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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
What Do Morticians Do With the Blood They Take Out of Dead Bodies?
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Zoe-Anne Barcellos:

The blood goes down the sink drain, into the sewer system.

I am not a mortician, but I work for a medical examiner/coroner. During an autopsy, most blood is drained from the decedent. This is not on purpose, but a result of gravity. Later a mortician may or may not embalm, depending on the wishes of the family.

Autopsies are done on a table that has a drain at one end; this drain is placed over a sink—a regular sink, with a garbage disposal in it. The blood and bodily fluids just drain down the table, into the sink, and down the drain. This goes into the sewer, like every other sink and toilet, and (usually) goes to a water treatment plant.

You may be thinking that this is biohazardous waste and needs to be treated differently. [If] we can’t put oil, or chemicals (like formalin) down the drains due to regulations, why is blood not treated similarly? I would assume because it is effectively handled by the water treatment plants. If it wasn’t, I am sure the regulations would be changed.

Now any items that are soiled with blood—those cannot be thrown away in the regular trash. Most clothing worn by the decedent is either retained for evidence or released with the decedent to the funeral home—even if they were bloody.

But any gauze, medical tubing, papers, etc. that have blood or bodily fluids on them must be thrown away into a biohazardous trash. These are lined with bright red trash liners, and these are placed in a specially marked box and taped closed. These boxes are stacked up in the garage until they are picked up by a specialty garbage company. I am not sure, but I am pretty sure they are incinerated.

Additionally anything sharp or pointy—like needles, scalpels, etc.—must go into a rigid “sharps” container. When they are 2/3 full we just toss these into one of the biotrash containers.

The biotrash is treated differently, as, if it went to a landfill, then the blood (and therefore the bloodborne pathogens like Hepatitis and HIV) could be exposed to people or animals. Rain could wash it into untreated water systems.

This post originally appeared on Quora. Click here to view.

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Big Questions
Why Does Asparagus Make Your Pee Smell Funny?
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The asparagus has a long and storied history. It was mentioned in the myths and the scholarly writings of ancient Greece, and its cultivation was the subject of a detailed lesson in Cato the Elder's treatise, On Agriculture. But it wasn't until the turn of the 18th century that discussion of the link between asparagus and odorous urine emerged. In 1731, John Arbuthnot, physician to Queen Anne, noted in a book about food that asparagus "affects the urine with a foetid smell ... and therefore have been suspected by some physicians as not friendly to the kidneys." Benjamin Franklin also noticed that eating asparagus "shall give our urine a disagreeable odor."

Since then, there has been debate over what is responsible for the stinky pee phenomenon. Polish chemist and doctor Marceli Nencki identified a compound called methanethiol as the cause in 1891, after a study that involved four men eating about three and a half pounds of asparagus apiece. In 1975, Robert H. White, a chemist at the University of California at San Diego, used gas chromatography to pin down several compounds known as S-methyl thioesters as the culprits. Other researchers have blamed various "sulfur-containing compounds" and, simply, "metabolites."

More recently, a study demonstrated that asparagusic acid taken orally by subjects known to produce stinky asparagus pee produced odorous urine, which contained the same volatile compounds found in their asparagus-induced odorous urine. Other subjects, who normally didn't experience asparagus-induced odorous urine, likewise were spared stinky pee after taking asparagusic acid.

The researchers concluded that asparagusic acid and its derivatives are the precursors of urinary odor (compared, in different scientific papers, to the smell of "rotten cabbage," "boiling cabbage" and "vegetable soup"). The various compounds that contribute to the distinct smell—and were sometimes blamed as the sole cause in the past—are metabolized from asparagusic acid.

Exactly how these compounds are produced as we digest asparagus remains unclear, so let's turn to an equally compelling, but more answerable question:

WHY DOESN'T ASPARAGUS MAKE YOUR PEE SMELL FUNNY?

Remember when I said that some people don't produce stinky asparagus pee? Several studies have shown that only some of us experience stinky pee (ranging from 20 to 40 percent of the subjects taking part in the study, depending on which paper you read), while the majority have never had the pleasure.

For a while, the world was divided into those whose pee stank after eating asparagus and those whose didn't. Then in 1980, a study complicated matters: Subjects whose pee stank sniffed the urine of subjects whose pee didn't. Guess what? The pee stank. It turns out we're not only divided by the ability to produce odorous asparagus pee, but the ability to smell it.

An anosmia—an inability to perceive a smell—keeps certain people from smelling the compounds that make up even the most offensive asparagus pee, and like the stinky pee non-producers, they're in the majority.

Producing and perceiving asparagus pee don't go hand-in-hand, either. The 1980 study found that some people who don't produce stinky pee could detect the rotten cabbage smell in another person's urine. On the flip side, some stink producers aren't able to pick up the scent in their own urine or the urine of others.

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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