This is reabsorption 1 – the proximal tubules.
Here, we’re going to review just briefly epithelial transport.
Epithelial transport, remember, is the process by which you move a solute
across the apical membrane
and then the basolateral membrane,
and then into the blood.
Here, we know there are 2 different pathways at which you can get through
the apical and basolateral membrane.
That involves the transcellular pathway
in where you first move that solute across with the transporter
into the cytosol of the epithelial cells,
across its basolateral membrane,
and then into the blood.
This can be contrast with the other type of transport,
which is paracellular.
Paracellular crosses the apical membrane,
and then goes into a cuniculi,
and then into the interstitial space in the blood.
This process of transport is important to think about.
We refer to earlier about tight and leaky epithelial junctions.
In the proximal tubule, this is considered a leaky epithelial junction.
Therefore, the paracellular path will be more important than it will be other areas of the segments of the nephron.
The whole process that generates this proximal tubule transport is almost always set-up by sodium.
How sodium set-up happens is you have a basolateral transporter.
This is the sodium-potassium ATPase, or sodium-potassium pump.
This is the main driver of the response.
It kicks out sodium into the interstitial space.
This creates a concentration gradient
so that sodium would want to travel across the apical membrane
in order to get to the interstitial space.
This concentration gradient will drive many of the transporters we’ll discuss in the proximal tubule.
The other item that the sodium-potassium ATPase does
is sets up an electro gradient between the apical side and basolateral
simply by having a very small millivolt difference across these membranes –
will allow some ions to travel towards the opposite pole.
So if you have a negatively charged molecule, it would want to travel towards the positive side.
If you have a positive charged molecule, it would want to travel towards the negative side.
Since this is a leaky-type epithelial junction,
water and other solutes will be able to move through that junction.
This process is sometimes called a solvent drag,
and that will move solutes along with the solvent into the interstitial space.
So you have two processes that are very closely related.
You have the fluid movement going through those epithelial junctions,
and sometimes they can drag along other particles with them.
But we have both of these different transport system set-up,
and sometimes the water will also travel through the cell or through a transcellular component
that usually involves an aquaporin.
When we look at proximal tubule transport,
it is probably best to first think about the bulk transport that we can do across the proximal tubule.
So if we put proximal tubule length along the X-axis,
and then the Y is going to be the amount or the tubule concentration as compared to plasma.
Why do we compare it to plasma?
So we understand which substances are reabsorbed and which substances are not.
If we didn’t compare them to something, we couldn’t make that judgement.
So let’s take our first substance, glucose.
Glucose is reabsorbed very quickly along the proximal tubule.
In fact, in the first 25 percent, the majority of the glucose is already reabsorbed.
Now, we’re going to go through later on,
conditions such as diabetes,
where you might have glucose spill over into the urine.
This only happens when plasma-glucose levels are very, very high.
Amino acids and peptides –
these also follow this transport process in which the majority of them
are transported very quickly across the proximal tubule and earlier on.
Bicarb is transported across also fairly rapidly.
And by the end of the proximal tubule,
most of the bicarb has been reabsorbed, but not quite all as happens with amino acids and glucose.
Then you have substances such as sodium and potassium.
These looks like a flat line.
Now, it doesn’t mean that there’s no sodium reabsorption occurring at this particular point,
but it’s being reabsorbed at the same level as what’s in the interstitial space
so the osmolality doesn’t change.
Chloride is also reabsorbed, but its only reabsorbed in a small amount
and it’s not reabsorbed in comparison to plasma.
Inulin, our last substance, does not undergo any reabsorption.
Therefore, whatever enters into Bowman’s space,
or the filtered load of inulin,
will then be excreted – none of it is reabsorbed.
So that gives us our parameters about what absorption happens in the various parts of the proximal tubule.
If you take inulin as one example, that is no reabsorption happening.
Sodium and potassium are being reabsorbed at the very same rate
that’s on the opposite side of the membrane.
Things like amino acids and glucose are reabsorbed almost totally from the renal tubule space.
Now, if we look at the structure of the proximal tubule,
you notice that there’s a convoluted component,
and then there’s a straight component.
So we oftentimes break these up into 2 different portions of the nephron.
But know that the majority of the reabsorption occurs in the proximal convoluted tubule –
just a little more gets done in the proximal straight.