Aldosterone and ADH | Renal system physiology | NCLEX-RN | Khan Academy

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

Not able to cross membranes very easily.

and what happens is that sodium, we know, is not permeable to membranes.

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.

because osmoles and volume are both affected and that they're proportional to one another

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.