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Your heart serves as the primary pump that circulates blood throughout your body. Every time it beats, the heart sends blood to critical areas, such as your muscles and bones, through a network of arteries, capillaries, and veins. As the blood moves through the arteries, it exerts varying amounts of pressure against the vessel walls. This pressure is what we refer to as your blood pressure (BP).
Each time your heart contracts to push blood to different parts of your body, your blood pressure rises—this is called the systolic number. Afterward, when the heart relaxes, your blood pressure decreases, known as the diastolic reading. Together, these two numbers are combined as a ratio, with systolic over diastolic: Your doctor might say your BP is “120 over 80.”
The American Heart Association (AHA) states that normal blood pressure should fall within a systolic range of 90 to 120, and a diastolic range of 60 to 80. To measure this, your doctor may use a tool called a sphygmomanometer—an inflatable cuff attached to a manual pump. When the cuff inflates on your arm, it temporarily cuts off the blood flow, and as it deflates, the blood flow resumes, revealing the two key numbers.
Today, doctors are encouraged to use an automatic blood pressure cuff, which operates using a different technique and is considered more accurate. While the manual cuff uses auscultation, where the doctor listens for the proper pressures with a stethoscope or microphone, automatic cuffs typically use oscillometric technology. As blood flows beneath the cuff, the arm slightly expands, and by measuring the oscillation amplitudes (hence oscillometric) across a range of pressures, blood pressure can be determined in a similar manner.
If you have high blood pressure, or hypertension—generally around 130/80 or higher for someone in average health—your heart has to work harder to circulate blood, which can be harmful. The AHA states [PDF] that elevated blood pressure ranges from 120–129/under 80; hypertension stage 1 is 130–139 (systolic) or 80–89 (diastolic); and hypertension stage 2 is 140 or above (systolic) or 90 or above (diastolic). If your blood pressure reaches 180/120, you're in a hypertensive crisis and should get help.
If you fall into these categories, your doctor will suggest adjustments to your diet and exercise, and possibly prescribe medication. High blood pressure often leads to heart disease or heart attacks and may be associated with other conditions like diabetes. However, your blood pressure can temporarily rise due to stress, pregnancy, or even common medications such as over-the-counter painkillers and antidepressants. One elevated reading doesn't automatically mean you have hypertension, but it's important to remain cautious.
Editor's note: This article was updated in July 2018 to reflect the new blood pressure guidelines from the AHA.
