What you want to start to think about is what
areas are involved in the
We talked a lot about the proximal tubule,
in fact, the proximal tubule transports
somewhere around 120
liters per day of solutes
So it's done a lot of the work.
But if we break it down on 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
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.
And this is based upon that principle that
we discussed earlier about a transport
maximum. But as long as you have a normal
blood glucose level, all of your
glucose will be reabsorbed.
Makes a lot of sense.
It took a lot to eat that food and get that
You might as well keep it in the body and
not get rid of it.
Amino acids and peptides are dealt with in a
primarily 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
About 80 % of it is reabsorbed in the
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 %
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
About 30 % of it is reabsorbed in the
thick, ascending limb,
then about 50 % is reabsorbed in the
And I know, I know we have some math wizards
out there, 50 plus
50 plus 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? Where does it occur?
Oh, 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 a greater
than 100 % because this is
going through the area 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.
Bicarbonate is one of those really important
to anions, we use it for acid base balance,
but how we retain this important in that
most of it is
reabsorbed in the proximal tubule,
about 80 %.
So most all of it is reabsorbed at that
about 10 % is reabsorbed in the thick
about 6 % from the distal convoluted
About 4 % occurs in the collecting
And that leaves none that is going to be in
So even though it occurs in four 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 two
divalent cations are
reabsorbed in 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
And that is highly regulated by parathyroid
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 % of
the proximal tubule. But it's real spot of
regulation and of
reabsorption is in the thick ascending limb.
And that can be seen here.
A 70 % is reabsorbed in that account,
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
So you see magnesium is lost to a greater
degree than calcium.
Potassium is an important can ion.
Potassium is a little bit dependent upon how
much you intake via
diet. So sometimes you will be able to
secrete it, sometimes you
reabsorb it. So let's go through this
Always you'll have about 80 %
reabsorption of potassium in the
You'll get a further 10 % in the thick
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
If you have a normal diet will be a little
bit of secretion, about
10 to 100 % in the convoluted tubing
that is primarily
controlled by Aldosterone.
Then you have 5 to 50 % that is
reabsorbed in the collecting duct.
So you notice there's high ranges there.
Why are 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
And so how do you do this?
Well, you reabsorb a bit in the distal
convoluted tubule, about 2 %
and then about 6 more percent in the
So if you take the 80 that is collected
through the proximal tubule, the 10 through
the thick ascending limb, the two from the
distal convoluted tube and the 6 %
through the 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
to 100 % of the dietary intake
could be excreted in the urine
sodium. One of our most important diets.
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
About 67 % of sodium is
reabsorbed in the proximal tubule.
We learn that that is primarily by a co
transport mechanisms with 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 tube,
you have another 5 %
being reabsorbed and that is done through an
exchanger between sodium chloride.
And finally, about 3 additional percent
are reabsorbed in the collecting duct.
This leads only about 1 % being lost
in the urine.
So sodium is reabsorbed.
99 % of it in the net from
the important regulatory
steps involve angiotensin two and
It will affect two different regions.
The proximal tubule and the thick ascending
Aldosterone affects the distal convoluted
And that's that 5 % at that local
will involve losing more sodium.
So instead of absorbing more, you'll reabsorb
And AMP is atrial nature 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 you 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.
And if you notice, this matches sodium quite
well, then 15 %
more is reabsorbed into 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 to
really what are the two regulatory
molecules that we need to be involved with
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 80 H.
Arginine vasopressin reabsorbed 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.