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How Do Blood Pressure Tests Work, And What Do Those Numbers Mean?

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Getting your blood pressure taken is a standard part of most visits to the doctor, but the details might seem mysterious—so read on.

What It Is

Blood pressure (BP) is the force exerted by circulating blood on the walls of the arteries as it's pumped from the heart. When we talk about it, we're usually referring specifically to the pressure as measured on the upper arm at the brachial artery.

Average blood pressure varies from person to person and is influenced by a number of factors, including age, gender, diet, stress, exercise and alcohol use. For an individual, blood pressure changes over the course of the day and even varies during a single heartbeat between a systolic (maximum) pressure, when the heart beats and pumps blood and the ventricles are contracting, and a diastolic (minimum) pressure, when the heart is at rest between beats and the ventricles are filled with blood.

That arm band the doctor uses to measure your BP is called a sphygmomanometer (from the Greek sphygmus ("pulse") + the scientific term manometer (pressure meter). The instrument consists of an inflatable cuff, a measuring unit and, for manual models, an inflation bulb and valve. The pressure of the cuff used to be measured on manual sphygmomanometers by observing a mercury column in the measuring unit and reading the BP as millimeters of mercury (mmHg). The risk of mercury leaks has led to the increased use of aneroid manual and even digital sphygmomanometers, but the mercury ones are still considered more accurate. Even if a mercury column isn't used, mmHg is still the unit of measurement for BP.

How It's Measured

During an exam, the cuff is placed around the upper arm at roughly the same height as the heart and inflated with the bulb until the artery is closed. Using the stethoscope, the doctor slowly releases the pressure in the cuff and listens. What they're listening for are the Korotkoff sounds, named for the Russian physician who described them in 1905. The first Korotkoff sound occurs when the pressure of the cuff is the same as the pressure produced by the heart and only some blood is able to pass through the upper arm in spurts, resulting in turbulence and an audible whooshing or pounding sound. The doctor records the pressure at which this sound is heard as the systolic blood pressure.

As the pressure in the cuff is further released, the sound changes in quality, becomes quieter (running through the second, third, and fourth Korotkoff sounds) and, when the cuff stops restricting blood flow enough to allow smooth flow with no turbulence, stops altogether. This silence is the fifth Korotkoff sound and the pressure at which it happens is recorded as the diastolic blood pressure.

The fraction that the doctor records as your blood pressure is the systolic pressure over the diastolic pressure, giving you the measure of both the pressure when your heart is exerting maximum pressure and when it's relaxed.

According to the American Heart Association, blood pressure readings break down like this:

Normal Blood Pressure: 120 systolic pressure and 80 diastolic pressure or less

Prehypertension:120-139 systolic pressure or 80-89 diastolic pressure

High Blood Pressure (Hypertension) Stage 1: 140-159 systolic pressure or 90-99 diastolic pressure

High Blood Pressure Stage 2: 160+ systolic pressure or 100+ diastolic pressure

Hypertensive Crisis (emergency care needed): 180+ systolic pressure or 110+ diastolic pressure

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Big Questions
Why Does Turkey Make You Tired?
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Why do people have such a hard time staying awake after Thanksgiving dinner? Most people blame tryptophan, but that's not really the main culprit. And what is tryptophan, anyway?

Tryptophan is an amino acid that the body uses in the processes of making vitamin B3 and serotonin, a neurotransmitter that helps regulate sleep. It can't be produced by our bodies, so we need to get it through our diet. From which foods, exactly? Turkey, of course, but also other meats, chocolate, bananas, mangoes, dairy products, eggs, chickpeas, peanuts, and a slew of other foods. Some of these foods, like cheddar cheese, have more tryptophan per gram than turkey. Tryptophan doesn't have much of an impact unless it's taken on an empty stomach and in an amount larger than what we're getting from our drumstick. So why does turkey get the rap as a one-way ticket to a nap?

The urge to snooze is more the fault of the average Thanksgiving meal and all the food and booze that go with it. Here are a few things that play into the nap factor:

Fats: That turkey skin is delicious, but fats take a lot of energy to digest, so the body redirects blood to the digestive system. Reduced blood flow in the rest of the body means reduced energy.

Alcohol: What Homer Simpson called the cause of—and solution to—all of life's problems is also a central nervous system depressant.

Overeating: Same deal as fats. It takes a lot of energy to digest a big feast (the average Thanksgiving meal contains 3000 calories and 229 grams of fat), so blood is sent to the digestive process system, leaving the brain a little tired.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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The balloons for this year's Macy’s Thanksgiving Day Parade range from the classics like Charlie Brown to more modern characters who have debuted in the past few years, including The Elf On The Shelf. New to the parade this year are Olaf from Disney's Frozen and Chase from Paw Patrol. But how does the retail giant choose which characters will appear in the lineup?

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