What is hypertension? The 'silent killer' that nearly one-third of Americans have but most don't realize it

• Hypertension occurs when you have blood pressure greater than 130/80 mm Hg.
• Nearly one-third of Americans have hypertension but most never exhibit symptoms.
• High blood pressure can be dangerous, with complications including blindness and stroke.

When your heart pumps blood throughout your body, the blood exerts a force against the walls of your arteries. The strength of this force — known as blood pressure — plays a critical role in your health.

High blood pressure, aka hypertension, can lead to heart disease or stroke if left uncontrolled. Here's what medical experts consider to be high blood pressure, how to know if you have it, and what it means for you.

Understanding hypertension

Nearly one in three American adults have high blood pressure, or hypertension, which is when your systolic blood pressure is greater than 130 mm Hg or your diastolic blood pressure is greater than 80 mm Hg.

During a blood pressure reading, the top number represents the phase when your heart is pumping and the arteries are filled with blood. This is your systolic blood pressure.

The bottom number represents the phase when your heart is relaxing in between beats. This is your diastolic blood pressure. Both of these two numbers are measured in millimeters of mercury (mm Hg).

Because many factors can affect the accuracy of a blood pressure reading — from your nervousness about a doctor's visit to smoking a cigarette beforehand — your physician must check your blood pressure more than once.

"You need at least two separate significantly elevated blood pressures on at least two different occasions to be diagnosed with hypertension," says Jason McKnight, MD, a primary care physician at Texas A&M University Family Care.

What is considered high blood pressure?

There are four general categories of blood pressure measurements:

• Normal blood pressure: Lower than 120/80 mm Hg.
• Elevated blood pressure: Systolic pressure ranges from 120 to 129 mm Hg and diastolic is below 80 mm Hg.
• Stage 1 hypertension: Systolic ranges from 130 to 139 mm Hg and diastolic ranges from 80 to 89 mm Hg. This is considered moderate hypertension.
• Stage 2 hypertension: Higher than 140/90 mm Hg. This is considered severe hypertension.

High blood pressure symptoms

According to McKnight, high blood pressure is when your heart is working too hard when it's pumping. But you likely won't feel any obvious symptoms — that's why hypertension is often known as the "silent killer."

"For the majority of Americans, you really don't know that high blood pressure is what's going on," McKnight says. "The only way to know is through routine visits with your physician or if you go sit down at the blood pressure machine at the supermarket."

If your blood pressure is dangerously high — like greater than 180/120 mm Hg — you may experience some symptoms including chest pain or even seizures.

But this is only for severely high blood pressure, also known as a hypertensive crisis. In these instances, you should seek medical attention immediately.

What if high blood pressure is left untreated?

If you have hypertension, what's happening is that the excess pressure is damaging the vessels that provide blood flow to certain regions of your body. And when high blood pressure isn't treated, it can pose dangers to the most vital parts of your body.

"Your heart, brain, eyes, and kidneys are four organ systems that can be highly affected by long-term, uncontrolled high blood pressure," McKnight says.

Hypertension may increase your risk of complications ranging from a type of blindness called hypertensive retinopathy to a heart attack or stroke. In fact, high blood pressure contributes to nearly 1,000 deaths a day, according to the CDC.

The higher the blood pressure and the longer it goes untreated, the more damage it can create. According to a 2008 study, people with uncontrolled hypertension had a greater risk of death when compared to people with controlled hypertension. However, getting diagnosed early and receiving treatment can help prevent the risks associated with hypertension.

How to control high blood pressure

Changing your lifestyle through a low-sodium diet and regular exercise can help control high blood pressure. This treatment route is typically recommended for anyone with hypertension, McKnight says.

Still, lifestyle modifications aren't always enough to properly manage high blood pressure. For these patients, doctors may also prescribe medications like diuretics or ACE inhibitors.

In addition to these aspects of treatment, McKnight says it's important to routinely visit your doctor to monitor your blood pressure and keep it under control.

Insider's takeaway

Hypertension is a serious condition that can lead to permanent health consequences including blindness, heart attack, and in severe cases, death.

Most people who have high blood pressure don't realize it because symptoms typically only manifest when blood pressure levels are extremely high.

That's why it's critical that if you are at risk of hypertension, you schedule regular visits with your doctor. You can also purchase a machine that will allow you to track your blood pressure at home.

Staying active and eating a healthy diet goes a long way toward preventing hypertension and promoting longevity.