No one wants high blood pressure. Here's the secret to keeping it low (but not too low).

Understanding blood pressure readings can be tricky. While most people know they don't want their blood pressure to get too high (hypertension) or too low (hypotension), many people don't know how to tell the difference. Some also don't understand what blood pressure means.

"Blood pressure is a measure of force necessary to move blood from the heart to vital organs and limbs around the body," explains Viet Le, associate professor of preventive cardiology and physician associate at Intermountain Health.

Measuring how the heart rests between heartbeats and the force of blood against the walls of one's arteries is what taking one's blood pressure is all about.

What do blood pressure numbers mean?

Blood pressure is measured using two numbers that are typically recorded as one number over another. The first, or top, number represents one's systolic blood pressure and measures "the force of your heart pumping blood through your arteries," says Barbara Olendzki, associate professor of population and quantitative health sciences at UMass Chan Medical School. "Certain conditions may make this higher, thus making your heart work harder."

The second, or bottom, number represents one's diastolic blood pressure – the pressure that occurs when the heart rests between heartbeats – the time when the heart fills with blood and obtains oxygen. "Both measures are important," Olendzki says.

What is a healthy blood pressure?

"Generally speaking, a healthy blood pressure is a systolic number of 120 or below, and a diastolic number in the 70s or 80s," explains Dr. Efrosini Barish, a clinical assistant professor in the Department of Medicine at NYU Grossman School of Medicine. "Though there is some fluctuation is those guidelines for individuals," she adds.

Broken down, the American Heart Association shows that normal systolic blood pressure is below 120, elevated blood pressure is when the top number is between 120 and 129 and high blood pressure is when the systolic number is 130 to 139. There's also a higher stage of blood pressure known as hypertension stage 2 which is a systolic number of 140 or more, plus a crisis level that's reached when one's systolic level climbs higher than 180. Any systolic blood pressure level that high requires immediate medical attention.

As for the bottom number, normal diastolic blood pressure is lower than 80 and high blood pressure starts when the diastolic number is between 80 and 89. Hypertension stage 2 occurs when that bottom number gets higher than 90.

In case you missed: High blood pressure can become a dangerous health problem: What you need to know to lower it

What happens when blood pressure is too high or too low?

While higher stages of hypertension are especially worrisome, Le notes that vascular-related disease such as heart attacks, aneurysms, strokes, eye damage and kidney damage "can occur when pressures are even slightly elevated (130/80) for sustained periods of time." And though hypertension is usually considered more dangerous than hypotension, low blood pressure can cause worrisome symptoms as well. These include fatigue, dizziness or lightheadedness, nausea, trouble thinking or blurred vision.

Le suggests thinking of high blood pressure like a garden hose being hooked up to a fire hydrant, and the damage that would cause to the hose (arteries) or any apparatuses (organs) it's connected to, and to think of low blood pressure as being similar to what a drought would do to lakes and streams. "The surrounding or downstream organs will suffer from not getting appropriate amounts of blood flow and may also become damaged," he explains.

Many factors affect one's blood pressure including heart disease, dehydration, activity levels, stress, medications and certain foods. "For those individuals who are struggling with high blood pressure, the number one cause seems to be from a high-sodium diet," says Doris Chan, a cardiologist at NYU Langone Hospital in Brooklyn. "Make a conscious effort to eliminate foods high in salt and watch your blood pressure start to normalize."

Barish agrees and also stresses the importance of having one's blood pressure checked often. "The best way to avoid problems is to see your doctor regularly," she advises. "Especially if you’re older than 40, have medical issues or have a family history of hypertension."

Heads up: Some people function normally with low blood pressure, but there are times for concern.

This article originally appeared on USA TODAY: What is a healthy blood pressure? Know what the numbers mean.