ADH effects on blood pressure | Renal system physiology | NCLEX-RN | Khan Academy
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ADH effects on blood pressure | Renal system physiology | NCLEX-RN | Khan Academy

August 30, 2019

We left off the story
of antidiuretic hormone when it was just
secreted into the blood vessels of the
posterior pituitary. So it was just
synthesized, just made. It’s a little hormone. And ADH was on its way to
different parts of the body. So let’s just pick up
the story right there. And figure out where
does it go next. So this little molecule is,
we said, a small peptide hormone made up of amino acids. And so I’m just going
to draw it here. And this little hormone
is going to go off to do a couple of
important things. So we know at the
end of the day, it really wants to
increase blood pressure. So one of the
places it visits is all of the vessels of the
body, all the arterial vessels of the body. And specifically, it
targets smooth muscle. So this hormone is
going to go and get this smooth muscle to constrict. And we know that when
smooth muscles constrict, the blood vessels are actually
going to tighten down, and we call that
vasoconstriction. So the blood vessels are
going to get tight and small, and that’s going to
increase resistance. And increased
resistance is going to relate to blood pressure. And we’ll talk about
how we know that. There’s that formula. I’m going to write it over
here– delta P equals flow. Q is flow times
resistance, is R. And you can actually
change that around to say arterial pressure
minus venous pressure equals– and we know
the flow is actually stroke volume times
heart rate, and it’s all multiplied by resistance. So if you look at this, and
if we assume for the moment that the venous
pressure is going to be basically
unchanged, then anything that increases the
resistance over here is going to increase
our pressure over here. So that’s why, in
this case, if ADH is able to cause constriction of
the blood vessels and increase resistance, our pressure
would go up as well. So that’s actually one of
the things that it does. And the other thing
that it does is it’s going to act on the kidney. So it’s going to have
an effect on the kidney. Here’s my kidney. And specifically
what it’s going to do is it’s going to cause
increased reabsorption of water. So increased
reabsorption of water is going to increase
our stroke volume. So now you can see the other
key effect it’s going to have. If it’s going to cause your
stroke volume to go up, then just as before, now
you have an increased stroke volume. So your arterial pressure
is going to go up, maybe doubly up. So it’s going to cause
the blood pressure to go up for a couple
different reasons. Now, let’s explore
this second point in a little bit more
detail, the whole idea of how it causes the
stroke volume to go up. So for that, what
I want to do is I’m actually going to create
a little bit of space. And I’m going to
draw out, again, as I’ve done
before, the efferent and afferent arteriole. So we know that blood is
going to enter the kidneys, and it’s going to do
this twisting on itself in the glomerulus. And so this is our
little glomerulus. And there’s the proximal
convoluted tubule. And there’s the loop of Henley. And this is the distal
convoluted tubule, and finally, a collecting duct. So we know that
this is basically what the nephron looks like. And I haven’t label
all the parts, but I’m going to label
the important part, which is this part right here. So this area here is
the collecting duct. And what I’m circling in blue
is what the ADH is actually going to work on. It’s going to work on this
area, the collecting duct. So it’s going to have its
effect here specifically. And let me try to draw this
a little bit larger so we can see exactly what goes on. So let’s imagine that you have,
let’s say, one cell there. And here’s another cell
here, something like that. And you have a blood vessel
going alongside of it. Now, we haven’t actually
talked about this before. But down in here– actually, let
me switch colors for a moment. We have urine going this way,
and blood going this way. So already, you might
be a little surprised. You’re thinking, well, why is
blood going up and urine going down? That makes no sense. Now, think about this. Before when we were talking
about blood and urine flowing in other
parts of the nephron, we were kind of separating
out the nephron, talking about this top bit. So we were talking
about this top bit here. And in here, the
concentration is around 300. And actually, the
units on that– I’ll just write the units
up here– are milliOsms. So it was around 300, but if
you go deeper, it’s about 600. And then if you go deeper
than that, it’s about 900. And if you go down
here, it’s about 1,200. So what’s happening
as you go deeper is that it’s basically
getting more and more salty. So it’s getting very salty. I’ll actually write that
sideways, very salty as you go down. And that saltiness is
really, really important, because what it
does is it allows us to concentrate our urine. And you’ll see why I say that. So keep that saltiness
in mind and the fact that there’s this big gradient. And I’m going to actually just
assume, right now, that we’re talking about something,
let’s say, at the 900 level. So we’re at this point
right here– 900 milliOsms. So we’ve got a pretty
salty area out here. Now, as I said, urine
is flowing through. And in these
collecting duct cells, we have something
called an aquaporin that basically sits like this. Let me actually show you
what it would look like. So these areas are not going
to allow water to go through. That’s actually the first
point that I want to make. Water cannot go
through these areas, except for when there’s a
little aquaporin channel. And I’m drawing the
channels for you. So you can see they’re
not on the surface, right? So there’s no way that water,
if water is sitting over here, there’s no way that it
can actually get through. It would actually
just bounce off because it’s not able
to permeate the cell. It can’t actually get in. So water just kind of
bounces back and basically goes down into the urine. Now, what ADH does– and
this is the neat thing. So ADH, what it will
do, is it will float up. So ADH is actually going
to float through the blood, because we said that ADH is
going to be all over the body. So this little molecule is
going to go through and float by this collecting duct cell. And it’s going to
have an effect on it. So it’s going to have an effect
on this collecting duct cell. What it’s going to
do exactly is it’s going to make those
little aquaporins. Let me write that out actually. This is an aquaporin. And you can see that’s
a really easy word to remember because it’s
literally aqua, meaning water, making a pore for water. So this aquaporin
vesicle is actually going to merge with the wall. It’s actually going to merge
with the wall like that. So let me actually
erase a little bit and show you what would happen. So now you have– instead of
this aquaporin sitting out here, you literally
have little channels that are now fused
in with the wall. So you can see how those
little vesicles just bumped right into the
wall and fused into it. And now water is going to
get a free ride across. It’s going to be able to just go
right through that channel just like that– boop–
and into the blood. And it’s going to
do it again here. And it’s going to go here. So all this water is just
gushing in to the blood. Look at all this water. And so this blood is going
to be loaded with water now, something that it
did not have before, because the water couldn’t
get across before. And so this blood
is going to go up, loaded with water,
because of the ADH. The ADH basically allowed
all that water to finally get across, and the blood
is now full of water. +And so now you can see how the
volume of blood is going to go up. And if the volume
of blood goes up, it’s going to create a larger
stroke volume for the heart. So that’s specifically how
the stroke volume goes up.

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  1. I've only watched 3 videos by far, but I've already learned more than during half-year physiology course on my University..

  2. This is fantastic. I always learn better if something is drawn out for me as opposed to just reading an explanation of what is going on.

  3. As someone who doesn't have an American accent, the way you said 'salty' made my day that much brighter. 

  4. Rishi is amazing instructor, he is among few instructors in Khan academy who is as good as Sal or even surpass him I think. What a beautiful lecture!

  5. Why aren't you talking about the billion trillion dollar question, how to LOWER blood pressure and not just the other way around scenarios ? Same in the last video.. just the raising volume, retaining water.
    Yes you said "just think the other way around" but I need help..

  6. I still don't understand why urine would move down the collecting duct. Shouldn't it move up to leave the kidney and enter the ureter?

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