Blood pressure basics: What do systolic and diastolic mean?

Every time you go to the clinic, you get your blood pressure checked. Someone tells you the result—ideally a higher number over a lower number—and that’s noted in your medical record. But what do those numbers mean and why are they important?

We asked Michael Sills, MD, a cardiologist on the medical staff at Baylor Scott & White Heart and Vascular Hospital – Dallas, to fill us in.

What is blood pressure?

Your heart pumps blood throughout your body in a system of arteries, veins and capillaries. The force with which that blood moves is called blood pressure.

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

What do the numbers mean?

“There are two numbers used to measure blood pressure,” he said. “The top number is called systolic. That 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.”

Your blood pressure is taken with an instrument called a sphygmomanometer. A rubber cuff is placed around your upper arm; a healthcare worker pumps up the cuff with air, listening for your heartbeat. When the thumping stops, he or she will pump it up further and then begin a slow release of the pressure. The cuff functions as a tourniquet, briefly stopping the flow of blood in your artery.

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

What is “normal” blood pressure?

These are 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. Dr. Sills said to think of the numbers as resistance, and what’s best is to have low numbers, representing low resistance to the heart’s efforts to pump blood throughout your body.

“Both the systolic and diastolic numbers are important for a healthy blood pressure,” Dr. Sills said. “You want your blood pressure to be less than 130/80 on average. Both numbers need to be in the ideal range. The old, conventional wisdom was that only the bottom number mattered. That’s not true. Heart disease, stroke and kidney disease can all be increased as much or more by a high systolic pressure as by elevated diastolic pressure.”

Blood pressure can vary a lot during the day, depending on a number of factors. Exercise, stress and anxiety can raise your blood pressure. Even in a single day, your blood pressure can fluctuate dramatically depending on what you’re doing.

If your blood pressure stays high, you might have a condition called 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. “This puts you at greater risk for heart disease, stroke, and kidney disease.”

The National Institutes of Health recommends screening adults for high blood pressure every two years if their blood pressure is normally less than 120/80 mmHg; however, the NIH recommends that adults with high blood pressure should have their blood pressure checked every year or more often.

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