So I talked a lot about transporters.
Now, what governs the transport?
Well, a transporter usually has an inherent maximal range,
and that means they can only transport X amount of solute per given amount of time.
So for something like glucose,
they can transport about 375 milligrams per minute,
and that is done by the SGLT1 and SGLT2.
However, if you have glucose levels that are above 375,
such as a diabetic who is in a hyperglycemic episode.
They may have a blood glucose of, let’s say, 400.
In that case, you will always have spillover of glucose into the urine
because its transport maximum is this given amount.
Now, many other substances have transport maximums.
It’s not just glucose.
Amino acids do, proteins do, phosphate does,
as well as some of the other transporters that we don’t talk about in as much detail,
such as the OATP transporters that will transport drugs across the apical and basolateral membrane.
If we want to think about excretion and reabsorption,
how would be best to process this information?
Well, we use this from a transport maximum point of view.
You can only reabsorb the amount of substance based upon its transport maximum.
So if we would deliver a freely filtered molecule
that is one that is filtered through the glomerular barrier into the tubule lumen space,
what is it dependent on?
Well, it depends on the plasma concentration of that substance.
So we have that now on our X-axis.
On our Y-axis, we’re going to have the amount of concentration
that is either reabsorbed, filtered, or excreted.
The higher the plasma concentration of a molecule that’s freely filtered,
will increase its concentration either in the urine or filtered.
For reabsorption purposes, the transport maximum governs how high reabsorption can occur.
So if we take our example, 375 for glucose –
if you filter more glucose than 375, you can’t reabsorb anymore.
So as plasma concentration increases, you see that flat line on reabsorption.
Now, just because there’s reabsorption that is pegged,
what happens to the filtered solute that’s above that particular amount?
Well, in this case, it gets excreted.
So whatever you don't reabsorb,
will continue down the renal tubule and eventually be excreted out of the body.
In this relationship, you can see there’s a little bit of an oddity in the reabsorption curve and excretion curve.
They start off curvilinear and then become linear – that is interesting.
This curvilinear component has a name associated with it,
and that name is called splay.
The reason why splay happens is because each individual nephron
has a slightly different transport maximum.
So when we say transport maximum for glucose is 375 mg/minute,
that’s an average across each individual nephron.
It’s not only one nephron.
So there are some nephrons that have a transport maximum that is less than 375 –
let’s say 350.
Therefore, if you have a blood glucose level in between 350 and 375,
it’s possible to still excrete a very small amount of glucose
because that individual nephron cannot reabsorb it because it hits its transport maximum.
So that is one of the reasons why you may get some values of glucose
that seemed to be positive in the urine dipstick test,
but the person is not diabetic or you don’t have a plasma glucose level above 375.
To review this process – because I know these 3 lines can be a little bit confusing –
remember that you have – if it’s a freely filtered molecule,
you will have a linear relationship between the plasma solute and the amount filtered.
The reabsorption component is governed by a transport maximum.
And then what is excreted is anything above the reabsorbed value.