Blood pressure monitors
Last updated: May 4, 2009.
Have you ever stood on a highway bridge watching the trucks all
rushing beneath you, some carrying goods to towns and cities, others running
back again empty? Our bodies are a bit like that too. If you could
stand "above" your own body and peer inside, you'd see millions
of blood vessels packed with cells carrying food and oxygen to your
internal organs, all of them dashing around like trucks on the road.
To live active lives and stay healthy, we need blood
to be pumping round our bodies under pressure, day and night.
Sometimes, though, that doesn't happen. If you have a poor diet or
don't exercise, or if you live a particularly stressful life, your
blood pressure may be outside the normal range—and that can
make you susceptible to serious medical conditions such as heart
attacks and strokes. One way to check your health is to
measure your blood pressure regularly. Once, that used to mean a trip
to the physician, but now simple, affordable electronic monitors are
available that can measure your blood pressure in moments. Let's take
a closer look at how they work.
Photo: Measuring your blood pressure with an electronic blood pressure monitor (also called a digital sphygmomanometer). This one is made by IBP Healthcare, but many other makes are available.
Why blood is under pressure
Imagine if you were like the Tin Man
in The Wizard of Oz and you didn't
have a heart. You might think your blood would just sit in your legs
all day, never going anywhere, with your head and upper body entirely
starved of oxygen. In fact, things would be even worse than that.
Since your blood wouldn't circulate without a heart, there'd be no
way to get food and oxygen to all the parts of your body. There'd be
nothing to supply your brain or your muscles so you wouldn't be able
to think or move!
Having a heart ensures blood can reach every part
of your body and provide the energy and
oyxgen it needs. To supply blood effectively, the heart has to pump quite fast—especially when
you're exercising hard. The pumping of your heart makes blood flow
under surprisingly high pressure, pushing against the walls of blood
vessels much like the air inside a bicycle
tire. You'll discover just
how much pressure if you're ever unlucky enough to cut through a
major blood vessel: the blood will literally spurt from your body!
Photo: Blood is amazing: it's the vital fluid that keeps you alive. Photo of blood samples in test tubes by courtesy of
Warren Grant Magnuson Clinical Center (CC) and
National Institutes of Health (NIH) Image Bank.
Blood pressure measurements
Your blood pressure can vary quite a lot during the day and night as
your level of activity changes, so a single, instantaneous measurement of blood
pressure isn't that useful. To get a sense of what your blood
pressure is really doing, you need to measure it repeatedly over
days, weeks, months, or even years.
When physicians measure blood pressure, they quote two numbers,
known as systolic and diastolic pressure. Systolic blood
pressure is
effectively your maximum blood pressure (while your heart is pumping
blood); diastolic blood pressure is your
minimum blood
pressure (when your heart is resting). Measuring both these things
indicates the range of your blood pressure (a bit like a maximum and
minimum thermometer that gives a
better idea of the weather by
showing a day's highest and lowest temperature readings). Blood
pressure measurements are quoted in relatively old-fashioned units
called millimeters of mercury (written
"mmHg"; see our
article on barometers, used for
measuring air pressure, if you're not
sure what this means), with the systolic measurement followed by the
diastolic measurement. So if your blood pressure is "120 over 80"
(often written 120/80) it means your systolic blood pressure is
120mmHg and your diastolic pressure is 80mmHg.
How to measure blood pressure
In science, pressure is defined as the force something exerts on a
certain area of a surface—and it's something we use in everyday life, all the
time. When you press on a thumbtack, for example, the large metal
head concentrates your pushing force onto the tiny pin behind it,
increasing the pressure so the tack enters the wall more easily. Cars
and bicycles use pressure too: they have tires filled with
pressurized air to make them ride more smoothly over bumps in the
road. When you measure the air pressure in a bicycle or car tire, you
put a little meter on the air valve and let
a tiny amount of air
escape. As the air rushes out, it exerts more or less force on the
meter and that tells you what pressure the air inside the tire is under.
Photo: Thumbtacks (sometimes called drawing pins) penetrate a wall with help from the science of pressure.
In theory, you could use the same trick to measure blood pressure:
you could poke a hole in a major blood vessel and see how much pressure the
blood exerted as it gushed out all over the floor. In practice, that would be
highly dangerous and very messy—so we have
to measure blood pressure a different way, without actually letting
any blood escape from the body. People generally prefer it that way!
Aneroid measurements
Photo: Blood-pressure measurement can be a little uncomfortable—the
cuff has to be tight enough to restrict the flow of blood in your arm—but it's not what
you'd call painful. Photo by Jason R. Zalasky courtesy of US Navy.
Until recently, blood pressure was almost always measured with an aneroid
sphygomomanometer. (A sphygmomanometer
is the name given
to any blood measuring instrument, while aneroid,
in this
context, simply means using a dial.) It has an inflatable cuff that
fits around your arm, a rubber bulb for
pumping in air, a little dial
on top for measuring pressure, and (often) a built-in stethoscope.
Once the cuff is fastened securely in place, the physician pumps the
rubber bulb to inflate the cuff rapidly and cut off the blood flow in
your arm. Then an air valve on the cuff is released so the blood
rushes back along your arm, while the physician uses the stethoscope
to listen to your pulse. When blood is first heard pumping, the
physician notes the pressure reading on the dial: that's your
systolic pressure. As the blood pumps back and the cuff deflates,
there comes a point where the blood pumping can no longer be heard
through the stethoscope: reading the dial again now gives the
diastolic pressure.
Photo: Left: It's a bit more tricky to have
your blood pressure measured the old-fashioned way.
Photo by Michael R. McCormick courtesy of US Navy. Right: Looking more closely at the apparatus. A traditional aneroid sphygmomanometer has (from left to right) a rubber air pump, inflatable arm cuff, pressure-measuring dial, and stethoscope. Picture courtesy of Wikimedia Commons.
Digital measurements
Aneroid instruments are generally very accurate but quite difficult
to use by yourself. That's why many people now monitor their pressure using
automatic, digital sphygmomanometers.
They're amazingly simple
to use and, at first sight, seem to consist of little more than a
fabric cuff with an electronic, liquid-crystal
display (LCD) panel on top. You can see a typical example in the top photo of this article.
To measure your blood pressure with one of these devices, you simply
fasten the cuff around your arm and press a button on the instrument panel. You
hear a kind of whirring noise (the sound of an electric motor) and a
tiny air pump inside the gadget makes
the cuff inflate. Once the cuff is
blown up, the machine automatically releases air through an
exhaust valve, measures your sistolic and diastolic pressure, and
displays the two values on the LCD screen. Some monitors have
electronic memories and keep a record of
your blood pressure
measurements, so you can see how your pressure is changing over a
day, week, or longer period of time.
Further reading
