So we’re going to talk about the causes,
the underlying abnormal physiology that leads
to hypertension. The so-called etiology.
In order to do this, we need to get some understanding
as to how the body controls blood pressure.
Obviously, blood pressure control is critical
because you want a nice, smooth blood pressure
and a smooth distribution of the blood to
all of the cells of the body. So this is a
very tightly-controlled and regulated system.
There are two mechanisms that are involved
in the control of blood pressure. There is
a neural – that is a central nervous system
set of mechanisms. And there is a hormonal
system. We’re going to talk first about
the neural mechanisms.
The neural mechanisms are centred in the brain.
Not surprising! The brain controls all kinds
of our activities. As you know, it has ways
of controlling our digestion, our blood pressure,
our sleeping and waking and so forth.
So what we see in the brain that is part
of the control system for blood pressure?
First of all, there is the autonomic nervous
system with the sympathetic nervous system
playing a major role in blood-pressure control.
The sympathetic nervous system is the part
of the nervous system that controls the release
of adrenaline or noradrenaline from nerve
endings throughout the body. And remember
when you release this hormone, it causes vasoconstriction
– that is contraction in the smooth muscle
in the arterioles and that increases peripheral
So a major determinant of peripheral resistance
and therefore blood pressure is the sympathetic
Another factor that is widely spread throughout
the arterial system are so-called baroreceptors.
These are pressure-sensitive cells that send
signals to the brain to say what the blood
pressure is. Is the blood pressure too low?
Is the blood pressure too high? Or is the
blood pressure just right? I think of them
sort of like thermostats in our house when
we set the temperature that we want the room
to be. Well the baroreceptors are part of
that thermostat system but they’re not
measuring heat. They’re measuring pressure.
And there is a hormone that is released from
the pituitary gland in the brain that decreases
the release of fluid by the kidney: the so-called
antidiuretic hormone, often abbreviated ADH.
This is a hormone that developed evolutionarily
over many, many millions of years. And it
protects animals who live on land from dehydration
or if there’s a hemorrhage. So if you lose
volume in your circulatory system either because
you’re dehydrated or because you’re bleeding,
the body has all kinds of mechanisms to preserve
fluid in the vascular system. And one of them
is to say, “Kidneys! Stop making urine.
We want to hold onto the fluid.” And that’s
organized by the pituitary.
Now you can imagine the thermostat in the
brain that’s receiving signals from the
baroreceptors is also going to be helping
to control the release of antidiuretic hormone.
That’s just one factor by the way in the
neural-mechanism system. And we’re going
to look at some others right now.
This diagram seems to be a little bit complicated
but let’s work our way through it here and
you can see.
This is for control of arterial blood pressure.
We already talked about cardiac output and
heart rate. If you push up the cardiac output
against no change in resistance, you’re
going to increase the blood pressure. But
the major control of blood pressure is not
so much in terms of changing cardiac output.
It’s in changing, as I’ve said before,
peripheral vascular resistance. And you see,
over on the right of the diagram, sympathetic
nervous activity is often the major factor
controlling the arterial wall constriction
or dilation that increases or decreases peripheral
The other component of the autonomic nervous
system – the vagus or parasympathetic part
– also has some vasodilating effects but
the major control of the arterioles is with
the sympathetic rather than the parasympathetic
part of the autonomic nervous system.
And then of course blood volume and salt and
water retention are all involved in controlling
the amount of volume in the cardiovascular
system and therefore contribute to the pressure
in the system.
Think of it this way: suppose we have a certain
volume of blood that’s circulating and we
suddenly double it. What’s going to happen
in the system if the system doesn’t enlarge
in any way? Blood pressure is going to go
up because, suddenly, there’s twice as much
volume travelling in a system that hasn’t
changed in size.
The kidney plays a major, major role in blood
pressure control and you see that down at
the bottom there of the diagram. That’s
a little cartoon of the kidney.
The kidney has a number of hormones and we’re
going to talk about this more in detail on
subsequent slides. But a number of hormones
can be released by the kidney that have effects
on blood pressure. The most important one
is the renin that we talked about when I mentioned
the Goldblatt kidney in the last lecture.
When there’s an abnormal flow into the kidney,
the kidney responds to this by releasing a
hormone called renin.
Renin gets into the circulation, goes through
a series of chemical reactions. That produces
a substance called angiotensin II. And angiotensin
II causes marked vasoconstriction of the arterioles.
In other words, increased peripheral vascular
resistance. Renin also goes to the adrenal
glands and causes the adrenal glands to release
a hormone called aldosterone. And aldosterone
feeds back to the kidney and says, “Hold
on to sodium! Hold on to sodium!”
At the same time, antidiuretic hormones
being released by the pituitary and that says,
“Hold on to water!”
So the kidney’s getting a signal from a
hormone from the adrenal glands and a signal
from a hormone from the pituitary gland that
say, “Hold on to salt and water!” And
you can see that in the diagram: a little
red arrow there to the left says: salt and
water is retained. That increases blood volume.
That increases venous return. And that may
help to increase blood pressure.
And all of this system was set up to help
animals survive in periods of dehydration
or when there’s blood loss. Unfortunately,
sometimes in this system the thermostat is
not set right and, in those patients, the
system is working even when the blood pressure
is normal and, consequently, peripheral resistance
goes up and, consequently, blood pressure
goes up and those patients are hypertensive.
We mentioned again that this is just further demonstration of
the central nervous system: neural mechanisms
for blood-pressure control. Just to review
it again: the autonomic nervous system, sympathetic
and parasympathetic. Parasympathetic can dilate
the blood vessels a little and slow the heart
rate and therefore decrease cardiac output
and decrease blood pressure. But the major
control factor is in the sympathetic system
which causes vasoconstriction in the arterioles
and increased peripheral vascular resistance.
And you see all of that listed on the slide.
And you can see that the control of blood
pressure relates partly to control of cardiac
output. But it’s mostly related to peripheral
vascular resistance which is mostly under
the control of the sympathetic nervous system.
The baroreceptors are scattered throughout.
You can see in this little diagram they’re
particularly common in the aorta and in the
carotid arteries. Interestingly enough, there
are other receptors – chemoreceptors, chemical
receptors – that are located with the baroreceptors
and they monitor oxygen in the blood, carbon
dioxide in the blood and hydrogen ions, or
acidity, in the blood. And so the body is
also regulating all of those and often part
of the regulation there relates to increasing
the respiratory rate – increasing how much,
how fast you’re breathing and also regulating
how much hydrogen ion – how much acid – the
kidneys are putting out.
So the body has very intricate and very, very
finely-tuned mechanisms for regulating the
internal environment. As we talked about in
the very first lecture, the homeostasis: maintaining
a smooth internal environment, normal blood
pressure, normal oxygen, normal carbon dioxide,
normal acidity. And of course we’re talking
here mostly about blood pressure regulation
and how that goes wrong in some people either
genetically – they have an abnormal thermostat
for regulating the blood pressure in the central
nervous system and they regulate it too high
– or many of the secondary factors that
can lead to high blood pressure. And we’re
going to talk about those as we go along.