It's not just high blood pressure you have to worry about—low blood pressure can be dangerous, too.

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It's one of the first things that happens during a checkup—a nurse slaps a black cuff on your upper arm, inflates it, then shouts out two numbers that later go into your chart. It's a blood pressure reading, of course—but those two numbers can cause quite a bit of confusion if you don't know what they mean.

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What's more: There's not usually anything that would tip you off to a potential high blood pressure reading, until you get the reading itself. "The problem with high blood pressure is that, contrary to popular belief, the vast majority of people will not experience any symptoms," Ian Del Conde Pozzi, M.D., a cardiologist and vascular medicine specialist for Baptist Health's Miami Cardiac & Vascular Institute, tells Health. "This is the reason high blood pressure has been dubbed 'the silent killer.'"

To help you prepare for your next doctor's visit—or if you've been tasked with monitoring your blood pressure at home—here's what you need to know about those all-important numbers, and how to keep your blood pressure in a healthy range.

What exactly is blood pressure?

"Blood pressure is the pressure that exists within our arteries and drives blood throughout our body," says Dr. Del Conde Pozzi. At normal levels, your heart creates just enough blood pressure to pump blood to vital organs like your eyes and kidneys without damaging your arteries over time.

If your blood pressure is elevated, though this hinders blood flow over time and can cause organ damage, hardening of your arteries, and plaque buildup within your arteries. These changes narrow and stiffen your arteries, increasing the risk of blood clots, heart disease, stroke, heart attack, and kidney disease, per the Centers for Disease Control & Prevention (CDC).

What do blood pressure numbers mean?

Blood pressure is measured with two key metrics: a top number called systolic blood pressure and a bottom number called diastolic blood pressure.

For example, a blood pressure of 120 systolic and 80 diastolic reads as "120 over 80" or "120/80 mm Hg" in doctor-speak ("mm Hg" means millimeters of mercury since the element was used in early blood pressure gauges and remains the standard unit of measurement), according to the American Heart Association (AHA).

"Systolic blood pressure is the peak pressure during a heartbeat while the heart is 'squeezing,' and diastolic blood pressure is the lowest pressure between two heartbeats while the heart is 'relaxed,'" Jennifer Wong, MD, a cardiologist and medical director of non-invasive cardiology at MemorialCare Heart and Vascular Institute at Orange Coast Medical Center in Fountain Valley, CA, tells Health.

What's a healthy blood pressure range?

A healthy blood pressure range is below 120/90 mm Hg and above 90/60 mm Hg, per the US National Library of Medicine (MedlinePlus). However, exactly what normal blood pressure should be gets a little more complicated on the lower end, as the tipping point for dangerously low blood pressure can vary person to person.

It's important to keep both your systolic and diastolic blood pressures in a normal range, Uyen Lam, MD, a cardiologist and director of the Cardiovascular Rehabilitation and Prevention Center at St. Elizabeth's Medical Center in Boston, tells Health. Often, the focus is on your systolic blood pressure (the top number) because it's a well-known risk factor for cardiovascular disease. But with every increase by 20 mm Hg systolic or 10 mm Hg diastolic, your risk of death from heart disease or stroke doubles, according to the AHA.

What does it mean to have high blood pressure?

High blood pressure or hypertension is a medical condition where your blood pressure numbers are consistently above normal.

However, your blood pressure naturally rises and falls throughout the day. It could go up in the doctor's office if you have "white coat hypertension" or anxiety around medical professionals. Or you could have "masked hypertension" if your blood pressure is normal during a checkup but becomes higher later at work or home. That's why to confirm a diagnosis of elevated blood pressure, yours will need to be measured with a blood pressure cuff multiple times under controlled conditions, says Dr. Lam.

A doctor can detect and treat high blood pressure depending on which stage you're in, per the AHA. Here's what each stage means:

Elevated

Elevated blood pressure is when your readings consistently fall in between 120–129 mm Hg systolic and less than 80 mm Hg diastolic. Shifts to a healthier lifestyle can help prevent the development of high blood pressure, but yours is likely to keep inching higher it if you don't make changes.

