Systolic and diastolic: Your guide to what your blood pressure numbers mean

Every time you visit the doctor, you get your blood pressure checked. You hear two numbers—like “120 over 80” —and they’re noted in your medical chart. But what do those blood pressure numbers mean, and why are they important?

We asked Michael Sills, MD, a cardiologist on the medical staff at Baylor Scott & White The Heart Hospital—Dallas, to break it down.

What is blood pressure?

Your heart pumps blood through a network of arteries, veins and capillaries that reach every part of your body. Blood pressure measures the force of that blood pushing against your artery walls.

“Think of it as resistance,” Dr. Sills said. “You want low numbers, which is low resistance.”

When there is too much resistance in the arteries, your heart has to work harder to push the blood into your body. Over time, high blood pressure can damage your heart, blood vessels and other organs.

What do systolic and diastolic mean?

A blood pressure reading consists of two numbers:

• Systolic (the top number) refers to the pressure inside the artery when the heart contracts and pumps blood through the body.
• Diastolic (the bottom number) refers to the pressure inside the artery when the heart is at rest.

“There are two numbers used to measure blood pressure,” Dr Sills said. “Systolic is the highest pressure achieved when your heart contracts and pushes the blood through your arteries—that’s about one-third of the time. The other two-thirds of the time, when the heart is refilling, is called the diastolic period.”

How is blood pressure measured?

Your blood pressure is taken with a device called a sphygmomanometer. A healthcare provider wraps an inflatable cuff around your upper arm, pumps it with air to temporarily stop blood flow, then slowly releases the pressure while listening to your heartbeat. The cuff functions as a tourniquet, briefly stopping the flow of blood in your artery.

The healthcare provider listens for the blood to begin flowing again. They will note the first heartbeat—the systolic pressure. Then listen for the final beat—the diastolic pressure—when there is no more resistance made by the cuff and the blood is flowing smoothly.

These numbers are then written as one number over the other, such as 120/70, and said as “120 over 70.” The numbers represent millimeters of mercury, which is abbreviated mmHg.

“Low numbers represent low resistance to the heart’s efforts to pump blood throughout your body,” Dr. Sills said.

What is normal blood pressure?

Normal blood pressure reading is less than 120/80 on average.

According to updated guidelines from the American Heart Association, here’s how blood pressure levels are classified:

Normal blood pressure

• Less than 120 mm Hg (systolic)
• Less than 80 mm Hg (diastolic)

Elevated blood pressure

• 120-129 mm Hg (systolic)
• Less than 80 mm Hg (diastolic)

High blood pressure

• 130 mm Hg or higher (systolic)
• 80 mm Hg or higher (diastolic)

“You want your blood pressure to be less than 120/80 on average. Both numbers need to be in the ideal range,” Dr. Sills said. “However, newer guidelines have suggested that we do not need to be as aggressive with blood pressure control as people age.” 

Your doctor can help you understand what numbers are right for you and how your lifestyle can support healthy blood pressure.

Do both numbers matter?

The short answer, yes. Both systolic and diastolic pressure are important for healthy blood pressure.

“The old, conventional wisdom was that only the bottom number mattered. That’s not true,” Dr. Sills said. "Heart disease, stroke and kidney disease can all be increased as much or more by a high systolic pressure as by elevated diastolic pressure.”

What can impact your blood pressure?

Your blood pressure isn’t static—it changes throughout the day based on a variety of factors.

Short-term factors that can raise your blood pressure include:

• Caffeine
• Certain medications
• Dehydration
• Exercise
• Stress and anxiety

If you measure your blood pressure and see elevated numbers, don’t panic. Even in a single day, your blood pressure can fluctuate dramatically based on what you’re doing.

Why is high blood pressure important?

If your blood pressure stays elevated over time, you may have a condition known as high blood pressure, or hypertension.

“Hypertension is dangerous because the heart then has to work much harder, and it causes injury to blood vessels,” Dr. Sills said.

High blood pressure often has no symptoms, which is why it’s sometimes called a “silent” condition. That’s why checking your blood pressure regularly is so important.

The good news? High blood pressure can often be managed with lifestyle changes and, when needed, medication.

How often should you check your blood pressure?

The National Institutes of Health recommends:

• Every two years if your blood pressure is normally less than 120/80 mmHg
• At least once a year (or more often) if you have high blood pressure or are at higher risk

If you have concerns about your blood pressure or aren’t sure what your numbers mean, talk with your primary care provider. Your doctor can tell you whether you're in a healthy range based on your medical history and other factors that can influence your overall health.

Don’t have a primary care provider? Find a doctor near you today to help you stay well.