What is blood pressure? | Circulatory system physiology | NCLEX-RN | Khan Academy

What is blood pressure? | Circulatory system physiology | NCLEX-RN | Khan Academy


So recently I went into
my doctor’s office, and I was told that my
blood pressure was 115/75. So I thought we would talk about
exactly what this means and try to figure out how to think
about blood pressure in general, using these numbers
and this experience as kind of a launching point. So the way I think
about blood pressure is I always imagine
kind of a tube and I imagine blood
going through that tube. And this tube is
like a blood vessel. So there’s blood
trying to get its way from one side to the other. And on its way, the neat
thing that it’s doing is as it flows,
it’s pushing out. So it’s forcing
against these walls, and specifically what I mean
by that is there are cells and there’s plasma, and all
that stuff is pushing out against the walls of
the blood vessels. So you’ve got a
force, and that force is being exerted on the surface
area of the blood vessel. So it’s force over
a surface area. And any time you see
force over an area, you know that equals a pressure. And in this case,
it’s a blood pressure because it’s the blood
that’s doing that work. So this is how I think of
blood pressure, specifically as those little blue arrows. And the two questions that kind
of pop into my mind anytime I’m thinking about
blood pressure are where is the blood
pressure being taken, and when are you taking it? So let’s start with the
first question, where? And by that I mean where
in the circulatory system. So you’ve got the heart– and
this is my Valentine’s Day heart– and you’ve got the
aorta coming off of the heart. And it’s got lots
of branches, but I’m going to just draw
one branch, which is the artery leading
off to my arms. This is called the brachial
artery going off to my arm. And usually, I’d
say 90% of the time, maybe even more, the
blood pressure that you’re getting recorded, or the number
that’s being told to you, is being checked at this point. I marked it with a
little x because that’s kind of the upper arm. So that’s usually where they’re
checking the blood pressure. And again, they’re
checking the force that the blood is putting
on the vessel walls. So these little blue arrows. So that answers
the where question. And certainly, you can
imagine if I checked blood pressure let’s say at some
other spot, let’s say over here or over here, you might get
a different blood pressure reading than if I checked
it at the yellow x. So really were just talking
about that one spot. Now the other question is
when are you checking it? So for this, let me show you
a little table or a figure, rather. So imagine that over
time, time is this way, you have different recordings
for blood pressure. So this will be blood pressure. And blood pressure
is usually measured in millimeters of mercury. So I’ll write millimeters
of mercury here. And let’s go from 200
all the way down to 0. And I’ll use my numbers
that I got from the doctor the other day to illustrate
what I’m trying to say. So right at that yellow
x, at the bottom, let’s say of my
reading I’ve got 75. So let’s start at 75,
which is about here. As the heart is pumping, the
pressure starts building up. And it gets up to about 115. So really what is
happening is blood is making its way from the heart
as it squeezes towards that x. And as it gets there, the force
that it’s putting on the walls is going up, up, up. And as it goes up, it goes
from 75 all the way to 115. So that all happens
during the squeezing part of the heart cycle. We call that systole. So this is all happening
during systole, which is when the heart
is squeezing down, and we know the heart is a pump. Now from that point forward,
the heart begins to relax. So at this point, at 115, the
heart is now relaxing slowly. And as it relaxes, the
pressure begins to fall. And it continues to fall
all the way out here. And the pressure gets
down to about 75 again. And this is diastole. So this is when the
heart is actually taking a break from squeezing
and is now refilling, and we call that diastole. So I’ll call that refilling. And so during systole, you
spend about 1/3 of the time. And in diastole, you spend
about 2/3 of the time. And I’m talking about
one heartbeat here. So this would be one heartbeat. So it’s not 50-50. And the easy way
to remember this is in a given day,
24 hour day, I spend about 8 hours working
and about 16 hours relaxing, or not working. And so I think of the
heart doing its squeezing, its work, for eight out
of 24 would be about 1/3. Kind of the same as me. About 1/3 of the time,
the heart is squeezing. So that’s how the heart
is spending its time, in systole and diastole. And the pressure is
ranging between 115 and 75. So really it’s never
just 115 or just 75. It’s somewhere between the two. And for example, if I
checked right here in time, it might be let’s say 85,
or here it might be 102, or here it might be 87. So it’s somewhere
between 115 and 75. So this number I wrote up
here becomes the range. This is the range
of blood pressure. So when I talk about
when, it really depends on when in
the cycle you’re at and you’re somewhere
in that range. And when I talk
about the where, we know that we’re talking
about a reading that’s taken in the upper arm. So those are the
two thoughts that should cross your mind every
time you hear a blood pressure recording. And of course, the
top number, 115 again, is the systolic blood pressure. And 75 is the diastolic
blood pressure.

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