Hypertension stage 1

Hypertension Stage 1 occurs when your readings consistently hit 130–139 mm Hg systolic or 80–89 mm Hg or higher diastolic. Depending on your risk of cardiovascular disease, your doctor may recommend lifestyle changes and possibly blood pressure medication, as well.

Hypertension Stage 2

Hypertension Stage 2 happens when your readings are 140 mm Hg systolic and higher or 90 mm Hg diastolic and higher. At this point, you're likely to get a prescription for lifestyle changes and medication alike.

Hypertensive crisis

At this stage, your systolic is higher than 180 mm Hg and/or your diastolic is higher than 120 mm Hg.

If your blood pressure is above 180/120 mm Hg and you're experiencing signs of potential organ damage like chest pain, shortness of breath, back pain, numbness, weakness, changes in vision, or trouble speaking, call 911 immediately. If your numbers are above 180/20 without these symptoms, wait five minutes and test again. If they're still up there, contact your doctor right away, per the AHA.

What does it mean to have low blood pressure?

Generally, low blood pressure or hypotension means that your blood pressure falls under 90 mm Hg systolic and 60 mm Hg diastolic. However, this should be assessed on an individual basis.

Some people have low blood pressure with no health problems, while others could have an underlying condition in need of treatment, according to the National Heart, Lung, and Blood Institute (NHLBI). Other factors that can lead to high blood pressure include:

  • Dehydration
  • Severe infection
  • Severe allergic reactions
  • Certain medications for anxiety, depression, pain, or surgery
  • Diuretics
  • Alcohol
  • Conditions like diabetes, heart failure, or an abnormal heart rhythm (arrhythmia)

If you or someone else has low blood pressure and passes out, call 911 or seek medical attention immediately. Dangerous dips in blood pressure deprive organs of oxygen and can lead to shock, a medical emergency.

If you have symptoms such as black stools, dizziness, blurred vision, chest pain, shortness of breath, or a fever above 101°F (38.3°C), call your doctor ASAP, according to MedlinePlus.

Treatment for low blood pressure involves addressing the root cause such as taking medication for an underlying condition or adjusting your current treatment plan to avoid this side effect, says Dr. Wong.

How can you keep your blood pressure in a healthy range?

To maintain your blood pressure or bring it down to a safer zone, the following lifestyle changes can make a big difference,

  • Eat less salt. The AHA suggests limiting your salt intake to no more than 2,300 mg (1 teaspoon) of salt per day or ideally less than 1,500 mg (⅔ teaspoon).
  • Move more. Aim for at least 150 minutes per week or about 30 minutes five days a week of moderate aerobic exercise, according to the CDC. Small habits like walking briskly, taking the stairs, and parking at the back of the lot can add up.
  • Adopt the DASH diet. DASH actually stands for "dietary approaches to stop hypertension," according to the NHLBI. The diet entails eating plenty of vegetables, fruits, low-fat dairy products, whole grains, poultry, fish, and nuts while cutting down on sweets, sugary drinks, and red meat can be as effective as taking blood pressure meds.
  • Limit alcohol. Men should cut themselves off at two drinks a day while women should have no more than one drink a day, per the AHA. Your doctor may also suggest at some point that you stop drinking alcohol altogether.
  • Quit smoking. Smoking's a top risk factor for heart disease, but you can quit with help such as the Smokefree community.
  • Add stress management tools to your day. Dial down the pressure with tools like daily self-care habits, therapy, support groups, and relaxation techniques, per the CDC.
  • Get enough sleep. Schedule a bedtime and wake-up time to ensure you're hitting 7–9 hours of shut-eye.

Finally, you can arm yourself with knowledge—that means knowing your numbers and if/how they're changing. know your numbers. "All adults should develop the habit of monitoring their blood pressure routinely," says Dr. Del Conde Pozzi. If yours are at healthy levels, check your blood pressure every month or two. If yours are consistently elevated, contact your doctor to figure out next steps.

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