We're going to compare Aldosterone and ADH or Antidiuretic Hormone side by side.
To do this I think it would be helpful if we just
do a little recap on how these two work because it's going to help
inform exactly what they do.
So if we have a little nephron here, a little tubule, and these are the cells lining the tubule
and it's going to eventually send the urine on its way out. Next to it I have a little
blood vessel, and just to save myself from drawing it twice I'm going to cut and paste
this over to this side right here.
In this tubule we know that on one side, on the aldosterone side, we have water permeability.
This membrane that seperates these two, these layer of cells are water permeable.
Over here on the ADH side, we know that they are NOT water permeable.
The reason I'm saying it's different is because we know that, although they look the same
these are different parts of the nephron.
The way that aldosterone works, the main thing it does,
it's going to pull in sodium and spit out into the urine potassium.
That's the main activity of aldosterone in terms of capturing sodium
and what happens is that sodium, we know, is not permeable to membranes.
Not able to cross membranes very easily.
That's actually really important because if it can't cross membranes,
then that means that it's going to contribute to tonicity because we know that
the ions that cannot cross membranes are the ones that
are the biggest contributors to tonicity.
In fact, this is actually very important because potassium
by comparison can "slightly" cross membranes.
So if you have one ion that cannot cross membranes at all
and you give away the ion that can slightly cross membranes,
then your tonicity goes up because overall you're getting
more ions that can stay in the blood vessel
and because they can stay in the blood vessel, it can contribute to tonicity.
Water is going to be driven into the blood vessels.
So that's really how aldosterone is dragging water into the blood vessel
through increased tonicity.
By comparison, the ADH is just using water channels
if water is unable to get across otherwise, if you through in some water channels
then you have no problem gathering water.
So these are the key differences: one of them uses an osmole
to drag water across and that's why we always say "Water follows sodium."
and the other is just using channels.
Let me make a little bit of space here. . . let's see if we can create some space down here
I'm going to create two categories: one category is Volume
and the other is Osmolarity.
We know osmolarity actually is simply a fraction. It's just osmoles divided by volume.
We have the same thing in the other column and I'm going to do that for this side as well:
osmolarity is osmoles divided by volume.
We're going to see how these two work and whether one or both will be effected by the hormones.
We know that the way that aldosterone works is by raising the osmoles.
It's going to change this, going to increase the osmoles
and as a result it will increase the volume.
So actually, both osmoles and volume are effected.
Let me start out by just circling this box because we know that the volume is effected.
we usually don't think of osmolarity being effected by aldosterone because both
the numerator and denominator are going to go up if there's a lot of aldosterone
or down if there's not any aldosterone around.
So osmolarity doesn't really get effected by aldosterone.
Now, ADH is a little different.
In ADH we have volume going up, that's really the primary thing that's happening
So we'd say we have a volume change here
but in terms of osmoles, you haven't really
changed the osmoles with ADH, not directly.
If you haven't changed the osmoles and you have changed the volume
then osmolarity is changed.
If you just change the denominator, but not the numerator then the number will change.
So that's why ADH effects osmolarity although it doesn't effect osmoles.
Kind of a tricky thing, but I think you can see it now
that the numerator doesn't change, but the overall fraction does.
So you have on this side increased volume and
you have increased volume on this side
and you have decreased osmolarity.
So these are the major changes from these hormones.
Let me make a little bit more space and we'll continue this line of reasoning.
So if these are the changes, now imagine the scenario
where you want to increase volume, but maintain the osmolarity.
So if you want to increase volume, but maintain osmolarity
which hormone would you use?
because we can't see the hormone [drawings above], let's just use aldosterone and ADH
and I'll just put "up" arrows and "down" arrows
These are the two hormones, I want to increase volume,
I would definitely use aldosterone because it
doesn't effect osmolartiy, but I would NOT use ADH
I'll put a little circle with a line through it.
I would NOT use ADH because again, I want to maintain osmolarity
I would not want to use ADH in that scenario.
Now let's say you wanted to increase volume, REGARDLESS of osmolarity
meaning you don't really care if osmolarity changes
and this could be, let's say you have a big car accident
and you're bleeding out and the only thing
you really care about right away is increasing
your blood volume, that's the only thing that
matters, so you really want to increase volume and you want to do it fast.
Well, in this scenario, you definitely want to use
everything that's available to you, aldosterone and ADH.
The fact the osmolarity will go down with ADH really doesn't matter
because we said regardless in this scenario
so because of that, I'm going to imploy ADH this time.
You can kind of get a sense for how this is going to work, right?
Now let's say you wanted to decrease osmlarity
regardless of volume, so I don't really care if the
volume changes a little bit here or there,
so in this case, regardless of volume, what would I do?
Well, if I didn't really care about the volume and I just wanted to decrease osmolarity
that seems like a no brainer, right?
That's exactly what ADH will do, it will decrease osmolarity
and really in this case, I don't need aldosterone.
I said in my phrase here that I don't care about volume changed so, that's fine
I'll just use ADH and I'll tolerate the increase in volume.
In the fourth scenario, let's say you want to decrease osmolarity and maintain
volume, you don't want it to go up or down.
This is kind of a tricky one right? because to decrease osmolarity, only one
hormone will do that so you've got to start with
some ADH, but if you want to maintain volume
you know that ADH will cause your volume to go up
a little bit and if you don't want it go up, you wanted to maintain volume, you
may actually have to decrease aldosterone just
a little smidge so that would maintain your volume.
Now you can see how the two hormones basically
have to work together to get the different outcomes.
Depending on what your volume status is and what
your osmolarity status is.
I can flip around all the arrows or I could say
well what about decreased volume and maintained osmolarity
or decreased volume regardless of osmolarity?
and you would basically just do the opposite of all these things.
Any tweak in volume and osmolarity can help you
predict what the aldosterone and ADH will be doing.