Hello! We’ll be covering secretion and do a combined review of reabsorption.
Let’s review quickly some of the primary renal functions.
We had filtration, reabsorption, secretion, and excretion.
So today, we’re going to be dealing with the third one - secretion.
Secretion is a very important process to be able to move solutes from the blood
into the renal tubules so you can get rid of them.
In this case, what would you like to get rid of?
Oftentimes, it’s things like endogenous or exogenous toxins.
So secretion is an important process
by which we can get rid of things we don’t want in the blood.
So how does this work?
Well, secretion is primarily going to be a process to get rid of something rather than to reabsorb it,
and this is important.
We mainly talked about reabsorption up until this point.
So reabsorption is bringing something from the renal tubule back into the blood.
Now, we are going to take things from the blood and put it in the renal tubule.
This occurs primarily in the late proximal tubule.
So the late proximal tubule is the proximal straight tubule.
So the proximal convoluted tubule is where a lot of the reabsorption took place
and the proximal straight is where secretion will take place.
To undergo secretion, we need some very specific transporters.
These are organic ion transporters, also abbreviated as OAT
and organic cation transporters.
These are very specialized transporters.
They will probably originally develop simply to get rid of endogenous toxins –
things that build up in the body that you want to get rid of.
But they also serve another purpose,
and that is to get rid of items that you would have taken in or exogenous toxins.
It might be a drug.
It might be a poison.
It might be a plant that was converted to a toxin that you need to get rid of.
How this process works is
if you remember our apical membrane is lining the tubule lumen side,
the basolateral membrane is close to the blood and the interstitial fluid.
If you have an organic ion – abbreviated here OA-
– you need to move it into the renal tubule cell,
and that is done here by a process using the ATP,
as well as these OAT transporters to move it across the basolateral membrane.
Then along the apical membrane,
you use a multidrug-resistant transporter to move it across into the tubule lumen.
You need to get the organic ion into the tubule lumen for you to be able to get rid of it.
OAT transporters are also used on the apical membrane,
along with these multidrug-resistant proteins.
Some of the endogenous ions that you can get rid of
are things like bile salts,
things like prostaglandins,
and some of the vitamins are removed in this manner.
So they’ll be partly freely filtered and excreted,
but then you will secrete them into the proximal tubule.
A number of drugs are also eliminated in this manner – things like ACTZ.
Some of the drugs like penicillin are also removed in this manner.
So you oftentimes wondered in pharmacology,
how do you get rid of a drug once you’ve taken it in?
Well, the kidney will clear it and also secrete it into the renal tubule.
Now, organic cations are handled in pretty much the same way.
The only thing is that they’re going to be a slightly different subset of various transporters.
So again, you need the sodium-potassium ATPase to set up this process,
and then you use an OCT transporter.
Finally, to get it across the apical membrane, use one of these multidrug-resistant proteins,
as well as organic cation transporters.
So this will get it from the blood across the basolateral membrane,
across the apical membrane.
So some of the endogenous cations that are removed by in this manner
are things like creatinine,
even your catecholamine, such as epinephrine and norepinephrine.
So those were the ones that were developed naturally.
But interestingly, these transporters are not that specific and will be able to transport other items
such as drugs,
like atropine or isoproterenol.
These drugs that were taken in to do a particular function can then be cleared out of the body.