges there.
Why do those high ranges occur?
Based upon how much dietary potassium you intake.
If you don’t take in enough potassium,
or you have a low dietary intake of potassium,
you’ll want to reabsorb them more.
And so, how do you do this?
Well, you reabsorb a bit in the distal convoluted tubule – about 2%.
And then about 6% more in the collecting tubule.
If you take 80 that is collected through the proximal tubule,
the 10% through the thick ascending limb,
the 2% from the distal convoluted tubule,
and the 6% through collecting duct,
it only leaves you 2% to excrete.
That’s much different than when you have a normal potassium diet
in which as much as 10-100% of the dietary intake could be excreted in the urine.
Sodium, one of our most important ions.
You always need to think about sodium because, where sodium goes, water want to travel along with it.
So let’s go through this important molecule now.
About 67% of sodium is reabsorbed in the proximal tubule.
We learn that, that is primarily via co-transport mechanism, so things like glucose and other molecules.
About 25% of it is reabsorbed in the thick ascending limb,
and that is done through these co-transporters that involve both chloride and potassium.
In the distal convoluted tubule, you have another 5% being reabsorbed,
and that is done through an exchanger between sodium and chloride.
And finally, about 3% additional are reabsorbed in the collecting duct.
This leaves only about 1% being lost in the urine.
So sodium is reabsorbed, 99% of it, in the nephron.
The important regulatory steps involve angiotensin II.
Angiotensin II, it will affect 2 different regions –
the proximal tubule and the thick ascending limb.
Aldosterone affects the distal convoluted tubule regulation,
and that’s that 5% at that locale.
ANP will involve losing more sodium.
So instead of reabsorbing more, you’ll reabsorb less,
and ANP is atrial natriuretic peptide.
So you can say atrial natriuretic peptide and aldosterone work at the same spot,
and these will be counter-regulatory.
Aldosterone will want to reabsorb
and atrial natriuretic peptide will want to release the sodium.
The reabsorption of water is one of our most important aspects in the kidney.
Water regulation involves things like controlling blood volume and controlling body fluid balance.
So where does this occur?
67% is brought off in the proximal convoluted tubule.
If you notice, this matches sodium quite well.
Then 15% more is reabsorbed in the thin descending limb of the loop of Henle,
so that yields quite a bit of the reabsorption already occurring.
Now, the last portion is a regulatory step,
and that is about 18% of the water is reabsorbed in the collecting tubule.
What are the 2 regulatory molecules that we need to be involved with here?
That’s arginine vasopressin, or antidiuretic hormone,
and that will stimulate this reabsorption of 18%.
The counter-regulatory hormone in this case is ANP, or atrial natriuretic peptide,
and that will reduce the amount of water that is reabsorbed.
So ADH, arginine vasopressin, reabsorb the water.
ANP allows for more to be released.
Usually, there’s only about 1% of the water in a normal hydrated state that is lost in the urine.
So 99% of it is reabsorbed.