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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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How Are Royal Babies Named?
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Jack Taylor, Getty Images

After much anticipation, England's royal family has finally received a tiny new addition. The birth of the Duke and Duchess of Cambridge's second son was confirmed by Kensington Palace on April 23, but the name of the royal newborn has yet to be announced. For the heir to the British throne and his wife, choosing a name for their third child—who is already fifth in line to the throne—likely won't be as easy as flipping through a baby name book; it's tradition for royals to select names that honor important figures from British history.

According to ABC WJLA, selecting three or four names is typical when naming a royal baby. Will and Kate followed this unwritten rule when naming their first child, George Alexander Louis, and their second, Charlotte Elizabeth Diana. Each name is an opportunity to pay homage to a different British royal who came before them. Some royal monikers have less savory connotations (Prince Harry's given name, Henry, is reminiscent of a certain wife-beheading monarch), but typically royal babies are named for people who held a significant and honorable spot in the family tree.

Because there's a limited pool of honorable monarchs from which to choose, placing bets on the royal baby name as the due date approaches has become a popular British pastime. One name that keeps cropping up this time around is James; the original King James ruled in the early 17th century, and it has been 330 years since a monarch named James wore the crown.

If the royal family does go with James for the first name of their youngest son, that still leaves at least a couple of slots to be filled. So far, the couple has stuck with three names each for their children, but there doesn't seem to be a limit; Edward VIII, who abdicated the throne to George VI in 1936, shouldered the full name of Edward Albert Christian George Andrew Patrick David.

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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Why Does the Queen Have Two Birthdays?
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CHRIS JACKSON, AFP/Getty Images

On April 21, Her Majesty Queen Elizabeth II will turn 92 years old. To mark the occasion, there are usually a series of gun salutes around London: a 41 gun salute in Hyde Park, a 21 gun salute in Windsor Great Park, and a 62 gun salute at the Tower of London. For the most part, the monarch celebrates her big day privately. But on June 9, 2018, Her Majesty will parade through London as part of an opulent birthday celebration known as Trooping the Colour.

Queen Elizabeth, like many British monarchs before her, has two birthdays: the actual anniversary of the day she was born, and a separate day that is labeled her "official" birthday (usually the second Saturday in June). Why? Because April 21 is usually too cold for a proper parade.

The tradition started in 1748, with King George II, who had the misfortune of being born in chilly November. Rather than have his subjects risk catching colds, he combined his birthday celebration with the Trooping the Colour.

The parade itself had been part of British culture for almost a century by that time. At first it was strictly a military event, at which regiments displayed their flags—or "colours"—so that soldiers could familiarize themselves. But George was known as a formidable general after having led troops at the Battle of Dettingen in 1743, so the military celebration seemed a fitting occasion onto which to graft his warm-weather birthday. Edward VII, who also had a November birthday, was the first to standardize the June Trooping the Colour and launched a tradition of a monarchical review of the troops that drew crowds of onlookers.

Even now, the date of the "official" birthday varies year to year. For the first seven years of her reign, Elizabeth II held her official birthday on a Thursday but has since switched over to Saturdays. And while the date is tied to the Trooping the Colour in the UK, Commonwealth nations around the world have their own criteria, which generally involve recognizing it as a public holiday.

Australia started recognizing an official birthday back in 1788, and all the provinces (save one) observe the Queen's Birthday on the second Monday in June, with Western Australia holding its celebrations on the last Monday of September or the first Monday of October.

In Canada, the official birthday has been set to align with the actual birth date of Queen Victoria—May 24, 1819—since 1845, and as such they celebrate so-called Victoria Day on May 24 or the Monday before.

In New Zealand, it's the first Monday in June, and in the Falkland Islands the actual day of the Queen's birth is celebrated publicly.

All in all, just another reason it's great to be Queen.

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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